Conventional BCS mechanism of HTSC in hydrides, a la' Ashcroft works remarkably well in H_3S, LaH_{10} etc. We pointed out (GB 2015) [1] that H-atom networks in hydrides is a Mott insulating subsystem, which supports RVB superconductivity via internal doping. We will present a case for occurrence of both BCS and RVB pairing to varying degrees, in different hydrides. It resolves several existing puzzles. In our theory, H-C-S and Lu-N-H systems have dominant RVB pairing, which favors competing orders create, encouraged by structural variations and disorder, wide variation in Tc (like in cuprates). On the other hand, H_3S and LaH_{10} is a dominant phonon mediated BCS system; consequently Tc is protected by Anderson theorem and varies less. We provide support for our electronic mechanism from the works of Ng et al. (1996) and Eder et al., (1997) on emergence of Kondo insulating states in LaH_3 and YH_3, via Kondo coupling of band electrons to localized H atom spins.

Superconducting qubits have emerged as the prominent building block of a scalable quantum computer. While inventing novel qubit designs has enabled the progress towards multiqubit systems that have demonstrated quantum advantage, there much more to be gained by properly engineering the qubit materials. Coherence time of the qubit, the relevant figure of merit, is limited by materials losses. In particular, the main limiting factor is the presence of “parasitic” two-level systems (TLS) that manifest in increased losses of the superconducting circuits at millikelvin temperatures and single-photon powers. Amorphous oxides, residues and transition layers at interfaces and grain boundaries are identified as sites harboring such TLS states. In this talk, I will present techniques to carefully address each interface, superconducting films, and substrate, and ultimately reduce the losses in the superconducting circuit in the quantum regime (millikelvin temperatures, single-photon occupancy). This will ultimately allow to push the envelope regarding coherence times and allow for quantum processors with larger number of qubits and larger quantum volume.

What is the connection between Aristotle and an ice cream? The answer is encoded in the Mpemba effect, the counterintuitive and controversial phenomenon that hot water cools faster than cold one. Here I will provide an analog quantum definition of this phenomenon, which has been studied in a quantum quench of a spin chain whose initially broken global U(1) symmetry is restored dynamically. Studying this evolution, we find that the more the symmetry is initially broken, the faster it is restored. We dub the measure detecting this effect entanglement asymmetry, which is a measure of symmetry breaking inspired by the theory of entanglement in many-body states. Although symmetry breaking is a pillar of modern quantum physics, quantifying how much a symmetry is broken is an issue that has received little attention so far: The merit of the entanglement asymmetry is to provide a quantitative tool for this.

Realization of artificial spin-orbit (SO) coupling in the cold atom systems has opened a broad spectrum of phenomena to be observed experimentally in bosonic systems. The physics of spontaneous symmetry breaking leads to the several exotic quantum phases such as superfluid and more recently the supersolid. In the latter case, both diagonal and off-diagonal long-range order, although being mutually exclusive, are present and it breaks two U(1) symmetries. One of the ways to realize this supersolid phase is using the condensates of Alkali atoms in a double well optical lattice. Such SO coupled systems have interesting phase diagrams depending on the interaction and SO coupling strength. In presence of weak coupling and negligible inter-species interaction the system remains in the phase known as striped phase or miscible phase. In this case, the Raman beam couples two condensates and provides a momentum kick along a perpendicular direction, which indeed leads to a one-dimensional density modulation due to interference.  Two continuous symmetries are broken by the superfluid phase and the breaking of translation symmetry along one direction, satisfying the definition of a supersolid. Spinor BECs and their relative phase coherence has been a topic of interest for quite some time. The dephasing of the relative phase has important implications in the density modulation and its experimental observation.  I will consider a specific realization of a pseudo-spin-1/2 BEC, in which the two spin components are represented by spatially separated low-lying states in a double-well potential. I will demonstrate that even if the system is initialized with a given relative phase between the two components, there is an apparent dephasing of the relative phase. The dephasing is a consequence of a very weak interaction between the spin species. Furthermore, I will demonstrate that the dephasing rate is a non-monotonic function of the Raman coupling strength. The minimum of the dephasing rate corresponds to the critical value of the Raman coupling, above which miscible to immiscible phase transition occurs.

In this talk, I will discuss energy dissipation and heating in slowly driven quantum systems, focusing on topological driving schemes. In the first part of my talk, I will present a system in which many-body dynamics leads to the emergence of a quasi-steady state with a high entropy density and yet robust topological transport. I will explain the mechanisms behind this phenomenon and demonstrate the emergence of the quasi-steady state on an exactly solvable strongly coupled fermionic model. In the second part of my talk, I will show that the dissipation of energy in nearly adiabatic quantum systems is linked to the quantum geometry of the problem. Interestingly, this result implies a topological bound on the energy dissipation rate in a class of topological systems. Our findings uncover new connections between topology and dissipation in slowly driven quantum systems, shedding light on their fundamental properties and potential for practical applications, such as the development of optimized driving protocols for topological drives.

Every animal cell is filled with a cytoskeleton, a dynamic gel made of inextensible filaments / bio-polymers, such as microtubules, actin filaments, and intermediate filaments, all suspended in a viscous fluid. Similar suspensions of elastic filaments or polymers are widely used in materials processing. Numerical simulation of such gels is challenging because the filament aspect ratios are very large.

We have recently developed new methods for rapidly computing the dynamics of non-Brownian and Brownian inextensible slender filaments in periodically-sheared Stokes flow [1,2,4]. We apply our formulation to a permanently1 and dynamically cross-linked actin mesh3 in a background oscillatory shear flow. We find that nonlocal hydrodynamics can change the visco-elastic moduli by as much as 40% at certain frequencies, especially in partially bundled networks [3,4].

I will focus on accounting for bending thermal fluctuations of the filaments by first establishing a mathematical formulation and numerical methods for simulating the dynamics of stiff but not rigid Brownian fibers in Stokes flow [4]. I will emphasize open questions for the community such as whether there is a continuum limit of the Brownian contribution to the stress tensor from the filaments.

References:

1. O. Maxian et al, Integral-based spectral method for inextensible slender fibers in Stokes flow,. Phys. Rev. Fluids, 6:014102, 2021
2. O. Maxian et al,. Hydrodynamics of a twisting, bending, inextensible fiber in Stokes flow, Phys. Rev. Fluids, 7:074101, 2022
3. O. Maxian et al, Interplay between Brownian motion and cross-linking controls bundling dynamics in actin networks, Biophysical J., 121:1230\u20131245, 2022.
4. O. Maxian et al., Bending fluctuations in semiflexible, inextensible, slender filaments in Stokes flow: towards a spectral discretization, ArXiv:2301.11123, to appear in J. Chem. Phys., 2023.

Two-dimensional hybrid organic-inorganic perovskites (HOIPs) have shown promising progress in light-emitting diodes applications. The three-dimensional (3D) HOIPs, commonly used as novel solar cells, exhibit extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects ^[1]. It has been experimentally shown that the reorientation of the polarized organic molecules can facilitate the polaron formation, where polaron is referred as a quasiparticle formed by the Coulomb interaction between an excess charge (an electron or a hole) and the ionic lattice, enhance the screening effects on charge carriers, and thus prolong the charge carrier lifetime ^[2-3]. Whereas the pure inorganic 3D perovskites without organic molecules, such as CsPbI₃, also show a moderate photovoltaic performance, which brings researcher’s efforts into investigations on dynamics of the inorganic perovskite framework ^[4]. An understanding of the microscopical mechanisms behind extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection in HOIPs is lacking. We performed time-of-flight neutron spectroscopy for two perovskites, butylammonium lead iodide (BA)₂PbI₄ (BA) and phenethyl-ammonium lead iodide (PEA)₂PbI₄ (PEA). From the obtained spectra we identified and quantitatively separated the rotational and phonon contribution for BA. Similar analysis would be performed for the PEA spectra in future. We try to understand how both inorganic vibrational dynamics and organic molecule rotational dynamics contribute to charge carrier lifetime and hence power conversion efficiency of solar cells. The study is important to get an idea on how to engineer new HOIPs by exploiting these dynamics for higher device performance. By examining the corresponding temperature dependence, we revealed that the rotational dynamics of organic molecules in these materials tends to suppress their photoluminescence quantum yield ^[5] while the vibrational dynamics did not show predominant correlations with their optoelectronic properties.

[1] Mei, Anyi, et al. science 345.6194 (2014): 295-298.

[2] Miyata, Kiyoshi, Timothy L. Atallah, and X-Y. Zhu. Science Advances 3.10 (2017): e1701469.

[3] Chen, Tianran, et al. Proceedings of the National Academy of Sciences 114.29 (2017): 7519-  7524.

[4] Wang, Kang, et al. Nature communications 9.1 (2018): 4544.

[5] Gong, Xiwen, et al. Nature materials 17.6 (2018): 550-556.

In recent years anomalous cooling and heating effects in the far from equilibrium limit have gained attention. One anomaly is the so called Mpemba Effect, in which the time to relax towards thermal equilibrium does not grow monotonically as a function of distance to the target. Instead, it has been proposed that there exist shortcuts in the relaxation process that allow both faster, and even exponentially faster heating and cooling. In this talk I will discuss recent works [1,2] that have progressed our understanding of such shortcuts by studying the Mpemba effect using Overdamped Langevin dynamics. I will show when and where you can get the effect, and that our models are in good agreement with experimental findings. Lastly, I will touch upon current works where we study the effect using Markovian jump processes on linear reaction networks.  

1. Anomalous thermal relaxation of Langevin particles in a piecewise-constant potential

Matthew R Walker and Marija Vucelja J. Stat. Mech. (2021) 113105

2. Mpemba effect in terms of mean first passage times for overdamped Langevin dynamics

Matthew R Walker and Marija Vucelja arXiv preprint arXiv:2212.07496 (2022)

Motivated by recent experimental observations of unconventional superconductivity in twisted bilayer and trilayer graphenes, we develop a theory describing the differential conductance between a normal STM tip and a 2D superconductor with an arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmission between the tip and the superconductor. Exploiting the dependence of Andreev reflection on the relative position of the STM tip with respect to the lattice symmetry points, we show that the structure of the superconducting gap can be extracted by combining weak- and strong-tunneling limits of differential conductance. Furthermore, the theory incorporates a tip/impurity-induced scattering potential within the 2D material, which allows us to describe subgap resonances.

Since many years, the canonincal classification of ordered magnets included noncollinear (with many further subdivisions) and two collinear

types: antiferromagnets (AF), which have net magnetization zero by symmetry, and ferro/ferrimagnets (FM), which do not have this property.

The two have distinctly different micro- and macroscopic properties. It was supposed, for instance, that only FM can exhibit spin-splitting of the electronic bands in absence of spin-orbit coupling AND lack of inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven by variation of the Berry phase), magnetooptical effects, suppressed Andreev scattering in contact with a singlet superconductor etc.

A surprisingly recent development (~2019) is that this classification is

incomplete: there are collinear magnets that would belong to AF by this classification, but show all characteristics of FM, *except the net spin polarization*! They were recently dubbed by Mainz group "altermagnets", AM. Incidentally, what has also not been fully appreciated was that there are also materials that have strictly zero net magnetization, but enforced not by symmetry, but by the Luttinger's theorem, and therefore truly belonging to the FM class ("Luttinger-compensated ferrimagnets").

In this talk I will present the new classification and explain, in specific examples, what are the symmetry conditions for AM, why these are a truly new class deserving a new name, and how their unusual properties appear.

The Mpemba effect describes the situation in which a hot system cools faster than an identical copy that is initiated at a colder temperature. In many of the experimental observations of the effect, e.g. in water and clathrate hydrates, it is defined by the phase transition timing. However, none of the theoretical investigations so far considered the timing of the phase transition, and most of the abstract models used to explore the Mpemba effect do not have a phase transition. In this talk, I will suggest a definition for the phase transition time in a non-equilibrium state using the Landau theory for phase transitions. Using this definition, I will show that a Mpemba effect with respect to phase transitions can exist in such models, namely that the hotter system undergoes the transition before the colder one when quenched to a cold temperature.

Simulation of quantum many-body physics, such as looking for ground state properties and real time dynamics, plays an important role in the study of condensed matter physics, high energy physics and quantum information science. The recent advancement of machine learning provides new opportunities for tackling challenges in simulating quantum many-body physics. In this talk, I will first discuss a class of wave functions via neural network transformation, neural network backflow,  which can fulfill the anti-symmetry property and capture the correlation and the sign structure for strongly-interacting fermionic physics. Next, I will talk about recent progress of simulating continuum quantum field theories with neural quantum field state [2], and lattice gauge theories such as 2+1D quantum electrodynamics with finite density dynamical fermions using gauge symmetric neural networks [3,4]. Finally, I will present a neural network representation based on positive-value-operator measurements for quantum circuit and open quantum system dynamics simulation [5].

Reference:
[1] Di Luo, Bryan K. Clark, Backflow Transformations via Neural Networks for Quantum Many-Body WaveFunctions, Phys. Rev. Lett. 122, 226401.
[2] John M. Martyn, Khadijeh Najafi, Di Luo, Variational Neural-Network Ansatz for Continuum Quantum Field Theory, https://arxiv.org/abs/2212.00782.
[3]  Di Luo, Giuseppe Carleo, Bryan K. Clark, James Stokes, Gauge Equivariant Neural Networks for Quantum Lattice Gauge Theories, Phys. Rev. Lett. 127, 276402.
[4] Zhuo Chen†, Di Luo†, Kaiwen Hu, Bryan K. Clark, Simulating 2+1D Lattice Quantum Electrodynamics at Finite Density with Neural Flow Wavefunctions, https://arxiv.org/abs/2212.06835.
[5] Di Luo†, Zhuo Chen†, Juan Carrasquilla, Bryan K. Clark, Autoregressive Neural Network for Simulating Open Quantum Systems via a Probabilistic Formulation, Phys. Rev. Lett. 128, 090501.

 

The Loudon-Fleury form of the Raman operator has been used to compute magnetic Raman responses inside magnetic insulators for decades. The formalism provided by Loudon and Fleury [1] only considers the light-induced direct hopping between two magnetic ions. However, this is an oversimplified scenario for spin-orbit coupled Mott insulators. For example, the microscopic origin of the superexchange interaction inside Kitaev materials shows multiple indirect superexchange paths involving ligand ions can also produce a significant anisotropic interaction [2]. Therefore, to correctly construct the Raman operator for spin-orbit coupled Mott insulators requires considering all the non-negligible direct and indirect superexchange paths.

In this talk, I will present our work on constructing the Raman operator for the spin-orbit coupled Mott insulators which involve multiple superexchange paths [3], and I will also show how our revised theory can be applied to the three-dimensional hyperhoneycomb Kitaev material β−Li2IrO3 [4], where we show a qualitative modification of the polarization dependence, including, e.g., the emergence of a sharp one-magnon peak at low energies, which is not expected in the traditional Loudon-Fleury theory.

[1] P. A. Fleury and R. Loudon, Phys. Rev. 166, 514 (1968).

[2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).

[3] Yang Yang, Mengqun Li, Ioannis Rousochatzakis, and Natalia B. Perkins Phys. Rev. B 104, 144412 (2021).

[4] Yang Yang, Yiping Wang, Ioannis Rousochatzakis, Alejandro Ruiz, James G. Analytis, Kenneth S. Burch, and Natalia B. Perkins Phys. Rev. B 105, L241101(2022).

Magnetic excitations in the spin-stripe phases of La-based 214-nickelates have been vigorously explored using inelastic neutron scattering (INS) study for almost last three decades and still have remained an exciting research field, especially to understand their differences yet of their structural similarities with high-Tc 214-cuprates. In view of so far reported two-dimensional antiferromagnetic nature, out-of-plane magnetic excitations are generally not expected in 214-nickelates. In this talk, I will present our recent results from INS measurements on the stripe discommensurated phases of Sr-doped Pr_3/2Sr_1/2NiO₄ samples, showing a compelling evidence for the presence of a sizable out-of-plane interaction suggesting a three-dimensional nature of magnetic excitations near the half-doped region [1,2]. The measured magnetic excitations are in good agreement with our linear spin wave (LSW) theory based calculations in the discommensurated spin stripe models. Additionally, I will discuss the effect of short-range vs. long-range spin stripe correlations on the spin wave dispersion by comparing the results with our INS study on an O-doped sample Pr₂NiO_4+δ [3].

Neutron scattering and its complementary X-ray scattering techniques help to probe many novel and
exotic quantum phenomena in materials and here, two case studies on spin dynamics in square lattice
Heisenberg antiferromagnets (AFM) are highlighted. Doped 214 -nickelates have served as a model
compound to understand its spin and charge stripe correlation in homologous superconducting
layered 214 -cuprates since last two decades. There are many similarities between La-based and
Pr-based 214-nickelates in terms of static stripe phases which emerge upon doping but, La-based
214 -nickelates have been treated so far as quassi 2D-XY AFM because of its negligible out-of-plane
exchange interaction. However, in our recent studies using inelastic neutron scattering (INS) and
linear spin wave theory (LSWT) calculations we have observed a sizeable out-of-plane exchange
interactions and the corresponding 3D spin wave dispersion in Pr1.5Sr0.5NiO4 [1, 2]. Besides static
charge and spin stripe degrees of freedom in Pr2−xSrxNiO4+δ, oxygen ordering on top of it plays
an important role and we have shown that presence of spin stripe fluctuation is not a prerequisite
for the formation of static charge stripe [3]. Another model compound multiferroic Ba2CoGe2O7
serves as an upstanding platform to study in gernal square lattice quantum physics with (S ≥ 1
2 ) besides its unconventional metal-ligand d-p hybridization mechanism which is responsible for giant
magnetoelectric effects. Using INS under applied high magnetic fields, we have demonstrated that
easy-plane-type single-ion anisotropy (SIA) can be tunable under fields and SIA is responsible for
the changes in the optical modes and unconventional electromagnon modes in 3D anisotropic spin
dispersion [4].

[1] A. Maity, R. Dutta, and W. Paulus, Stripe discommensuration and spin dynamics of half-doped Pr3/2Sr1/2NiO4, Phys. Rev.
Lett. 124, 147202 (2020).
[2] R. Dutta, A. Maity, A. Marsicano, J. R. Stewart, M. Opel, and W. Paulus, Direct evidence for anisotropic three-dimensional
magnetic excitations in a hole-doped antiferromagnet, Phys. Rev. B 102, 165130 (2020).
[3] A. Maity, R. Dutta, and W. Paulus, Spin stripe fluctuations in antiferromagnetic Pr2−xSrxNiO4+δ, Phys. Rev. B 106,
024414 (2022).
[4] R. Dutta, H. Thoma, I. Radelytskyi, A. Schneidewind, V. Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura, and V. Hutanu, Spin
dynamics study and experimental realization of tunable

Swirling spin textures, such as helices and vortices, appear in chiral magnets, and these noncolinear and noncoplaner spin textures bring about characteristic resonance structures and quantum transports. Recently, a one-dimensional chiral magnet hosting a conical spin texture and a chiral soliton lattice (CSL) have attracted a lot of interest since resonance frequencies as well as a magnetic modulation period can be flexibly controlled by an external magnetic field [1,2]. However, the dynamical properties of these spin textures associated with the oscillating magnetic field and the resultant emergent electromagnetic phenomena have not been systematically clarified. 

In this study, we theoretically study the resonance modes and the emergent electromagnetic phenomena in a one-dimensional chiral magnet by numerically solving the Landau-Lifshitz-Gilbert equation (LLG eq.) and by using the linear spin wave theory. We systematically clarified the magnon band structure by varying the external magnetic field and find that the band gaps increase with the magnetic field perpendicular to the chiral axis. We also find the edge modes appear within the band gap and clarify that the swirling spin textures penetrate into the system from the edges by activating the edge modes. In the talk, we will also discuss the enhancement of the emergent electric phenomena and AC magnetic field drive of swirling spin textures.

Information technology backed by sophisticated semiconductor devices have deeply changed our society in the past two decades, with AI-driven tools such as ChatGPT posited to bring even deeper impact. However, in the physics governing devices behind most of these applications, we have only utilized the charge carried by electrons, while ignoring the other inherent quantum property, the spin.  My research focuses on the understanding of the spin degree of freedom of electrons at nanoscales. First I will give an introduction on a few key phenomena in the field of spintronics, such as the coherent tunneling of spins by controlling the symmetry of the wavefunctions, and the exchange scattering effect in materials with compensated magnetization where the spins can be manipulated in the picosecond time scale. Then I will present in detail one of our research directions in which we attempt to control the order parameter of magnetic systems by using electric fields, instead of magnetic fields or spin-polarized currents. Through the voltage controlled magnetic anisotropy effect where the energy of the system can be modified by the redistribution of wavefunctions induced by external electric potentials, a 100-fold reduction in switching current density has been realized. We have demonstrated that both the magnetic anisotropy and saturation magnetization of a metallic ferromagnet can be controlled by voltage, leading to a new method to modify the interlayer exchange coupling of the system, directly verified by in-situ X-ray magnetic circular dichroism experiment. In addition to the ferromagnetic order, the antiferromagnetic order can also be effectively manipulated by electric fields. Finally, I will describe our recent effort to reduce the switching energy of magnetic tunnel junctions. By controlling the spin-orbit interaction of the system using a remote doping technique, we have achieved a record-low switching energy of ~3 fJ using sub-ns voltage pulses.

The Heisenberg antiferromagnet on the kagome lattice is an archetypal example of how large ground state degeneracies arise, and how they may get resolved by thermal and quantum fluctuations. Augmenting the Heisenberg model by chiral spin interactions has proved to be of particular interest in the discovery of chiral quantum spin liquids. I will focus on the classical variant of this chiral kagome model, which exhibits, similar to the classical Heisenberg antiferromagnet, a remarkably large and structured ground-state manifold combining continuous and discrete degrees of freedom. This allows for a rich set of order-by-disorder phenomena. Degeneracy lifting by thermal and quantum fluctuations occurs in a highly selective way that, among other interesting effects, provides a semiclassical route to an emergent Z_2 spin liquid. 

Height models and random tiling are well-studied objects in classical statistical mechanics and combinatorics that lead to many interesting phenomena, such as arctic curve, limit shape and Kadar-Parisi-Zhang scaling. We introduce quantum dynamics to the classical hexagonal dimer, and six-vertex model to construct frustration-free Hamiltonians with unique ground state being a superposition of tiling configurations subject to a particular boundary configuration. An internal degree of freedom of color is further introduced to generate long range entanglement that makes area law violation of entanglement entropy possible. The scaling of entanglement entropy between half systems is analysed with the surface tension theory of random surfaces and under a q-deformation that weighs random surfaces in the ground state superposition by the volume below, it undergoes a phase transition from area law to volume scaling. At the critical point, the scaling is L logL due to the so-called "entropic repulsion” of Gaussian free fields conditioned to be positive. An exact holographic tensor network description of the ground state is give with one extra dimension perpendicular to the lattice. We also discuss an alternative realisation with six-vertex model, inhomogeneous deformation to obtain sub-volume intermediate scaling, and possible generalisations to higher dimension.

Since the discovery of cuprate superconductors, layered compounds have been one of important target systems for developing new exotic superconductors and studying unconventional mechanisms. In this seminar, I will talk about material development of layered BiCh₂-based (Ch: S, Se) superconductors, which were first discovered in 2012 [1,2]. In the BiCh₂-based compounds, intrinsic local structural disorder (local distortion) has been observed by various experimental probes, and the suppression of local disorder is essential for the emergence of bulk superconductivity in the system [3]. After obtaining bulk superconductors with less local disorder, we could see some interesting superconducting properties in BiCh₂-based superconductors: absence of isotope effect [4], nematic superconductivity [5], and huge upper critical fields [6] in La(O,F)Bi(S,Se)₂ will be briefly explained.

[1] Y. Mizuguchi et al., PRB 86, 220510 (2012)

[2] Y. Mizuguchi et al., JPSJ 81, 114725 (2012)

[3] (review article) Y. Mizuguchi, JPSJ 88, 041001 (2019)

[4] K. Hoshi, Y. Goto, Y. Mizuguchi, PRB 97, 094509 (2018)

[5] K. Hoshi et al., JPSJ 88, 033704 (2019)

[6] K. Hoshi et al., Sci. Rep. 12, 288 (2022)

The  quantum  spin  Hall  insulator, also  known  as  a  two-dimensional  (2D) topological  insulator,  is  a  topological  state  of  matter  supporting  the  helical edge states, which are counter-propagating, spin-momentum locked 1D modes protected  by  time  reversal  symmetry. It  exhibits  special  magneto-transport properties under external magnetic field. In my talk, I will construct modified Anderson impurity models to study the magneto-conductance of the quantum spin hall insulator. Firstly, I will solve the transmission through single impurity on helical Luttinger liquid in the presence of magnetic field using Lippmann-Schwinger equation.  I will show the analytical expression for trans- mission and reflection coefficient in terms of the difference between energy of particle and the impurity level, the hybridization coefficient and the magnetic field. Then, I will show the effect of Coulomb interaction on helical Luttinger liquid at Hartree-Fock level. Using renormalization group, I will derive the temperature dependence of conductance.  Lastly, I will show the coherent transport of transmission through many impurities with different energy and hybridization coefficient in the presence of magnetic field.  I will compare my theoretical result with experiment.

Quantum gas microscopes have expanded the understanding of many-particle physics with their unique ability of single atom resolved imaging. Quantum gas microscopes provide microscopic information of quantum many-body states through spatial correlation functions. Relying on the unique tunability of ultracold atoms in atomic interactions via Feshbach resonances, density, and spin-imbalance, we study a wide parameter range in the phase diagram. Interestingly, a triangular lattice is the simplest example of geometric frustration because three spins with antiferromagnetic interactions cannot be antiparallel, leading to large degeneracies in the many-body ground state [1]. In this talk, I present a Mott insulator of lithium-6 on a symmetric triangular lattice with a lattice spacing of 1003 nm. The lattice is imaged via a Raman sideband cooling technique with imaging fidelity of 98% [2]. We calibrated tunneling by extracting lattice depth from band excitation and the interaction is determined using doublon formation. We can access single-species singles components with the use of doublon hiding [3] and spin removal techniques [4] to detect spin-spin correlations. We compare the results to Determinantal Quantum Monte Carlo calculations, plan to investigate 120° Neel ordering in Heisenberg antiferromagnets, and search for quantum spin liquids in the triangular lattice Hubbard system.

[1] L. Balents, Nature 464, 7286 (2010).

[2] J. Yang, et al., PRX Quantum 2, 020344 (2021).

[3] P. T. Brown, et al., Science 357, 6358 (2017).

[4] M. F. Parsons, et al., Science 353, 1253 (2016). 

In the Kitaev's toy model with a constant chemical potential, the Majorana zero modes (MZMs) can exist but stay localized at the edges of a 1D spinless chain. In this talk, I will introduce the Kitaev's toy model with a site-dependent chemical potential. In this case, one is able to create segments of topological and normal superconducting phases. The MZMs exist at the domain walls between the two phases, but not necessarily at the edges of the whole chain. By tuning the chemical potential such that the domain walls can change in space, the MZMs are able to move in space as well. We call this motion of MZMs the Majorana pump and argue that it leads to px+ipy superconductivity. I will present how the py pairing emerges, and how to realize this model in experiments.

P.W. Anderson envisaged a novel situation of quantum paramagnetic (quantum spin liquid) phase of low spin Mott insulators, back in 1973 and described it using Pauling's  resonating valence bonds states. RVB idea got a resurgence, with the discovery of high Tc superconductivity by Bednorz and Muller in 1986. Low dimensionality and frustrations enhance quantum fluctuations in low spin systems, resulting in a variety of spin liquids and many ideas -  emergent gauge fields, Majorana Fermi sea, to topological phases etc. I will discuss a delightful and exactly solvable model by Kitaev, which realized dreams of RVB theorists and more, in an exact fashion. This fertile model is experimentally realized now, thanks to Khaliullin and Jackeli's prediction. There are continuing surprises, including very recent discovery of anomalous non-linear susceptibility by Shivaram and collaborators.

Electron microscopy is transforming the physical sciences. Aided by a new generation of direct imaging detectors, cryo-electron microscopy won the 2017 Nobel Prize in Chemistry for advancements in visualization of biomolecules.  To go beyond traditional electron microscopy, new detectors must also be developed for the diffraction imaging; here, the scattered electron beam encodes a wealth of information about the structure, chemistry, electrical, optical, and magnetic properties of matter. During my PhD, I co-invented the electron microscopy pixel array detector (EMPAD), a fast, highly efficient detector designed to capture the full scattered electron information. The EMPAD has been licensed to Thermo Fisher Scientific and sold around the world. In my talk, I will highlight how the EMPAD enables new characterization techniques for imaging topological magnetic and ferroelectric structures.  These approaches can be used to uncover polarization fields, orbital angular momentum and chirality of polar and magnetic textures. By developing new characterization methods in combination with theoretical predictions, new physics in emerging quantum materials can be revealed with electron microscopy at atomic resolution.

: The perfectly linear temperature dependence of the resistivity observed as T→0 in a variety of metals close to a quantum critical point is a major puzzle of condensed matter physics. In cuprates, this phenomenon is observed in the vicinity of the pseudogap critical point p*. Using high magnetic fields to suppress superconductivity, one can access the normal state properties down to T→0 close to this critical point. I will present high-field magneto-transport measurements of two hole-doped cuprates, near their respective p*, supporting that T-linear resistivity as T→0 is a generic property of cuprates, associated with a universal scattering rate. We measured the low-T resistivity of Bi2Sr2CaCu2O8+δ just above p* [1] and found that it exhibits a T-linear dependence, quantitatively similar to other very different cuprates. We also observed, using the Drude formula, that in various cuprates showing this low-T phenomenon the slope of this T-linear resistivity is given by a universal relation implying a specific scattering rate for charge carriers: 1/�� = αh/2πk_BT (corresponding to what is called the Planckian limit [2]), where h is Planck’s constant, k_B is the Boltzmann constant and α a constant of order unity. Finally, we directly measured the scattering rate in La_1.6−xNd_0.4Sr_xCuO₄, just above p* and in the low-T limit, using angle-dependent magneto-resistance measurements [4]: these experiments reveal an inelastic scattering rate which is isotropic and linear in temperature, and whose magnitude is consistent with Planckian dissipation.
[1] Legros et al., Nat. Phys. 15, 142 (2019)
[2] Zaanen, SciPost Phys. 6, 061 (2019)
[3] Grissonnanche et al., Nature 595, 667 (2021)

We consider the entanglement entropies of energy eigenstates in quantum many-body systems. For the typical models that allow for a field-theoretical description of the long-range physics, we find that the entanglement entropy of (almost) all eigenstates is described by a single crossover function. The eigenstate thermalization hypothesis (ETH) implies that such crossover functions can be deduced from subsystem entropies of thermal ensembles and that they assume universal scaling forms in quantum-critical regimes. They describe the full crossover from the groundstate entanglement scaling for low energies and small subsystem size (area or log-area law) to the extensive volume-law regime for high energies or large subsystem size. For critical 1d systems, the scaling function follows from conformal field theory (CFT). We use it to also deduce the scaling function for Fermi liquids in d>1 dimensions. These analytical results are complemented by numerics for large non-interacting systems of fermions in d=1,2,3 and the harmonic lattice model (free scalar field theory) in d=1,2. Lastly, we demonstrate ETH for entanglement entropies and the validity of the scaling arguments in integrable and non-integrable interacting spin chains.

References: PRL 127, 040603 (2021); PRA 104, 022414 (2021); arXiv:2010.07265.
 

The field of Quantum Information is of great excitement in both fundamental physics and industry. One promising platform for quantum computing is gate defined quantum dot in semiconductors. The greatest limiting factor currently is that delicate quantum states can lose their quantum nature due to interactions with their environment. Other open challenges are to develop methods to entangle quantum bits that are separated by significant distances and can be measured quickly with high fidelity.

Silicon-based materials are promising due to the long lifetimes of electrons’ quantum states, but also challenging due to the difficulty in fabrication and valley degeneracy. I will report a singlet-triplet qubit with a qubit gate that is assisted by the valley states. This work would potentially relax the  design and fabrication requirement for scaling. Moreover, this research field has achieved strong coupling between electron spins and photons in hybrid circuit-QED architecture. Quantum optics, long distance quantum entanglement and communication via photons are promised. To address that, I will present my project on indium arsenate (InAs) double quantum dots (DQD) that are embedded in circuit-QED architecture. We demonstrated the direct evidence of photon emission from a DQD in the microwave regime and further achieved stimulated emission in a similar system. By achieving stimulated emission from one DQD in these works, we invented a semiconductor single atom maser that can be tuned in situ.  I will demonstrate that a semiconductor based quantum dot is a promising platform for quantum information as well as for fundamental physics.

WTe₂ is an example of a two-dimensional semimetal. It shows incredibly diverse and intriguing behavior such as the quantum spin Hall effect (QSH), superconductivity, ferroelectricity, and excitonic insulator, providing a new platform for studying the interplay between topology and correlations. In this talk I will discuss the helical nature of the QSH edge state in monolayer WTe₂ and the proximity effect of a magnet upon it. In the first part, I will describe how we explore the spin-momentum locking in the QSH edge state and determine the spin axis by studying the magnetic anisotropy. In the second part, I will discuss the magnetic coupling between a two-dimensional antiferromagnet, CrI₃, and the QSH edge state.

Optical photons are excellent flying qubits for long-distance quantum networks due to negligible thermal noise and decoherence at room temperature. In this talk, I will discuss how frequency encoding can be combined with nonlinear optics and fiber and integrated photonic technologies to address challenges in scaling future photonic quantum networks. Frequency multiplexing has had a profound impact on classical telecommunication networks, creating low loss and inexpensive hardware that can be exploited for quantum applications. I will describe quantum photonic applications where frequency encoding provides a distinct advantage in terms of scaling losses and resource overhead compared to polarization, spatial or temporal mode encoding.

Coherent manipulation of light in the frequency domain at the single-photon level requires a strong, noise-free nonlinear process. I will discuss our implementation of four-wave mixing (FWM) in a commercial dispersion-shifted fiber to achieve quantum frequency conversion with near-unity efficiency and low noise. I will discuss how we used this process as an active "frequency switch" to realize a low-loss multiplexed single-photon source that can be scaled to the deterministic regime. Next, I will discuss how we used this process as a frequency beam-splitter to demonstrate two-photon Hong-Ou-Mandel type interference between entangled photons of different colors- a hallmark of quantum indistinguishability. Finally, I will discuss our realization of a FWM-based "time lens" for the generation and detection of single-photon waveforms with picosecond r​esolution.

Based on Joshi et al., Nat. Comm. 9, 847 (2018), Joshi et al. Phys. Rev. Lett. 124, 143601(2020)

The recently proposed map [arXiv:2011.01415] between the hydrodynamic equations governing the two-dimensional triangular cold-bosonic breathers [Phys. Rev. X 9, 021035 (2019)] and the high-density zero-temperature triangular free-fermionic clouds, both trapped harmonically, perfectly explains the former phenomenon but leaves uninterpreted the nature of the initial (t=0) singularity. This singularity is a density discontinuity that leads, in the bosonic case, to an infinite force at the cloud edge. The map itself becomes invalid at time t=T/4. Here, we first map -- using the scale invariance of the problem -- the trapped motion to an untrapped one. Then we show that in the new representation, the solution [arXiv:2011.01415] becomes, along a ray in the direction normal to one of the three edges of the initial cloud, a freely propagating one-dimensional shock wave of a class proposed by Damski in [Phys. Rev. A 69, 043610 (2004)]. There, for a broad class of initial conditions, the one-dimensional hydrodynamic equations can be mapped to the inviscid Burgers' equation, a nonlinear transport equation. More specifically, under the Damski map, the t=0 singularity of the original problem becomes, verbatim, the initial condition for the wave catastrophe solution found by Chandrasekhar in 1943 [Ballistic Research Laboratory Report No. 423 (1943)]. At t=T/8, our interpretation ceases to exist: at this instance, all three effectively one-dimensional shock waves emanating from each of the three sides of the initial triangle collide at the origin, and the 2D-1D correspondence between the solution of [arXiv:2011.01415] and the Damski-Chandrasekhar shock wave becomes invalid.

Due to the low atomic mass and high electron-phonon coupling strength in hydrogen-rich materials, hydride compounds under extremely high pressures are most promising in the search of high-Tc superconductors. First-principles-based computational search has become extremely important not only in predicting new materials but also in guiding high-pressure experimental measurements. In this work, we have developed a super-efficient and fast method for searching high-T hydride superconductors. We introduce new "metrics" that are strongly correlated to strong electron-phonon coupling and T but it is much faster to calculate them. Using our new method, we have searched more than 100,000 binary hydride systems and discovered several new high-T superconductors. Among them, we report our prediction of high-temperature superconductivity at relatively low pressure in a novel binary metal hydride which may break the current record. A detailed mechanism of the superconductivity, phonons, and electron-phonon coupling, anharmonicity, as well as the abnormal T -pressure dependence, will be also discussed.

The accelerated discovery and design of new quantum materials requires atomic-level information about chemical reactions, phase transformations, mechanical deformations and other collective and emergent quantum phenomena. Several techniques have been developed recently that can learn the potential energy surface (PES) of complex materials. Machine Learning (ML) models, particularly deep neural networks, have proven capable of learning highly complex non-linear relationships between atomic structure and properties and theory and experiments. In this talk, I will describe two examples of ML-driven MD called neural-network quantum molecular dynamics (NNQMD) to tackle problems related to large systems and long trajectories that cannot be investigated by Quantum Molecular Dynamics (QMD).

First, we use NNQMD for quantitatively characterizing the intermediate range order, manifested as first sharp diffraction peak in GeSe₂. In the second example, we compute the dielectric constant, ε₀, and its temperature dependence for liquid water using fluctuations in macroscopic polarization using two coupled neural network models. The first network, NNQMD, learns the PES of liquid water from QMD training data. The second network, neural-network maximally localized Wannier functions, NNMLWF, is trained to predict dipole moments.

I will also briefly discuss applications of ML to discovery of new dielectric polymer materials with high breakdown strengths and to optimization of chemical vapor deposition synthesis of quantum materials.

Analogy between the Coulomb law of interaction between charges and the Newton law of gravitational attraction between masses is familiar to every physics student.  In this talk I demonstrate that this analogy implies that a system of identical charges can evolve with time in a manner that parallels cosmological evolution of the physical Universe with hallmarks such as Hubble's law and Friedmann-type dynamics present.  The Coulomb and Newton laws are also dissimilar because the electrostatic force is many orders of magnitude larger than the gravitational force whose manifestations are only noticeable on astronomical scale.  On the other hand, analog cosmological evolutions driven by Coulomb interactions are predicted to be observable in laboratory experiments involving Coulomb explosions and electron density oscillations in conductors.

The pressure variable opens the door towards the synthesis of materials with unique properties, e.g. superconductivity, hydrogen storage media, high-energy density and superhard materials. Under pressure elements that would not normally combine may form stable compounds or they may adopt novel stoichiometries. As a result, we cannot use our chemical intuition developed at 1 atm to predict phases that become stable when compressed.

To facilitate the prediction of the crystal structures of novel materials, without any experimental information, we have deve loped XtalOpt, an evolutionary algorithm for crystal structure prediction. XtalOpt has been applied to predict the structures of hydrides with unique compositions that become stable at pressures attainable in diamond anvil cells. In the ternary hydride system two different classes of superconductors composed of S and H atoms have been discovered - methane intercalated H3S perovskites with the CSH7 stoichiometry, and phases containing SH honeyco mb sheets. We also predict a superconducting RbB3Si3 phase in the bipartite sodalite structure that could be synthesized at mild pressures and quenched to 1 atm.

Thermal quenching is the process of rapidly cooling or heating a material. It has been practiced since ancient times to obtain desirable mechanical properties in materials, especially metals. The dynamics in play during quenching fall in the regime of non-equilibrium dynamics and is a subject of interest as most processes in nature happen out of equilibrium. A curious phenomenon during out of equilibrium processes is the so called Mpemba effect. The Mpemba effect is a phenomenon where a system prepared at a hot temperature (Thot) “overtakes” an identical system prepared at a warm temperature (Twarm) and cools down faster to be in equilibrium with a cold environment (Thot > Twarm > Tenvironment). My project involves studying the dynamics and behavior of linear chemical reaction networks during this kind of out of equilibrium process. Chemical reaction networks are a good model to study various biochemical processes, which are integral to the study of biochemical pathways and thus the functioning of cells. I am especially searching for the existence of a Mpemba like behavior in these kinds of systems and trying to characterize their behavior and dependence on the different parameters of the linear chemical reaction network. In this seminar I will be detailing on the methods used to study the out of equilibrium dynamics of linear chemical reaction networks and will be presenting the preliminary results which indicates the existence of Mpemba like behavior. This understanding will eventually lead to the optimization of chemical production for industrial application and characterization of biochemical pathways.

The Gutzwiller approximation is a method for strongly-correlated systems, it is the simplest theory that successfully captures the correlated induced metal-insulator transition, i.e. mott transition. Density function theory (DFT) is a very efficient method to deal with many-electron systems, thus currently quantum molecular dynamics (QMD) simulations are dominantly based on DFT, however DFT fails to describe many strong electron correlation phenomenon, for example the mott transition.

We proposed a new scheme of quantum molecular dynamics based on the Gutzwiller method, the Gutzwiller quantum molecular dynamics (GQMD). A liquid Hubbard model is studied by GQMD, two schemes of mott metal-insulator transition is found at different densities, based on which a phase diagram can be given to describe different states of the Hubbard liquid system. An effort to apply GQMD to real materials is also made on hydrogen system at high temperature and pressure conditions.

The Falicov-Kimball (FK) model was initially introduced as a statistical model for metal-insulator transition in correlated electron systems. It can be exactly solved by combining the classical Monte Carlo method for the lattice gas and exact diagonalization (ED) for the itinerant electrons. However, direct ED calculation, which is required in each time-step of dynamical simulations of the FK model, is very time-consuming. Here we apply the modern machine learning (ML) technique to enable the first-ever large-scale kinetic Monte Carlo (kMC) simulations of FK model. Using our neural-network model on a system of unprecedented 10⁵ lattice sites, we uncover an intriguing hidden sub-lattice symmetry breaking in the phase separation dynamics of FK model.

I present a new computational paradigm to simulate time and momentum resolved inelastic scattering spectroscopies in correlated systems. The conventional calculation of scattering cross sections relies on a treatment based on time-dependent perturbation theory, that provides formulation in terms of Green’s functions. In equilibrium, it boils down to evaluating a simple spectral function equivalent to Fermi’s golden rule, which can be solved efficiently by a number of numerical methods. However, away from equilibrium, the resulting expressions require a full knowledge of the excitation spectrum and eigenvectors to account for all the possible allowed transitions, a seemingly unsurmountable complication. Similar problems arise when the quantity of interest originates from higher order processes, such as in Auger, Raman, or resonant inelastic X-ray scattering (RIXS). To circumvent these hurdles, we introduce a time-dependent approach that does not require a full diagonalization of the Hamiltonian: we simulate the full scattering process, including the incident and outgoing particles (neutron, electron, photon) and the interaction terms with the sample, and we solve the time-dependent Schrödinger equation. The spectrum is recovered by measuring the momentum and energy lost by the scattered particles, akin an actual energy-loss experiment. The method can be used to study transient dynamics and spectral signatures of correlation-driven non-equilibrium processes, as I illustrate with several examples and experimental proposals using the time-dependent density matrix renormalization group method as a solver. Even in equilibrium, we find higher order contributions to the spectra that can potentially be detected by modern instruments.

We study the long term behavior of lattice fermions undergoing repeated particle detection, extraction, or injection interspersed with unitary evolution in two specific regimes.  First, we investigate the wake pattern formed behind a moving probe performing these operations.  These disturbances show dramatically different behavior where, notably, at half-filling the “measurement wake” vanishes and the “extraction wake” becomes temperature independent.   Second, in analogy with the edge modes found in topologically trivial systems when undergoing floquet driving, we provide a protocol of repeated local density measurements that induces edge modes in a topologically trivial system while the hamiltonian remains time independent.  In the limit of rapid measurements, the so-called Zeno limit, we connect this system to a novel stochastic dynamical system and discover an interesting double step structure in the charge transport in this regime.    

The magnetic skyrmions are topologically protected spin configuration, which stabilized by Dzyaloshinskii-Moriya interaction (DMI). Due to skyrmions’ ability to be small, stable, and controllable by electric current [1], they have considerable potential for high-density data storage applications. It is theoretically predicted that ferrimagnetic materials prefer holding small skyrmions at room temperature (RT) [2,3]. 10-15nm ferrimagnetic CoGd heterostructures and 10-15nm ferrimagnetic Mn₄N heterostructures were fabricated by magnetron sputtering for holding small skyrmions at RT. Magnetic force microscope images show skyrmions. A designed compound layer is capping on the top of the magnetic layer to adjust the interfacial DMI, thus tune the size of skyrmions. The micromagnetic simulation was performed to study the effect of DMI on the size of skyrmions Mn₄N.

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect 
formation via the one-dimensional transverse-field Ising model on two 
different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain  quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe  physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system. 

We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.

Ref.:
Yuki Bando, Yuki Susa, Hiroki Oshiyama, Naokazu Shibata, Masayuki 
Ohzeki, Fernando Javier Gómez-Ruiz, Daniel A. Lidar, Sei Suzuki, Adolfo 
del Campo, and Hidetoshi Nishimori, Phys. Rev. Research 2, 033369 (2020)

Metal halide perovskites (MHPs) have come to the forefront in the photovoltaic and light-emitting devices due to their attractive optoelectronic properties, such as tunable bandgaps, long charge carrier lifetime, and bright luminescence. The interactions between charge carriers and crystal lattice, such as electron-phonon coupling, polaron formation and low thermal conductivity, are proposed to be the major reasons for their extraordinary device performance. Our previous studies have demonstrated the significant role of the organic cation mobility in screening the excited charge carriers and further extending the charge carrier lifetime in three-dimensional (3D) perovskites. Compared with 3D perovskites, however, 2D perovskites exhibit a one-order-of-magnitude longer degradation time and some of them could have extremely high photoluminescence quantum yields, which make them popular in the LED field. This talk will summarize the experimental investigations on a 2D MHP and examine the relationships between the lattice dynamics and optoelectronic properties in this type of 2D perovskites.

Skyrmions are topologically protected magnetic quasi-particles. An isolated skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions has a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. Materials with different symmetry groups can support different kinds of skyrmions (Bloch, Neel, Anti-skyrmion). We will see how these different types of symmetries can be used to control movements of a skyrmion. Skyrmion and domain wall racetracks can be used for temporal memories in race logic. Locally synchronized racetracks can spatially store relative timings of wavefronts and provide non-destructive read-out.

Studies on transition metal dichalcogenides (TMD) is of great significance due to their interesting topological properties and remarkable electronic behavior. Among these materials,1T- TaX₂ class of TMDs, where X = S, Se, has spurred considerable interest due to their multiple first order phase transitions between different charge density wave (CDW) states. The effects of CDW formation in these compounds are attributed primarily to in-plane re-orientation of Ta atoms to Star-of-David formation. But this alone doesn’t explain the notable electronic behavior of 1T-TaS₂ and 1T-TaSe₂ and why they differ from one another despite having the same trigonal symmetry. At very low temperatures, 1T-TaS₂ undergoes a metal - insulator transition and is proposed to harbor a quantum spin liquid behavior whereas 1T-TaSe₂ remains metallic. Investigating the local structure of pristine 1T-TaS₂ and 1T-TaSe₂ in the CDW regime could tell us the differences in local atomic correlations in these compounds.

Mo_1-xW_xTe₂ belongs to the family of layered transition metal dichalcogenides (TMD) that are of intense interest recently because of their fascinating topological properties. The end members of this series, MoTe₂ and WTe₂ are Weyl semimetals upon cooling to the orthorhombic Td phase. Mo_1-xW_xTe₂ undergoes a structural phase transition from a high-temperature monoclinic 1T' phase, to a non-centrosymmetric orthorhombic T_d phase at low temperatures through a first-order structural phase transition. Both 1T' and T_d phases are comprised of weakly-bound layers, and differ mainly by shifts of the layers along the c-axis. Despite much research, the structural properties of Mo_1-xW_xTe₂ have not been thoroughly investigated. Neutron scattering experiments at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory were carried out on single crystals of Mo_1-xW_xTe₂. Structural changes including changes in interlayer disorder were observed by focusing on the elastic scattering along (2, 0, L) on cooling and warming through the hysteresis loop at the transition. A T_d* phase was discovered for the first time across the T_d-1T’ phase boundary in Mo_1-xW_xTe₂ with x up to ~0.2. In WTe₂, a sharp transition from T_d to 1T′ was observed at ambient pressure in the single crystal near ∼565 K, the transition proceeds without hysteresis. These results should clarify in microscopic detail the nature of these phase transitions.

Living systems respond to external stimuli by utilizing chemical reaction networks that function as cellular information processors. What can we learn from living systems as the design principle of intelligent active materials? One salient example of a biological intelligence is the single-cell circadian clock (i.e. the Kai-ABC oscillator in cyanobacteria). Such circadian clocks process external signal (sunlight intensity) and computes the time during the day/night. These microscopic computers are naturally challenged by two main sources of uncertainty the internal thermal fluctuations and the external noisy signal. To optimize its performance, we find that a clock must make a tradeoff between resisting internal thermal fluctuations and external signal noise. This noise tradeoff relation can be explained through the geometry of its energy landscape. I will also discuss the use of the energy landscape in designing intelligent responses into mechanochemical materials.

't Hooft anomalies provide a unique handle to study the nonperturbative dynamics of strongly-coupled theories.  Although this type of anomalies was known since the 80's, recently it has been realized that one can generalize them by turning on 't Hooft twists in the color, flavor, and baryon number directions.  Such generalized anomalies put severe constraints on the possible realizations of the global symmetries of a given theory in the infrared. In this talk, I will explain how one can construct such 't Hooft twists and give examples of the constrains the generalized anomalies can impose on strongly coupled gauge theories.

5-dimensional superconformal field theories play an intriguing role in the general understanding of quantum field theory. They provide strongly-coupled UV fixed points for many perturbatively non-renormalizable 5-dimensional gauge theories, and upon compactification they provide new insights into many lower-dimensional theories. This makes them both useful and interesting in their own right. In this talk I will discuss recent progress in the understanding of 5-dimensional superconformal field theories through AdS/CFT dualities and non-perturbative field theory methods, leading to interesting exact results and many cross checks.

Despite its many successes, the Standard Model of particle physics cannot be the final description of nature at the most fundamental level. Additional elementary particles and interactions are an absolute necessity but have so far evaded our experimental efforts. I will highlight the importance of searches for processes that are forbidden within the Standard Model, as these make for clean signatures of new physics. Important examples are searches for lepton flavor violation and baryon number violation, which will be tested to unprecedented levels in upcoming experiments.

Soft Collinear Effective Theory (SCET) is a framework for modeling the infrared structure of theories whose long distance behavior is dominated by soft and collinear divergences. SCET is utilized to compute processes in Jet physics, WIMP annihilations, and more. My collaborators and I showed that SCET can be made compatible with supersymmetry, and that such a theory can be conveniently formulated in "Collinear Superspace". In this talk I will introduce a new set of effective field theory rules for constructing Lagrangians in collinear superspace. This new formalism represents a general way to derive on-shell superspace Lagrangians directly from the symmetries of the theory. However, I will also demonstrate how the non-propagating off shell degrees of freedom i.e. F and D terms, can be reintroduced into the theory. This framework paves the way to constructing theories with N > 1 supersymmetry directly from low-energy considerations, and has potential implications for supergravity, the Scattering Amplitudes program, and more.

Since the original papers of Drell-Levy-Yan,  there has been a desire to unite the process of production of hadrons in the final state to the process of probing the structure of a hadronic initial state. Specifically, one wants to relate the process of single inclusive annihilation (SIA) to deep in-elastic scattering (DIS) via crossing and analyticity. It has long been known that any straightforward relation between the two processes fails, however, pursuing this relation has lead to a deeper and richer connection between SIA and DIS, now known as the space-time reciprocity relation (arXiv hep-th/0612247). I shall give an introduction to this space-time reciprocity relation, and argue that the relationship is a consequence of the deeper connection between final and initial state dynamics governed by an underlying conformal symmetry which maps between the two, up-to anomalous terms which should cancel as regulators are removed, but from which one is never free due to the initial conditions of the scaling evolution. As a consequence, I will give a time-like BFKL equation that resums the soft region of the fragmentation function of QCD, which maps to the space-like BFKL equation that governs the soft region of the parton distribution function. Time permitting, I will discuss both possible formal implications and phenomenological applications. This will be a blackboard talk.

Hydrodynamics is a theory of the collective properties of fluids and gases that can also be successfully applied to the description of the dynamics of quark-gluon plasma. It is an effective field theory formulated in terms of an infinite-order gradient expansion. For any collective physical mode, hydrodynamics will predict a dispersion relation that expresses this mode’s frequency in terms of an infinite series in powers of momentum. By using the theory of complex spectral curves from the mathematical field of algebraic geometry, I will describe how these dispersion relations can be understood as Puiseux series in (fractional powers of) complex momentum. The series have finite radii of convergence determined by the critical points of the associated spectral curves. For theories that admit a dual gravitational description through holography, the critical points correspond to level-crossings in the quasinormal spectrum of a dual black hole. Interestingly, holography implies that the convergence radii can be orders of magnitude larger than what may be naively expected. This fact could help explain the “unreasonable effectiveness of hydrodynamics” in describing the evolution of quark-gluon plasma. In the second part of my talk, I will discuss a recently discovered phenomenon called “pole-skipping” that relates hydrodynamics to the underlying microscopic quantum many-body chaos. This new and special property of quantum correlation functions allows for a precise analytic connection between resummed, all-order hydrodynamics and the properties of quantum chaos (the Lyapunov exponent and the butterfly velocity).

Although the existence of magnetic monopoles is admitted by​ ​the fundamental laws, the real monopoles in nature remain elusive.​ ​Nevertheless, variations of monopoles appear in realistic condensed​ ​matter systems, from quantum Hall effects​ to ​​topological​ ​superconductivity​, which spur a race to discover new exotic​ ​topological phases of matter. In this talk, we will present a​ ​dramatic effect ​arising from topological​ Fermi surface​s -- a novel topological class of superconductivity and density-wave orders​: ​When ​the ordered pairs acquire​ ​non-trivial two-particle Berry phases, their pairing phases cannot be​ globally well-defined in the momentum space. Therefore, the​ conventional description of superconducting pairing symmetries in​ terms of spherical harmonics (e.g. s-, p-, d-waves) ceases to apply.​ Instead, they are characterized by topologically protected nodal gap functions represented by monopole harmonic functions. This so-called “monopole harmonic order” is expected to be realized and detected in Weyl semimetal materials.

We obtain a new relation between the distributions μ_t at different times t ≥ 0 of the continuous-time TASEP (Totally Asymmetric Simple Exclusion Process) started from the step initial configuration. Namely, we present a continuous-time Markov process with local interactions and particle-dependent rates which maps the TASEP distributions μ_t backwards in time. Under the backwards process, particles jump to the left, and the dynamics can be viewed as a ver- sion of the discrete-space Hammersley process. Combined with the forward TASEP evolution, this leads to a stationary Markov dynamics preserving μ_t which in turn brings new identities for expectations with respect to μ_t. Based on a joint work with Axel Saenz.

TBA

The recent discoveries of ferromagnetism in single atomic layers have opened a new avenue for two-dimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for long-range magnetic order in low-dimensional systems, but they also provide a new platform for the development of spintronic devices. In this talk, I will present a series of studies on the family of layered ferromagnetic semiconductors, CrX₃ (X = I, Br, Cl), in the atomically thin limit. By incorporating these materials as tunnel barriers between graphene electrodes, we are able to achieve extremely large tunnel magnetoresistance as well as robust memritive switching that is tunable with magnetic field. Tunneling spectroscopy further allows for direct observation of their spin wave excitations, or magnons, from which we are able to derive a simple microscopic Hamiltonian for all three spin systems. These results show that strong exchange anisotropy is not necessary to stabilize ferromagnetism in the monolayer limit.

I shall review the history of Sagnac interferometers and give a geometric description of light rays propagation in flexible optical fibers in Minkowski space. Based on joint works with Amos Ori and Oded Kenneth.

Quantum effect is expected to dictate the behavior of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy (TEE) and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z2 topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of off-diagonal and diagonal quantum processes leading to the quasi-degeneracy of states and effectively, the classical degeneracy is restored.

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. In these systems, organic cations not only serve as spacers between slabs consisting of corner-sharing metal-halide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organic-inorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Furthermore, by means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm⁻¹) optical phonons involving motion in the lead iodide layers. This supports our conclusion that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. Excitonic correlations (exciton and biexciton binding energies) and exciton dynamics (e.g. uni- and bimolecular population decay mechanisms, pure dephasing processes, excitation-induced dephasing, etc.) reflect the polar solvation-like processes induced by organic cation components of the hybrid lattice in a broad structural space. I will address how ultrafast nonlinear spectroscopies yield deep insight on the multiparticle properties in compelx semiconductor materials.

Through string theory, people found interesting relations in particle theory. For example, Kawai-Lewellen-Tye (KLT) relation relates the scattering amplitudes of QCD and gravity. However, these kinds of relation are completely mysterious from the point view of Quantum Field Theory since the gravity Lagrangian seems totally unrelated to the Yang-Mills Lagrangian. On the other hand, these kinds of relations are nevertheless true and can be checked by computing the amplitudes using Feynman diagrams order by order. Thus the Feynman diagrammatic expansion does not capture everything of interest, there are still hidden relations between different field theories. Worldline approach, born as a first quantized approach to calculate amplitudes, shares a lot of similarities to string theory.  In this talk, I will show how worldline approach works and how it helps shed some light on the problems we are interested in. I will also discuss the subtlety and limitation of the approach and the possibility of generalizing it to "worldgraph approach".

Bi-Sb alloys have shown promising thermoelectric (TE) properties at cryogenic temperature (<200 K). Over six decades, the figure of merit zT of n-type polycrystalline Bi-Sb has plateaued at ~0.4, while its p-type counterpart has remained even lower at ~0.1. We have studied the TE properties of melt-spun and spark plasma sintered (SPS) Bi-Sb alloys. We obtained a zT of 0.55 @100-150 K for n-type undoped Bi₈₅Sb₁₅ based on a low thermal conductivity 1.5 W/(m*K) measured with the hot-disk method. For p-type Bi-Sb, doping effects of Ge, Sn, and Pb were investigated. A high doping level of Ge and a high doping efficiency of Pb were obtained with the help of a low-temperature SPS processing. The transport properties (resistivity and Seebeck coefficient) of n-type undoped and p-type doped Bi₈₅Sb₁₅ were analyzed using the two-band effective mass model within the Boltzmann transport theory. A band gap decreasing phenomenon was observed which poses challenges to the improvement of p-type Bi-Sb’s zT.

An interesting theme in topological materials has been classification and prediction of symmetry protected topological(SPT) phases. Despite the Altland-Zirnbauer(AZ) classification under time-reversal symmetry, particle-hole symmetry and chiral symmetry, a system can also be invariant under a combined symmetry composed by two distinct operations. In this talk I will discuss the classification of nodal topological phases under two-fold spatial antiunitary symmetries. We also generalize SPT phases to non-Hermitian system with two-fold spatial antiunitary symmetries and give an example of dissipative topological superconductors.

The three-body problem dates back to the 1680s. Isaac Newton had
already shown that his law of gravity could always predict the orbit
of two bodies held together by gravity, such as a star and a planet,
with complete accuracy. The periodic two-body orbit is always an
ellipse (circle). For two centuries, scientists tried different tacks
to find similar solution for three-body problem, until the German
mathematician Heinrich Bruns pointed out that the search for a general
solution for the three-body problem was futile, and that only specific
solutions that work only under particular conditions, were possible.
Only three families of such collisionless periodic orbits were known
until recently: 1) the Lagrange-Euler (1772); 2) the Broucke-Henon
(1975); and 3) Cris Moore's (1993) periodic orbit of three bodies
moving on a "figure-8" trajectory. Few years ago we reported the
discovery of 13 new families of periodic orbits. Meanwhile, hundreds
of new topologically different solutions have been reported by our and
other groups. We discuss the numerical methods used to find orbits and
to distinguish them from others. Additionally, we found that period T
of an orbit depends on its topology. This dependence is a simple
linear one, when expressed in terms of appropriate variables,
suggesting an exact mathematical law. This is the first known relation
between topological and kinematical properties of three-body systems.

https://scholar.google.com/citations?hl=en&user=dEJ0ThoAAAAJ&view_op=list_works&sortby=pubdate
 

Skyrmions are topologically protected magnetic quasi-particles. An isolate skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions have a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. I will talk about how we are trying to calculate lifetime of skyrmion in candidate Heuslers compounds. Materials with different symmetry groups can support different kind of skyrmions (Bloch, Neel, Anti-skyrmion). We will see how this different types of symmetries can be used to control movements of a skyrmion. Also, we will look at how presence of point defects can effect dynamics of skyrmion, either for movement or nucleation. The ultimate goal is to figure out a compact analytical model for describing skyrmion movement and critical spin current needed for nucleation.

Unconventional magnetic states such as spin liquids and spin glasses continue to attract the interest of researchers in magnetism.These materials retain their magnetic disorder even at zero temperatures. We study two different cases of frustrated systems  1)In the case of Kitaev-type models frustration originates from highly anisotropic exchange interactions. We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spin-orbit magnets where a dominant off-diagonal exchange, the so-called Γ term, results in a macroscopic ground-state degeneracy at the classical level [1]. We show that this phase transition actually corresponds to plaquette ordering of hexagonal fluxes. We also study the dynamical behavior of fluxes.  2) We study the deterministic spin precession dynamics using energy conserving Landau-Lifshitz equation on a geometrically frustrated magnet. The lattice constitutes of a triangular arrangement of bipyramids with classical antiferromagnetic Heisenberg interaction. Such a lattice structure is realized in frustrated SrCr9Ga12-9pO19 [SCGO(p)] compounds [2]. Monte Carlo simulations are used to thermalize the system, which is then used as the initial state for the dynamical studies. We explore the temperature, wave vector and frequency dependence in the dynamical structure factor and the corresponding time dependent correlation functions of the model. Dynamics simulations is further used to estimate the extent to which transport of spin excitations in the lattice conform with phenomenological concept of spin diffusion [1]I. Rousochatzakis and N. B. Perkins Phys. Rev. Lett. 118, 147204 (2017). [2]T. Arimori and H. Kawamura J. Phys. Soc. Jpn. 70, 3695 (2001)

Members of UVa's Advanced Research Computing Services group will be presenting the second of two information/Q&A sessions about rivanna (UVa's supercomputing cluster) on Thursday, April 4 at 11am in Physics 313. 

                Rivanna is a 7,000-core cluster with more than a petabyte of storage.  It includes a subset of GPU-equipped nodes. Please drop in if you have any interest in high-performance computing.

                Slides from the talk can be found here:

http://galileo.phys.virginia.edu/compfac/faq/IntroductionToRivanna.pdf

The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum many-body dynamics. While there are many well-developed theoretical tools to study many-body quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements.
First, I will present a study that probes the relaxation of density modulations in the doped Fermi-Hubbard model. This leads to a hydrodynamic description that allows us to determine the conductivity. We observe bad metallic behavior that we compare to predictions from finite-temperature Lanczos calculations and dynamical mean field theory.
Second, I introduce a new platform to study the 2D quantum Ising model. Via optical coupling of atoms in an optical lattice to a low-lying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations.

Ge2Sb2Te5 (GST-225) is a phase change material which has wide use in fabricating random access memories. With different quenching process, the GST-225 could have three different phases: an amorphous phase, an intermediate cubic phase, a crystalline hexagonal phase. The fast transition between amorphous and crystalline phase makes GST-225 an ideal material for RAM with fast speed. In our project, the Se-doped GST-225 materials are grown and studied. At x=0.9 in liquid nitrogen quenched samples, we observe a phase transition from crystalline to amorphous. The transport measurement also confirm that there are metal-insulator transition happen for both furnace cooled samples and liquid N2 quenched samples at this limit. A tentative hypothesis is proposed to explain this metal-insulator transition. 

The study of low-dimensional quantum systems has proven to be a particularly fertile field for discovering novel types of quantum matter. The tensor network's utility in studying short range correlated states in 1D have been thoroughly investigated. Yet, despite the large number of works investigating these networks and their relations to physical models, examples of exact correspondence between the ground state of a quantum critical system and an appropriate scale-invariant tensor network have eluded us so far. Here we show that the features of the quantum-critical Motzkin model can be faithfully captured by an analytic tensor network that exactly represents the ground state of the physical Hamiltonian. In particular, our network offers a two-dimensional representation of this state by a correspondence between walks and a type of tiling of a square lattice. We discuss connections to renormalization and holography.

A topic of interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of iron-based superconductors. In my talk I will present a theory of disordered unconventional superconductor with competing magnetic order. My discussion will be based on the results obtained for on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering induced by doping. Within the quasi-classical approximation I will present the analysis of the quasi-classical Eilenberger’s equations which include weak external magnetic field. I will demonstrate that disorder has a crucial effect on the temperature dependence of the magnetic penetration depth as well as critical current, which is especially pronounced in the coexistence phase.

The Kitaev model on a honeycomb lattice predicts a special quantum spin liquid (QSL) ground state with excitations resembling Majorana Fermions and gauge flux excitations. These emergent features are exciting prospects to both basic physics and applications towards a lossless technology for quantum qubits.  In this talk, I will describe our recent range of experiments on the magnetic Mott insulator alpha-RuCl3 which has honeycomb layers held together with weak van-der-Waals  interactions. A strong spin-orbit coupling and an octahedral crystal field makes the Kitaev interactions arguably the leading order term in the Hamiltonian. Prominently, despite a long-range ordered ground state, our neutron scattering measurements reveal a continuum of fractionalized excitations resembling predictions from Majorana Fermions, confirming that the material is proximate to a QSL. In a 8T  magnetic field the long-range order vanishes and the continuum becomes gapped, supporting a state where a direct evidence of non-Abelian excitations can be measured. I will describe the present and future endeavors that may help to stabilize the coherent quantum excitations allow a better understanding of the underlying physics, as well as experiments to complete the understanding of the phase diagram of this material.

Quantum spin liquids evade long-range magnetic order down to absolute zero temperature. These anarchic, yet highly entangled states break no symmetry but have remarkable properties such as fractional excitations. In this talk, I will first give an example of spin liquid using  a compass model relevant to recently discovered honeycomb antiferromagnet NaNi2BiO6. Then I will introduce a new model, the dipolar Heisenberg model, motivated by recent experiments on artificial many-spin systems based on interacting dipoles. I will argue that long-range magnetic order can be suppressed by simply tuning the direction of the dipoles using an external field. The classical, semiclassical, and quantum phase diagram of this frustrated spin model will be presented to show an extended region where the ground state is a quantum paramagnet. By comparing to DMRG, I will argue that it is likely a quantum spin liquid.

In this talk I plan to overview measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities with an emphasis on magnetoresistance and drag resistance. As many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium we will concentrate on physics associated with strong correlation effects and develop hydrodynamic theory of transport. We will apply these ideas to composite fermions of quantum Hall bilayers in hydrodynamic regime.  

Recent years have witnessed enormous progress toward harnessing the power of quantum mechanics and integrating it into novel technologies capable of performing tasks far beyond present-day capabilities. Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information in a process known as decoherence. I will present recent progress in understanding and modeling the effects of multiple noise sources on the evolution of a quantum bit and show how this can be used to develop new ways to slow down decoherence. I will then describe a new general theory for dynamically combatting decoherence by driving quantum bits in such a way that noise effects destructively interfere and cancel out, enabling the high level of control needed to realize quantum information technologies.

Materials in condensed matter have recently been testbeds for several exotic particles, predicted but never realized, in high energy physics. The examples are skyrmions observed in magnetic textures. Weyl fermions in the low energy electronic excitations of Weyl semimetals and Majorana fermions in topological superconductors. These discoveries have not only allowed access to the fundamental physics of the rare particles but also driven large interest in the application of such exotic states to future technologies such as spin based electronics and quantum computation. Discoveries of topological states in materials have largely benefited from the precision of the electronic structure calculations in the weakly correlated systems. In the first part of this talk, I will discuss our resent results on two such predicted materials – 1) NbAs, one of the first generation Weyl semimetals [1-3] and 2) Pd₃Pb, a novel topological material hosting multiple Dirac points and surface states [4]. While calculations are pretty accurate in weakly correlated systems, the topological states in presence of strong electron correlations are still not well understood. As such, materials can take a lead in this field. In the second part of the talk, I will briefly highlight our recent efforts in this area, driven by specific materials design criteria. As an illustration, I will discuss our study on the chiral-lattice antiferromagnet CoNb₃S₆ that has topological character in the electronic band structure, and manifests an unusually large anomalous Hall effect [5].

[1] N. J. Ghimire et al. J. Phys.: Condens. Matter 27, 152201 (2015).

[2] Y. Luo et al. Phys. Rev. B 92, 205134 (2015)

[3] P. J. W. Moll et al., Nat. Communs. 7, 12492 (2016).

[4] N. J. Ghimire et al., Phys. Rev. Materials 2, 081201(R) (2018)

[5] N. J. Ghimire et al., Nat. Communs. 9, 3280 (2018)

Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically non-trivial properties. Among these are geometrically frustrated magnets, which harbor spin liquid phases with fractional excitations.

On the experimental front, this has motivated the search for new low dimensional quantum materials and on the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum many-body systems.

I present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the frustrated kagome geometry, which has seen a flurry of research activity owing to several near-ideal material realizations. On the theoretical front, the kagome problem has a rich history and poses multiple theoretical puzzles which continue to baffle the community. First, I present a study of the spin-1 antiferromagnet, where our numerical calculations indicate that the ground state is a trimerized valence bond (simplex) solid with a spin gap [1], contrary to previous proposals. I show evidence from recent experiments that support our findings but also pose new questions. The second part of the talk follows from an unexpected outcome of my general investigations in the area for the well-studied spin-1/2 case [2]. I explain the existence of an exactly solvable point in the XXZ-Heisenberg model for the ratio of Ising to transverse coupling $J_z/J=-1/2$ [3]. This point in the phase diagram, previously unreported in the literature, has "three-coloring" states as its exact quantum ground states and is macroscopically degenerate. It exists for all magnetizations and is the origin or "mother" of many of the observed phases of the kagome antiferromagnet. I revisit aspects of the contentious and experimentally relevant Heisenberg case and discuss its relationship to the newly discovered point [3,4].

[1] H. J. Changlani, A.M. Lauchli, Phys. Rev. B 91, 100407(R) (2015).
[2] K. Kumar, H. J. Changlani, B. K. Clark, E. Fradkin, Phys. Rev. B 94, 134410 (2016).

[3] H. J. Changlani, D. Kochkov, K. Kumar, B. K. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018).

[4] H. J. Changlani, S. Pujari, C.M. Chung, B. K. Clark, under preparation.

In this talk, I will explain the notion of anisotropic gravitational Weyl anomalies in Lifshitz field theories and their potential applications. Anomalies are quantum violations of classical symmetries, in this case under a non-relativistic (non-Lorentz-invariant) symmetry group. More specifically, I will focus on the only anomaly found in 1+1-dimensional Lifshitz field theories that arise after coupling to Newton-Cartan geometry with torsion, and it's relation to the Lorentz (diffeomorphism) anomaly in the quantum effective action of a 1+1 CFT. At the heart of this discussion, I will emphasize the role of temporal torsion in generating this anomaly and show how such an anomaly can be derived from a 2+1-dimensional Weyl-invariant Chern-Simons Horava-Lifshitz theory of gravity. I will finally comment briefly on how the 1+1 Lifshitz Weyl anomaly can potentially be connected to thermal Hall transport and some other potential applications.

Weyl semimetals (WSMs) have a three-dimensional (3D) bulk band structure in which the conduction and valence bands meet at discrete points, i.e. Weyl nodes. Projections of Weyl points with opposite chirality can be connected by Fermi arcs at a surface. Topological Dirac semimetals (DSMs) have 3D Dirac points which can be viewed as superimposed copies of Weyl points and are stabilized by rotational symmetry. When an external magnetic field is applied to a DSM, Dirac points can be separated into multiple Weyl points and so a WSM phase can be driven. DSMs and WSMs have received a lot of attention because of the chiral anomaly and novel magneto-transport signatures. We develop a tight-binding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM Na3Bi. We add spin-orbit coupling and Zeeman terms in the tight-binding model. Upon magnetic field along the rotational axis, we find that each Dirac node splits into two single Weyl nodes and two double Weyl nodes with opposite chirality, in contrast to  common belief. Our calculations also reveal an interesting evolution of Fermi-arc surface states and other topological surface states as a function of chemical potential in the presence of the external magnetic field.

We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang,^1 a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as non-equilibrium stochastic growth, optimal paths in ill-condensed matter, as well as the extremal statistics of random matrices & increasing subsequences in random permutations.

1. J. Stat. Phys. 160, 794 (2015).

We present an extensive numerical study of a new type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a cross-linking network of Kondo or double-exchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n = 1/2$ and 2/3. In particular, we show that an intriguing glassy magnetic state characterized by ordering wavevectors $\mathbf q = \langle \frac{1}{3},\frac{1}{3}, 1\rangle$ gives a rather satisfactory description of the low temperature phase recently observed in spinel~GeFe$_2$O$_4$.

Arguably the biggest challenge of the high-efficiency perovskite solar cells, such as CH₃NH₃PbI₃ and CH(NH₂)₂PbI₃, is their device instability. A recent study of 2D perovskite compounds, butylammonium methylammonium lead iodide perovskite, [CH₃(CH₂)₃NH₃]₂(CH₃NH₃)_n-1Pb_nI_3n+1, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52%, without any obvious degradation over thousands of hours under standard light illumination and humidity test. This talk focuses on the study of temperature-dependent crystal structures, along with the photovoltaic properties of the 2D 1-layer (n = 1) perovskite material. We have performed elastic and inelastic neutron scattering, Raman scattering, and photoluminescence measurements on a powder sample of the 1-layer system ([CH₃(CH₂)₃NH₃]₂PbI₄). Our analysis of the data illuminates the evolution of the lattice structure, rotational and vibrational dynamics with temperature, and their connection to the charge carrier lifetime of the solar cell will be discussed.

Quantum materials will arguably be the key materials to push forward the forefront energy relevant technologies of the future. Having two powerful neutron sources at the Oak Ridge National Laboratory, enables us to be positioned strongly to use neutron scattering in unveiling the structure of, and dynamics in quantum systems that lead to fundamental understanding and control of quantum phenomena such as coherence, entanglement and novel emergent states. quantum critical phenomena among others, with a range of correlation strength in them. With the increasing progress in instrumentation, the instruments at ORNL are able to study systems in extreme environments of ultra-low temperatures, high magnetic field as well as high pressure. In this talk, I will give several examples of quantum materials problems where neutron scattering played a crucial role with some prospects for possible studies in molecular magnets in particular.

The discovery of high critical temperature superconductivity in iron-based pnictides and chalcogenides brought to the  forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understandingof iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include:
i) The development of spin-fermion models [1] that allow studies in the difficult nematic regime with a finite
short-range antiferromagnetic correlation length above the ordering critical temperatures. This type of studies
also allow the inclussion of doping, quenched disorder, and the study of transport and real-frequency responses;
ii) The application of the Density Matrix Renormalization Group (DMRG) approach to multi-orbital Hubbard
models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder
iron-based compounds such as BaFe₂S₃ and BaFe₂Se₃. In this context,
the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed;
iii) Results for a newly developed multi-orbital spin-fermion model for the CuO₂ planes in high Tc cuprates.[3]


 [1] S.Liang {\it et al.}, Phys.Rev.Lett.{\bf 109}, 047001 (2012) and Phys. Rev. Lett. {\bf 111} 047004 (2013); Phys. Rev. B{\bf 92} 104512 (2015); C. Bishop {\it et al.}, Phys. Rev. Lett. {\bf 117} 117201 (2016); Phys. Rev. B{\bf 96} 035144 (2017). [2] N.D. Patel {\it et al.}, Phys. Rev. B{\bf 96}, 024520(2017). See also  N.D. Patel {\it et al.}, Phys. Rev. B{\bf 94}, 075119(2016). [3] Mostafa Hussein et al., in preparation.

Transition metal dichalcogenide (TMDC) materials exhibit a wide variety of interesting physical phenomena. This diverse family of materials forms a quasi-two dimensional layered hexagonal structure of X-M-X sandwiches (M= Ti, Mo, Hf, W, etc.., X= S, Se, Te) that, depending on composition, may be semiconducting, metallic, or superconducting and many undergo charge density wave transitions. As the materials are layered and can be exfoliated, interest in the TMDCs has increased due to the search for graphene-like materials and the importance of thin film applications. One particularly interesting material is TiSe₂, which forms a prototypical commensurate CDW that occurs in the vicinity of superconductivity. The origin of this CDW phase is controversial and has alternatively been attributed to exciton condensation or several possible Jahn-Teller type mechanisms. I will discuss how neutron scattering and local structure refinements give insight into the effect of the lattice on CDW formation in TiSe₂ and the doping series in which Te is substituted for Se.

(PbZrO3)1-x-(PbTiO3)x (PZT) solid solutions probably represent the most studied group of functional dielectrics materials. For many years the main efforts were devote to the study of the PZT compounds at the morphotropic boundary region (x≈0.5). However recently Zr-rich (x<0.06) compounds attracted new attention due to their potential for the electric energy storage and electricaloric application. Beside possible application related interest these crystals demonstrate extremely reach phase diagram including antiferroelectric, ferroelectric and incommensurate phases.

In my presentation I would like to concentrate on the dynamical features related to the phase transitions in the Zr-rich PZT (including PbZrO3 itself) and in the PbHfO3. In the papers [1,2] we demonstrated that the antiferroelectric phase transition In PbZrO3 with order parameter described by the wavevector q_AFE=(¼ ¼ 0) can be considered as a missed incommensurate transition with some arbitrary wavevector, corresponding to the flat part of the dispersion curve of the TA mode. Later in PbHfO₃ and PbZrO₃ at high pressure the minima at the TA dispersion curves were found [3,4], resulting in the realization of the incommensurate phases.

Extremely complicated diffraction pattern is observed in the PZr_1-xTi_xO₃ crystals with x<0.06. In the intermediate phase between the paraelectric and antiferroelectric phases incommensurate phase is sometime observed similar to that in the PbZrO₃ under high presuure. And in addition complicated system of the satellite peaks in the vicinity of the q_M=(½ ½ 0) including first order and second order satellites exists. In addition to the satellite peaks near the M-points we found second order satellites near the main Bragg peaks.

Observed diffraction pattern can be fully described by the incommensurate structure determined by 2 wavevectors from the same star: q₁=(0.5+δ 0.5-δ -δ)  and q₂ =(0.5-δ δ 0.5+ δ). Combination of the q1 and q2 describes all observed superstructure peaks.

Creation of the true incommensurate phase can be attributed to the mode softening not at q_M, but at a position shifted from the zone boundary. Such unusual soft mode can be described in terms of the coupling of 2 modes in the vicinity of M-point, namely TA mode and oxygen tilt mode. Such coupling is forbidden at q_M but became allowed aside of it. Proposed model provides qualitative agreement with the results of the inelastic and diffuse X-ray scattering measurements 

References
[1] A. K. Tagantsev et al., Nat. Commun., 4, 2229 (2013) [2]R. G. Burkovsky, et al. Phys. Rev. B 90, 144301 (2014) [3] R.G. Burkovsky, et al.. J. Phys.: Condens. Matter,  27, 335901 (2015) [4] R.G. Burkovsky, et al.., Sci. Reports, 7, 41512 (2017)

The curve of cosmic ray energy spectrum is turned around PeV region, which is called "the knee" of the cosmic ray spectrum. The origin of the PeV knee of cosmic ray spectrum remains a puzzle since its first discovery near 70 years ago. Searching for the origin of the knee is one of main aims of LHAASO experiment, a hybrid cosmic ray observatory, which is building on Haizi mountain, south of China. In this talk, the puzzle of the knee of cosmic ray spectrum will be introduced briefly together with the LHAASO experiment introduction. Then I will focus on the technique of scintillator detector of LHAASO and the photomultiplier tube used in this detector. The PMT test bench of Shandong University will also will be introduced.

Heterostructures combining topological and non-topological materials constitute the next frontier in the effort to incorporate topological insulators (TIs) into functional electronic devices. I show that the properties of the interface states appearing at the planar boundary between a topologically-trivial semiconductor (SE) and a TI are qualitatively different from those at the vacuum surface, and are controlled by the symmetry of the interface. In contrast to the well-studied helical Dirac surface states, SE-TI interface states exhibit elliptical contours of constant energy and complex spin textures with broken helicity. Experimental signatures include out of plane spin accumulation under a transport current and the opening of a spectral gap that depends on the direction of an applied in-plane magnetic field. I will also discuss how symmetry breaking at the interface controls proximity-induced superconductivity of the TI surface state. 

Carbon is one of the most fascinating elements in the periodic table. It not only forms the basis of all life on the Earth but also it is important to technology. The unique properties of carbon emerge from its ability to form diverse spⁿ (1 < n < 3) bonds. Until 1960’s graphite with sp² and diamond with sp³ bonding were the most common forms of carbon known. The discovery of one-dimensional (1D) chain-like polymer called “carbyne” in 1960 and later zero-dimensional (0D) carbon fullerenes, 1D carbon nanotube, and two-dimensional (2D) graphene, all with novel properties characteristic of their reduced dimensionality and size, has ushered a new era in carbon science. In recent years many new meta-stable forms of carbon exhibiting a mixture sp¹, sp² and/or sp³ bonding pattern have also emerged.  In this talk I will focus on the carbon allotropes that have been studied in our group^1-7. These include functionalized C₆₀ fullerenes for hydrogen storage^{1, 2}, semi-hydrogenated graphene for metal-free ferromagnet³, metal-organic complexes with large electron affinity⁴, 3D metallic carbon made of hybridized sp² and sp³ bonded atoms⁵, a Cairo-tilling inspired quasi-2D penta-graphene made of only carbon pentagons,⁶ and its thermal conductivity⁷.   All calculations have been carried out using gradient corrected density functional theory. Thermodynamic stability of the above carbon allotropes is confirmed by total energy calculations as well as quantum molecular dynamics. Potential applications of some of these carbon allotropes will be discussed. 

1. Sun, Q., Jena, P., Wang, Q., and Marquez, M.: “First-principles study of hydrogen storage on Li₁₂C₆₀”, J. Am. Chem. Soc. 128, 9741 (2006).
2. Berseth, P. A., Harter, A. G., Zidan, R., Blomqvist, A., Araujo, C. M., Scheicher, R. H., Ahuja, A., and Jena, P.: “Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH₄”, Nano Letters. 9, 1501 (2009).
3. Zhou, J., Wang, Q., Sun, Q., Chen, X. S., Kawazoe, Y., and Jena, P.: “Ferromagnetism in semihydrogenated graphene”, Nano Letters 9, 3867 (2009).
4. Giri, S., Child, B., Zhou, J., and Jena, P: “Unusual Stability of Multiply Charged Organo-metalic Complexes”, RSC Advances 5, 44003 (2015).
5. Zhang, S., Wang, Q., Chen, X., and Jena, P.: “Stable Metallic 3D Metallic Phase of Carbon with Interlocking Hexagons”, Proc. Nat. Acad. Sci. 110, 18809 (2013).

The formation of traffic jams on highways, the clustering of particles in shaken granular gases, and the emergence of macroscopically-linked hubs in complex networks are all examples of real-space condensation. This real-space analogue of Bose-Einstein condensation is rather ubiquitous in nonequilibrium systems. In this talk, I shall present some of the insights into this phenomenon garnered from the study of prototypical toy models. After reviewing static properties of the condensation phase transition, I shall focus on two unexpected features recently discovered: (1) Spatial correlations, which generically exist in driven systems, may give rise to a collective motion of the condensate through the system. Using simplified models, the mechanism behind this motion is explained and shown to be rather robust. (2) Rare fluctuations with extremely atypical currents may lead to condensate formation in systems that otherwise do not condense. I will present microscopic and macroscopic approaches to analyze this novel scenario of condensation.

Near equilibrium, linear response theory has proven to be a powerful tool.  At its core is the fluctuation-dissipation theorem, which dictates that the variance of small fluctuations is intimately related to dissipation.  However, far from equilibrium no such equality exists.  I will show that arbitrarily far from equilibrium dissipation still plays a dominant role in shaping fluctuations, both small and large, through some novel inequalities.  In particular, I will discuss the thermodynamic uncertainty relation and its variants, which are universal nonequilibrium constraints between the variance of fluctuations and dissipation.  These predictions offer general design principles for engineering artificial and natural nano-devices under energy restrictions.

http://jordanmhorowitz.mit.edu/

Since the initial discovery of skyrmion lattices in chiral magnets [1], there has been a tremendous growth in this field as an increasing number of compounds are found to have extended regions of stable skyrmion lattices [2] even close to room temperature [3].  These systems have significant promise for applications due to their size scale and the low currents or drives needed to move the skyrmions [4].  Another interesting aspect of skyrmions is that the equations of motion have significant non-dissipative terms or a Magnus effect which makes them unique in terms of collective driven dynamics as compared to other systems such as vortex lattices in type-II superconductors, sliding charge density waves, and frictional systems.  We examine the driven dynamics of skyrmions interacting with random and periodic substrate potentials using both continuum based modelling and particle based simulations. In clean systems we examine the range in which skyrmion motion can be explored as a function of the magnetic field and current and show that there can be a current-induced creation or destruction of skyrmions.  In systems with random pinning we find that there is a finite depinning threshold and that the Hall angle shows a strong dependence on the disorder strength. We also show that features in the transport curves correlate with different types of skyrmion flow regimes including a skyrmion glass depinning/skyrmion plastic flow region as well as a transition to a dynamically reordered skyrmioncrystal at higher drives. We find that increasing the Magnus term produces a low depinning threshold which is due to a combination of skyrmions forming complex orbits within the pinning sites and  skyrmion-skyrmion scattering effects.  If the skyrmions are moving over a periodic substrate, with increasing drive the Hall angle changes in quantized steps which correspond to periodic trajectories of the skyrmion that lock to symmetry directions of the substrate potential.

[1] S. Muhlbauer et al Science 323 915 (2009).
[2] X. Z.  Yu et al. Nature 465, 901–904 (2010).
[3] X.Z. Yu et al Nature Materials, 10, 106 (2011).
[4] A. Fert, V. Cros, and J. Sampaio Nature Nanotechnology 8, 152 (2013).

The study of topological invariants in band theory has led to many insights into the behavior of electrons in insulators and superconductors. In 2013, Kane and Lubensky pointed out that certain "isostatic" mechanical systems can also admit topological boundary modes [1].  This has led to the design and realization of several families of "topological mechanical metamaterials". In my talk I will introduce the "topological polarization" of Kane and Lubensky and then explain how it can be realized in certain mechanical structures, called rigid origami and kirigami, which consist of rigid plates joined by hinges meeting at vertices [2]. Mysteriously, we found in [2] that triangulated origami structures always seem to be unpolarizable, that is, despite the lack of any apparent symmetry, all of these structures have a vanishing polarization invariant. I will describe recent work with Zeb Rocklin (Georgia Tech), Louis Theran (St. Andrews) and Chris Santangelo (UMass Amherst) which explains this via a "motion to stress" correspondence, that generalizes the 19th-century Maxwell-Cremona correspondence in several directions.

[1] C.L. Kane and T.C. Lubensky, Nat Phys 10, 39–45 (2014).

[2] B.G. Chen, B. Liu, A.A. Evans, J. Paulose, I. Cohen, V. Vitelli, and C. Santangelo, Phys Rev Lett 116, 135501 (2016).

I will discuss the topological and geometric aspects of optical
and transport phenomena in metals with nontrivial band geometry, and outline
the full theory of linear-in-q contribution to the non-local conductivity in
a disordered metal. Physical applications of the theory include the natural
optical activity of metals and the dynamic chiral magnetic effect, as well
as the kinetic magnetoelectric effect/the current-induced magnetization in
metallic systems. The theory is similar in spirit to the one of the
anomalous Hall effect in metals, and can be used for the analysis of the
typical optical and transport measurements (e.g. Faraday rotation,
current-induced magnetization) in the THz frequency range.

 

The behavior of hydrogen in metals has attracted much attention in fundamental and applied research areas. Palladium hydride (PdHx) is a typical metal-hydrogen system and has been studied for many decades. Pd has remarkable abilities to absorb plenty of H atoms and the H atoms are highly mobile in the Pd lattice. It is interesting to examine how the properties are changed as the particle size is reduced to a nanometer-scale. We have investigated structure, diffusion and vibrational dynamics by means of neutron scattering techniques for both bulk and nanocrystalline PdHx with a diameter of 8nm. Neutron diffraction work on nanocrystalline sample demonstrated that some of hydrogen atoms are accommodated at the tetrahedral (T) sites. This is in contrast to bulk PdHx with octahedral (O) occupation. In quasielastic scattering measurements, we found an additional fast diffusion process with a small potential barrier in nanocrystalline PdHx. Furthermore, our inelastic scattering works revealed that nanocrystalline PdHx exhibits two distinct vibrational excitations; one resembles that observed in bulk PdHx and the other is the excitations appeared at higher energies. The additional diffusion process and vibrational states are attributed to the H atoms at T sites near the surface of nanoparticles. The potential shape around the T site near the surface will be discussed in the seminar.

In the recent experiments, PRB 94, 054504 (2016), the unusual oscillatory magnetore- sistance of superconductors was discovered with a periodicity essentially independent on magnetic field direction and even material parameters. The nearly universal period points to a subatomic mechanism of the phenomenon. This mechanism is related to formation inside samples of subatomically thin (10−11cm size) threads in the form of rings of the interatomic radius. Electron states of rings go over into conduction electrons which carry the same spin imbalance in energy as the ring. The imbalance occurs due to the spin interaction with the orbital momentum of the ring. The conductivity neat Tc is determined by fluctuating Cooper pairs consisting of electrons with shifted energies. Due to different angular momenta of rings these energies periodically depend on magnetic field resulting in the observed oscil- latory magnetoresistance. Calculated universal positions of peaks (n + 1)∆H (∆H '"" 0.18T and n = 0, 1, 2, ...) on the R(H) curve are in a good agreement with measurements.

Fractional topological insulators (FTI) are electronic topological phases in (3 + 1) dimensions enriched by time reversal (TR) and charge U(1) conservation symmetries. We focus on the simplest series of fermionic FTI, whose bulk quasiparticles consist of decon ned partons that  carry fractional electric charges in integral units of e = e=(2n + 1) and couple to a discrete Z2n+1 gauge theory. We propose massive  symmetry preserving or breaking FTI surface states. Combining the long-ranged entangled bulk with these topological surface states, we  deduce the novel topological order of quasi-(2 + 1) dimensional FTI slabs as well as their corresponding edge conformal eld theories. We will also describe some past work on coupled wire constructions of Weyl/Dirac semimetals, and some new work about mechanical topology and conformal field theory.

Classical and quantum spin systems have played an enormously important role in the study of magnetism. They provide excellent models to understand the ground state magnetic structure and low energy excitations of magnetic systems. In addition, they provide a rich arena for studying the physics of interacting many-body systems. In this talk I will discuss a very specific problem, namely the observation of an unusual ordering of spins, the so called uudd states, in quasi one-dimensional Heisenberg spin chains and anisotropic two-dimensional Heisenberg spin systems. Theoretical work to understand this and some related issues using both model spin Hamiltonians with competing interactions, exchange anisotropy, biquadratic exchange etc. and ab initio electronic structure calculations, will be discussed. 

Topological  Weyl and Dirac semimetals in three dimensions have gapless (massless) Weyl fermions. These semimetals were predicted theoretically in 2011 (Vishwanath et al, Burkov and Balents) and have been experimentally discovered recently in TaAs, TaP, NbAs, NbP (Weng et al., Huang et al., Shekhar et al., 2015). Although these gapless systems can be gapped (given mass) trivially by symmetry breaking terms, a more interesting problem is if they can be gapped without breaking symmetries? We show that you can indeed do this by introducing many body interactions. We also show that this symmetry-preserving gapping gives point-like and line-like anyonic excitations in the gapped bulk. 

Population annealing is a sequential Monte Carlo method for studying the equilibrium properties of glassy systems characterized by rough free energy landscapes.  In this talk I will introduce two glassy systems, the Ising spin glass and a glass-forming binary mixture of hard spheres. I will describe population annealing and some of its useful features, and then present results from simulations of these glassy systems.

http://people.umass.edu/machta/

Small organic molecules have had a dramatic impact on our health and daily life over the past century. Small molecule pharmaceuticals have increased human lifespans, and organic pigments have expanded in use in textiles and displays. This impact is set to accelerate in the near future, as small molecules are explored for novel applications such as organic electronics, or metal-organic frameworks for chemical separations, catalysis and sensing. One of the major barriers for using small organic molecules for new applications is the limited understanding we possess on how molecules aggregate together to form different crystal habits and phases. Different crystal structures and morphologies can have wildly varying physical, chemical and physiological properties. Thus, if we do not control the crystallization organic molecules, we cannot predict its behavior for the aforementioned applications. Understanding the crystallization process can also help form metastable phases. These metastable phases can be more useful than the equilibrium phase for many applications. Metastable phases permit tunable optical bandgap for optoelectronics, control over pore size and shape in metal organic frameworks (MOF), and increased bioavailability in pharmaceuticals. General methods used to create metastable phases, like confinement or rapid cooling, require small length scales and extreme rates of heat and mass transfer. Moreover, these processes need precise control to get reliable results. This talk will focus on flow coating and microfluidic methods of controlling organic molecule and MOF crystallization characteristics, and the use of these materials for various applications.

I first consider an elevator as a system that lifts a load from a ground floor to a top floor by consuming an input energy. I ask: what amount of load enables the system to perform maximum amount of power? Can I design a mode of operation where I can utilize good enough amount of the input energy better than that of energy utilized at maximum power?

I will then go over to a molecular dynamics simulation study of a heat engine where the working substance is a real gas and try to address similar questions as the elevator system.

Spin chains with PSU(n) symmetry are known to have symmetry protected topological phases distinguished by boundary states. We determine the parent Hamiltonians of some of these phases at the AKLT point. In the adjoint representation we use a graphical method that produces expressions for arbitrary N. Finally we discuss the fate of the Haldane conjecture.

Quantum spin liquids (QSLs) have achieved great interest in both theoretical and experimental condensed matter physics due to their remarkable topological properties. Among many different candidates, the Kitaev model on the honeycomb lattice is a 2D prototypical QSL which can be experimentally studied in materials based on iridium or ruthenium.. Here we study the spin-1/2 Kitaev model using classical Monte-Carlo and semiclassical spin dynamics of classical spins on a honeycomb lattice. Both real and reciprocal space pictures highlighting the differences and similarities of the results to the linear spin wave theory will be discussed in terms dispersion relations of the pure-Kitaev limit and beyond. Interestingly, this technique could capture some of the salient features of the exact quantum solution of the Kitaev model, such as features resembling the Majorana-like mode comparable to the Kitaev energy, which is spectrally narrowed compared to the quantum result, can be explained by magnon excitations on fluctuating one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of a magnon to Majorana fermions. The calculations will be directly compared with our neutron scattering data on α-RuCl3 which is a prime candidate for experimental realization of Kitaev physics. 

Topological phases in two and three dimensions can be theoretically constructed by coupled-wire models whose fundamental constituents are electronic channels along strings. On the other hand the collective topological phases support further fractionalized emergent quasi-string excitations or defects such as flux vortices. In this talk I will describe topological superconductors and Dirac (or Weyl) semimetals using coupled-wire models, and discuss the fractional behavior of emergent topological strings.

We propose that resonant inelastic X-ray scattering (RIXS) is an effective probe to detect spin-liquid character in potential material incarnations of the Kitaev spin liquid (such as the honeycomb iridates and ruthenium chloride). Calculating the exact RIXS response of the Kitaev honeycomb model, we find that the fundamental RIXS channels, the spin-conserving (SC) and the non-spin-conserving (NSC) ones, can probe the fractionalized excitations of the Kitaev spin liquid separately. In particular, SC RIXS picks up the gapless Majorana excitations with a pronounced momentum dispersion, while NSC RIXS creates immobile flux excitations, thereby rendering the response weakly momentum dependent.

I will first introduce formation probability as a quantity which can determine the universality class of a quantum critical system. In other words, by calculating this quantity one can find the central charge and critical exponents of the quantum system. We will show that calculating this quantity boils down to finding Casimir energy of two needles. Then we will  briefly talk about Shannon mutual information as another quantity which can play similar role. Finally, we will introduce post-measurement entanglement entropy as a tripartite measure of entanglement. We will show that this quantity is related to the Casimir energy of needles on Riemann surfaces and can be calculated exactly for conformal field theories.

Ref:
1: MAR, Europhysics Letters, 112, 66001 (2015), J. Stat. Mech. (2016) 123101 and K. Najafi, MAR, Phys. Rev. B 93, 125139 (2016).
2: F. C. Alcaraz, MAR, Phys. Rev. Lett. 111, 017201(2013), Phys. Rev. B, 90, 075132 (2014).
3: MAR, Phys. Rev. B 92, 075108 (2015) , J. Stat. Mech. (2016) 063109 and MAR,
K. Najafi, MAR, JHEP12(2016)124.

Rigid spherical particles in oscillating fluid flows form interesting patterns as a result of fluid mediated interactions. Here, through both experiments and simulations, we show that two spheres under horizontal vibration align themselves at right angles to the oscillation and sit with a gap between them, which scales in a non-classical way with the boundary layer thickness. A large number of spherical particles form strings perpendicular to the direction of oscillation. Investigating the details of the interactions we find that the driving force is the nonlinear hydrodynamic effect of steady streaming. We then design a simple swimmer (two-spheres-and-a-spring) that utilizes steady streaming in order to propel itself and discuss the nature of the transition at the onset of swimming as the Reynolds number gradually increases. We discuss implications and connections to biological systems, motility, and collective behavior of swimmers.

http://dklotsa.wixsite.com/dklotsa

The progress of modern technology is increasingly driven by the development and evaluation of novel materials. Layered two-dimensional (2D) materials, collectively referred to as van der Waals solids, are receiving intense interest in the community due to their unconventional and diverse electronic behaviors. Transition metal dichalcogenides (TMDs) are a class of 2D material following the basic chemical formula MX2, where M = (Mo, W, Nb, Re, …) and X = (S, Se, Te, …). Varying the chemical composition allows access to semiconducting, semi-metallic, superconducting, or magnetic behaviors in the 2D limit. Of recent interest are telluride based TMDs such as MoTe2 and WTe2 which are predicted exhibit controllable structural phase transitions appropriate for phase change memory applications as well as topologically protected and spin polarized electronic states.

In this colloquium, I will present our efforts to understand the temperature-dependent optical properties of MoTe2 and MoxW1-xTe2 using temperature-dependent and polarization-resolved Raman spectroscopy. We have used this technique to identify the anharmonic contributions to the optical phonon modes in bulk MoTe2 occupying the distorted orthorhombic (T­d) lattice structure. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature however, below 100 K we observe nonlinear frequency shifts in some modes. We show that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near 250 K that correlate well with a structural phase transition.

We also explore the composition-dependent optical properties of MoxW1-xTe2 alloys. Our observations identify signatures of the hexagonal (H), monoclinc (1T’) and Td structural phases. Polarization-resolved Raman measurements allow for the assignment of all vibrational modes as well as the evolution of mode symmetry and frequency with composition. We discover a previously unobserved WTe2 mode as well as a Raman-forbidden MoTe2 mode that is activated by compositional disorder. The primary WTe2 Raman peak is asymmetric for x <= 0.1, and is well fit by the spatial correlation model. From these fits, we extract the spatial phonon correlation length which serves as an indirect measure of the WTe2 domain size. Our study is foundational for future studies of MoxW1-xTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides.

By applying a magnetic field, a superconducting proximity nanowire in the presence of spin-orbital coupling can pass through topological phase transition and possesses Majorana bound states on the ends. One of the promising platforms to detect the Majorana modes is a coulomb blockade island by measuring its two-terminal conductance (S. M. Albrecht et al., Nature (London) 531, 206 (2016)). Here, we study the transportation of a single electron across the superconducting Coulomb blockade nanowire at finite temperature to obtain the generic conductance equation. By considering all possible scenarios that Majorana modes appear in the nanowire, we compute the nanowire conductance as the magnetic field and the gate voltage of the nanowire vary. The oscillation behavior of the conductance peak is temperature independent and the  oscillation amplitude of the conductance peak spacings increases as the magnetic field increases. 

A grand challenge in the field of topological physics is to understand the role of interaction and to realize interacting topological phases in realistic materials. In this talk, we will discuss the possible realization of interacting topological states in bilayer graphene under a strong magnetic field. We start from a fermonic two-channel quantum spin Hall state with two copies of helical edge states, which have been demonstrated experimentally in bilayer graphene. By introducing interaction into two channel helical liquids, we demonstrate that all the fermion degrees are gapped out and only one bosonic mode remains, thus yielding a bosonic version of topological insulator, dubbed “bosonic symmetry protected topological state”. Physically, the two dual boson fields of this bosonic mode carry charge-2e and spin-1, respectively, due to the helical nature. Thus, we dubbed them “bosonic helical liquids”. We further study the transport of a quantum point contact for bosonic helical liquids and compare them to fermonic two-channel helical liquids. A novel charge insulator/spin conductor phase is identified in the weak repulsive interaction regime for bosonic helical liquids while charge insulator/spin insulator or charge conductor/spin conductor phase is present for fermonic two-channel helical liquids. Thus, a quantum point contact experiment will allow us to identify the bosonic symmetry protected topological states unambiguously. Similar physics can also emerge in topological mirror Kondo insulators, such as SmB6.

Reference:

[1] Bilayer Graphene as a platform for Bosonic Symmetry Protected Topological States, Zhen Bi, Ruixing Zhang, Yi-Zhuang You, Andrea Young, Leon Balents, Chao-Xing Liu, Cenke Xu, arXiv:1602.03190v1, 2016

[2] Interacting topological phases in thin films of topological mirror Kondo insulators, Rui-Xing Zhang, Cenke Xu, Chao-Xing Liu, arXiv: 1607.06073, 2016

[3] Fingerprints of bosonic symmetry protected topological state in a quantum point contact, Rui-xing Zhang, Chao-xing Liu, arxiv: 1610.01236, 2016

The spin and orbital degrees of freedom play a crucial role in determining the remarkable properties of transition metal oxide materials. In this talk, I will describe how resonant inelastic x-ray scattering (RIXS) opens up important new possibilities for measuring these degrees of freedom even in challenging cases such a heterostructures and transient states [1]. This includes determining how orbitals are modified within LaNiO3-based heterostructures [2] and characterizing the spin behavior within the ultra-fast transient state of photo-doped Sr2IrO4 [3].

References

1. M. P. M. Dean et al., Nature Materials 11, 850 (2012); M. P. M. Dean et al., Nature Materials 12, 1019–1023 (2013)

2. G. Fabbris et al., Phys. Rev. Lett. 117, 147401 (2016)

3. M. P. M. Dean et al., Nature Materials 15, 601-605 (2016)

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. We revealed that the stability of the layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. Our work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond the material database and provides new insight into the principles of electride design.

Many experimental systems can spend extended periods of time relative to their natural time scale in localized regions of phase space, transiting infrequently between them.  This display of metastability can arise in stochastically driven systems due to the presence of large energy barriers, or in deterministic systems due to the presence of narrow passages in phase space.  To investigate metastability in these different cases, I take a Langevin equation and determine the effects of small damping, small noise, and dimensionality on the dynamics and mean transition time.  Of particular interest is what happens in the infinite dimensional limit, a stochastic partial differential equation, and the question of what ensemble this system appears to sample over time.  Both analytical and numerical results will be presented.

http://knewhall.web.unc.edu/

Under certain conditions, it takes a shorter time to cool a hot system than to cool the same system initiated at a lower temperature. This phenomenon – the “Mpemba Effect” – is well known in water, and has recently been observed in other systems as well. However, there is no single generic mechanism that explains this counter-intuitive behavior. Using the theoretical framework of non-equilibrium thermodynamics, we present a widely applicable mechanism for this effect, derive a sufficient condition for its appearance in Markovian dynamics, and predict an inverse Mpemba effect in heating: under proper conditions, a cold system can heat up faster than the same system initiated at a higher temperature. Our results suggest that it should be possible to observe the Mpemba effect and its inverse in a variety of systems, where they have never been demonstrated before.

Geometric frustration in Mott insulators permits perturbative electron fluctuations controlled by local spin configurations [1]. An equilateral triangle (“trimer”) of spins with S = 1/2 is the simplest example, in which low-energy degrees of freedom consist of built-in magnetic and electric dipoles arising from the frustrated exchange interaction. Such trimers can be weakly coupled to make multiferroics by design [2]. An organic molecular magnet known as TNN [3], with three S = 1/2 nitronyl nitroxide radicals in a perfect C3 symmetric arrangement, is an ideal building block as demonstrated by recent experiments on a single crystal comprising TNN and CH3CN. The fascinating thermodynamic phase diagram of this molecular crystal, TNN·CH3CN, is in excellent agreement with our theory, which predicts multiferroic behavior and strong magnetoelectric effects arising from an interplay between magnetic and orbital degrees of freedom [4]. Our study thus opens up new avenues for designing multiferroic materials using frustrated molecular magnets.

References:

[1] L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008).

[2] Y. Kamiya and C. D. Batista, Phys. Rev. Lett. 108, 097202 (2012).

[3] Y. Nakano et al., Polyhedron 24, 2147 (2005).

[4] Y. Kamiya et al.,  in preparation.

https://sites.google.com/site/yoshitomokamiya/

The study of metal-insulator, structural and magnetic phase transitions in materials with strongly interacting electrons is a challenging frontier of condensed matter physics. The challenge is to disentangle the contributions of charge, lattice, spin and orbital degrees of freedom to phase transitions. I will report on the optical properties of two materials that undergo thermally-induced metal-insulator transitions accompanied by structural and/or magnetic instabilities: vanadium dioxide (VO2) and the manganite La0.67Sr0.33MnO3. Infrared micro-spectroscopy and micro-ellipsometry measurements on crystals of VO2 reveal that its insulating phases are Mott-Hubbard insulators, not Peierls insulators. Scanning near-field infrared microscopy (SNIM) allows us to directly image nano-scale metallic puddles that appear at the onset of the first-order metal-insulator transition (MIT) in VO2 films. We find that the patterns of metallic domains are reproducible upon repeated thermal cycles across the MIT and point to the important role of imperfections in films.  In contrast to VO2, recent SNIM data shows time dependence of near-field infrared amplitude in a La0.67Sr0.33MnO3 film which we attribute to fluctuating conductivity in the vicinity of its second order metal-insulator transition.

Fathoming interplay between symmetry and topology of many-electron wave-functions has deepened understanding of quantum many body systems, especially after the discovery of topological insulators. Topology of electron wave-functions enforces and protects emergent gapless excitations, and symmetry is intrinsically tied to the topological protection in a certain class. Namely, unless the symmetry is broken, the topological nature is intact. We show novel interplay phenomena between symmetry and topology in topological phase transitions associated with line-nodal superconductors. The interplay may induce an exotic universality class in sharp contrast to that of the phenomenological Landau-Ginzburg theory. Hyper-scaling violation and emergent relativistic scaling are main characteristics, and the interplay even induces unusually large quantum critical region. We propose characteristic experimental signatures around the phase transitions in three spatial dimensions, for example, a linear phase boundary in a temperature-tuning parameter phase-diagram.