Condensed Matter Seminars This Term

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.

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.

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

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].

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).

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 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.

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.

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.

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)

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