Condensed Matter Seminars

TBA

The goal of this seminar is to provide an overview of our recent work. We will discuss connections between operator space entanglement of unitary dynamics, OTOCs, and entropy production as introduced in [1]. How to 'disentangle' contributions from decoherence vs. scrambling for open quantum systems [2]. How finite temperature 'regularized' OTOCs are related to spectral form factors and the entanglement of the time-evolved thermofield double state [3]. An intermezzo about the connection between the quantum coherence-generating power of a unitary, OTOCs of random diagonal unitaries, and distances between maximally abelian subalgebras [4]. A unifying operator algebraic framework for scrambling that is useful for both closed and open quantum systems [5]. How OTOCs behave in local non-Hermitian Hamiltonians (effective models for measurement-induced phase transitions) [6]. And finally, an application of some of these techniques to a dual-unitary classical shadow tomography scheme [7]

[1] Phys. Rev. Lett. 126, 030601 (2021)

[2] Phys. Rev. A 103, 062214 (2021)

[3] Quantum 6, 746 (2022)

[4] Phys. Rev. Research 3, 023214 (2021)

[5] Phys. Rev. A 107, 042217 (2023) [Editors' Suggestion]

[6] Phys. Rev. B 108, 134305 (2023) [Editors' Suggestion]

[7] arXiv:2404.01068 (2024) [quant-ph]

Recently, ladder-shaped arrays of Rydberg atoms have been used to simulate scalar QED in 1+1 dimensions. This type of analog quantum simulator has a rich phase diagram which has been explored experimentally using QuEra facilities. We report recent results regarding the observation of a floating phase (2401.08087) and the derivation of an effective Hamiltonian (2312.04436) for the simulator. Entanglement entropy plays a crucial role in the determination of the phase diagram. We describe a recent proposal (2404.09935 ) to extract the entanglement entropy with user-driven QuEra measurements of a single copy of the system instead of the twin copy method. We show empirically that this estimation provides a lower bound which becomes tighter when the empirical probabilities are filtered. 

Second-order differential equations are ubiquitous in physics, but every once in a while there are special cases where the order of these equations may be reduced. One example of such a reduction is the quantum harmonic oscillator as solved by P. Dirac. Dirac introduced the annihilation and creation operators, which permit solving of the second-order problem presented by the linear Schrödinger equation by reduction to a first-order linear equation. In the context of superconductivity, a system of non-linear second-order differential equations describing vortex lines in superconductors may be reduced to first-order at a particular value of the Ginzburg-Landau parameter. This reduction can be achieved by two procedures. One approach uses an ingenious minimization of the Ginzburg-Landau free energy functional tailored to the presence of a topological defect (the Bogomol’nyi procedure). The other uses the Ginzburg-Landau equations and operators analogous to Dirac’s annihilation and creation operators (the Sarma procedure). The Bogomol’nyi procedure leads to the famous Bogomol’nyi equations, published in 1976. The Sarma procedure was never published in a journal but is hinted at in P. G. de Gennes’s 1966 classic Superconductivity of Metals and Alloys.  We will show that, in the particular case of the vortex line, the Sarma procedure recovers the Bogomol’nyi equations and that the Sarma procedure is in fact more general, as to be applicable it does not rely upon the presence of a topological defect.

Topological spin textures like magnetic skyrmions exhibit a stable particle nature due to their topological protection, leading to their utilization as an information carrier in magnetic memory devices. Although exploring the fundamental current-induced dynamics is a pivotal topic for such applications, the studies have been mostly limited to two-dimensional skyrmions [1] and the systematic investigation for three-dimensional (3D) magnetic hedgehogs remains largely unexplored. The topological defect at the core of the hedgehog where spin length vanishes, corresponding to monopoles of the emergent magnetic field for electrons, could bring about current-induced dynamics distinct from skyrmions [2]. 
In this study, we theoretically investigate the drift motions of the hedgehogs caused by an electric current, focusing on periodic arrangements of hedgehogs and antihedgehogs called the hedgehog lattices (HLs) [2]. Through the numerical simulations based on the Landau-Lifshitz-Gilbert equation, we find that a drift motion of the HLs requires a nonzero threshold current density depending on the magnetic field and the direction of the electric current relative to the Dirac strings connecting the hedgehogs and antihedgehogs. Furthermore, we reveal that the HLs exhibit a drift motion transverse to the current direction, which we call the monopole Hall effect. Notably, the threshold current for the transverse motion can be smaller than that of the longitudinal motion, resulting in the “perfect” monopole Hall effect with the maximally large Hall angle of π/2. Our study makes an important step toward understanding the current-induced dynamics of the HLs and would shed light on the applications of the 3D topological spin textures to novel spintronic devices.
[1] F. Jonietz et al., Science 330, 1648 (2010). 
[2] N. Kanazawa et al., Nat. Commun. 7, 11622 (2016).

The transition of many materials of interest for the condensed matter community, particularly in energy-related applications, from crystalline materials to heterostructures and complex architectures, where different compounds are combined, stacked, interfaced and twisted in different ways, has forced the experimental techniques to evolve accordingly. Angle-resolved photoemission (ARPES), which has been for decades a spectroscopy of reference for accessing the low energy excitations in solids, had to implement a number of updates to keep pace with the progress made in materials fabrication. This talk will discuss some of the latest trends in ARPES, how it has advanced and to where it is moving in order to remain an invaluable tool for studying the electronic properties of materials.

Analogues of pixels to two-dimensional (2D) pictures, voxels –– in the form of small cubes or spheres –– are the basic units of three-dimensional (3D) objects. Digital assembly of bio-ink voxels may provide an approach to engineering heterogeneous yet tightly organized 3D tissue mimics. However, this approach requires precisely manipulating highly viscoelastic bio-ink voxels in 3D space, which represents a grand challenge in both soft matter science and biomanufacturing. In this talk, I will introduce a voxelated bioprinting technology that enables the Digital Assembly of Spherical bio-ink Particles (DASP). First, I will discuss the criteria for the on-demand generation, disposition, and assembly of viscoelastic bio-ink droplets in an aqueous environment without the help of large interfacial tension. Second, I will describe how to use DASP to create 3D structures consisting of interconnected yet distinguishable bio-ink particles. Finally, I will share our recent progress in applying DASP to encapsulate islets into multiscale porous scaffolds to treat type 1 diabetes. I will also discuss immediate applications and emerging challenges associated with voxelated bioprinting.

High Entropy Alloys (HEAs), also known as Compositionally Complex Alloys (CCAs), embody a transformative class of materials consisting of at least four elemental components. These new types of material open new horizons in alloy design for exploring new structural and functional material properties unknown in traditional alloys. However, the combinatorial nature of HEAs can result in compositional possibilities reaching into the billions or even trillions, making traditional studies challenging. This talk presents an in-depth exploration of HEAs, starting with a foundational understanding of their unique characteristics and the importance of their complex phase behavior. It highlights the inherent challenges posed by the expansive compositional space and limitations of conventional materials discovery and design methodologies. The talk emphasizes the pivotal role of computational techniques that provides a strategic blueprint for high-throughput alloy design that accelerates the exploration and optimization of HEAs but also provides a deeper insight into their fundamental behaviors.

Linear temperature dependence of transport coefficients in metals is habitually ascribed to non-Fermi-liquid physics. In this talk we establish this behavior for 2D electron fluids, systems in which carrier collisions assist conduction, leading to resistance decreasing with temperature. As we will see, electron fluids with simple Fermi surfaces obey nonclassical hydrodynamics described by a loop representing the Fermi surface shape evolving in space and time. Replacing the fluid velocity dynamics with an amoeba-like loop dynamics leads to a large family of long-lived excitations manifest as multiple viscous modes. A cascade of these modes results in a linear T dependence that extends down to lowest temperatures, as well as a Kolmogorov-like fractional power -5/3 scaling of conductivity vs. wavenumber. These dependences provide a smoking gun for nonclassical hydrodynamics and are expected to be generic for strongly-correlated 2D systems with near-circular Fermi surfaces.

Pairing PbS quantum dots (QDs) with photochromic molecules (PCMs) allows for the synthesis of efficient and reversible near infrared photoluminescence (PL) photo-switches. In our work, we explore the utility space of this hybrid system by systematically comparing and contrasting different types of PCMs and different sizes of QDs. We demonstrate that the amount of photo-switching observed can be affected by (1) varying the size of the QDs, (2) varying the length of the PCMs, (3) fluorinating the PCMs, (4) varying the end group of the PCMs. We further investigate this system to parse out the mechanisms which may be responsible for this behavior. We present strong evidence to suggest that the mechanism driving this switching effect is an inter-QD tunneling process. We demonstrate a possible link between the energy levels of the PCMs and the magnitude of the switching effect and outline a rough empirical model which can guide the future design of QD/PCM photo switches to produce customized switching properties.

In this talk, I will discuss our recent work on transport phenomena stemming from the topological properties of magnetic textures. As a specific illustrative case, we study the transport of vorticity on curved dynamical two-dimensional magnetic membranes. We find that topological transport can be controlled by geometrically reducing symmetries, which enables processes that are not present in flat magnetic systems. To this end, we construct a vorticity 3-current obeying a continuity equation, which is immune to arbitrary local disturbances of the magnetic texture as well as spatiotemporal fluctuations of the membrane. We show how electric current can manipulate vortex transport in geometrically nontrivial magnetic systems. As an example, we propose a minimal setup that realizes an experimentally feasible energy storage device and discuss its thermodynamic efficiency in terms of a vorticity-transport counterpart of the thermoelectric “ZT” figure of merit.

Despite identical R3 crystal structures, honeycomb layered MTiO₃ ilmenites exhibit diverse magnetic orders and transition temperatures (T_N): G-type antiferromagnetic for MnTiO₃ (T_N=68 K) and A-type antiferromagnetic for CoTiO₃(T_N=38 K) and NiTiO₃ (T_N=22 K). This work focuses on this intriguing interplay between local structure, electronic properties, and magnetic configurations. CoTiO₃ has two magnon peaks around 5-14 meV with a distinct gapless Dirac node nestled between them are observed and the magnon modes are renormalized to lower energies. For CoTiO₃, magnetic excitations attributed to spin-orbit exciton multiplet transitions show the same temperature dependance as magnon with the intensity dissipating quickly above T_N.  The energy levels arising from crystal field and spin-orbit coupling are gradually thermally populated through T and reaching maximum at 100 K. However, the NiTiO₃ system shows a single low energy magnon peak around 2-4 meV which is renormalized into lower energies, but it does not show Dirac magnon properties. The calculated exchange interactions using SpinW confirm the weaker inter-plane interaction in CoTiO3 than NiTiO3. Across three system, both transition metal M⁺² ion and Ti⁺⁴ ions are in distorted octahedra environment, and the first four nearest neighbors are Ti-O < M-O < Ti-O < M-O with the given bond length order. Across three systems Ti-O and short M-O bond length variation is minimum. However, M-O bond length (MnTiO₃=2.28 Å, CoTiO₃=2.17 Å and NiTiO₃=2.12 Å) variation is significant which follows the same variation as reported dielectric constants (MnTiO₃=20.4, CoTiO₃=19.5 and NiTiO₃=17.8 ) and T_N and confirms the interplay between these parameters. Within the measured 100 K to 500 K, temperature dependance of local structure is insignificant and for the reported relative dielectric values, the variation is almost constant. This suggests that the interplay between local geometry and magnetic interactions governs the diverse behaviors observed in these honeycomb materials.

Quantum phases such as superconductivity and ferromagnetism are among the most important topics in condensed matter physics research. Recently, a family of two-dimensional flat band systems, including magic-angle twisted graphene, uncovered an abundance of symmetry breaking and novel quantum phases.

In this talk, I will introduce the recent advances in these materials and give two examples of how we engineered and revealed new quantum phases of matter in twisted graphene. These include an orbital ferromagnetic state induced by spin-orbit coupling and a zero-field superconducting diode effect. Towards the end of the talk, I will mention our on-going effort of studying a new type of Coulomb-driven rotational symmetry breaking state in the moiré-less bilayer graphene. These examples establish the two-dimensional flat band systems as a versatile platform with multiple tuning knobs, where new physics emerges from the interplay between various quantum phases.

Recent measurements on the tetragonal phase of the iron-based superconductor FeSe,S support the existence of a remarkable phase where the superconducting state supports a finite residual  density of states arising from patchlike nodal surfaces[1,2].  This ``ultranodal"> state can arise in situations where conventional intraband spin singlet pairing is highly anisotropic and coexists with time-reversal symmetry breaking  interband spin triplet interactions [3].  Here I present a  microscopic scenario including ferromagnetic interactions that can account for nonunitary pairing and C4 symmetry breaking in the superconducting state that is also observed in recent experiments.

1) Sato, Y. et al. Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1-xSx. Proc. Natl Acad. Sci. 115, 1227??1231 (2018).

2) Hanaguri, T. et al. Two distinct superconducting pairing states divided by the nematic end point in FeSe1-xSx. Sci. Adv. 4, eaar6419 (2018).

3) ``Topologically protected ultranodal state in iron-based superonductors", S. Setty, S.

Bhattacharyya, Y. Cao, A. Kreisel and P.J. Hirschfeld,  Nat. Comm. 11, 523 (2020).

Characterization and quantum control of complex quantum matter is one of the shared goals for condensed matter and quantum information science research. Toward this end, my research uses van der Waals materials to synthesize topological and correlated states, and quantum sensors based on spin defects to uncover their microscopic picture. Focusing on superconductivity as the theme of this talk, I will first present pathways to program the electron correlation by exploiting the lattice degree of freedom, both in the planar and vertical directions of moiré materials. The challenges to fully characterize the moiré superconductivity will be discussed. In the second part, I will show new experimental observables unlocked by the nanoscale quantum sensing platform can uncover hidden physics. As an example, quantitative visualization of the super current flow in a Josephson junction is used to reveal electrically configurable ground states in the zero-resistance regime. A surprising role of the kinetic inductance and the implications for the Josephson diode effect will be discussed. Finally, I will share my vision to explore intertwined topology and correlation by integrating the programmable quantum materials with nanoscale quantum sensors.

The quest for novel quantum states in condensed matter physics often hinges on the reduction of system dimensionality. In particular, one-dimensional systems are theoretically predicted to host a range of fractionalized excitations. These include the Tomonaga-Luttinger liquid, which exhibits spin and charge separation, and the Majorana particle, a cornerstone for fault-tolerant quantum computing. However, fabricating near-perfect one-dimensional quantum wires has been a significant challenge, especially those involving strongly correlated electrons.

In our research, we have developed a novel method to fabricate quantum wires of a Mott insulator on graphite substrates using pulsed-laser deposition, achieving structures such as stripes, junctions, and nanorings. These single-crystalline wires are one unit cell in thickness and precisely two to four unit cells in width, and can extend to several micrometers in length. The spectroscopy measurements along with theoretical calculations reveal the existence of strong electron correlations in this system. Moreover, our findings emphasize the importance of nonequilibrium reaction-diffusion processes in atomic-scale self-organization, opening up exciting avenues for the exploration of exotic fractionalized states in purely one-dimensional quantum wires.

If one can control the atomic symmetries of a material at will, the intrinsic properties of the material will be significantly modified. However, the atomic structures of conventional materials are often constrained by the equilibrium phase of matter. Here, we overcome this fundamental limitation using recent advances in twistronics, enabling precise control over all the individual point group symmetry elements – inversion, mirror, and rotational symmetries – in twisted van der Waals (vdW) material in a new 3D configuration [1]. The resulting 3D twisted materials exhibit emerging optical responses that are fundamentally different from those of natural vdW materials. This novel approach to control symmetries can enable nearly infinite vdW quasicrystalline phases, promising a practical platform to study less-explored structure-property relationships of quasicrystals. In addition, we will discuss an ultraclean vdW crystal synthesis method [2]. A self-flux synthesis method we developed has yielded vdW materials with ~ 2 orders of magnitude lower point defect density compared to commercial vdW materials grown by a chemical vapor transport method. These ultraclean vdW materials reveal intrinsic excitonic properties that were previously obscured by low-quality materials. The combination of these ultraclean materials with the symmetry design approach holds great promise for the development of high-performance artificial material systems for next-generation technologies.

References:

1. Bumho Kim, Jicheng Jin, Zhi Wang, Li He, Thomas Christensen, Eugene J. Mele, and Bo Zhen, Nature Photonics 18, 91-98 (2024).
2. Bumho Kim, Yue Luo, Daniel Rhodes, Yusong Bai, Jue Wang, Song Liu, Abraham Jordan, Baili Huang, Zhaochen Li, Takashi Taniguchi, Kenji Watanabe, Jonathan Owen, Stefan Strauf, Katayun Barmak, Xiaoyang Zhu, and James Hone, ACS Nano 16, 140-147 (2022).

Topological states of matter combine quantum physics with topology—a branch of mathematics that explores geometric properties preserved under deformation. Quantum topology can lead to incredible properties. For instance, in a topological insulator, conducting edge states exist within an insulating bulk. Despite continuing progress, the search for such new quantum phases remains a central theme of condensed matter physics. In this talk, I will introduce two of the most sought-after quantum states—room-temperature topology and topological exciton insulator. I will first discuss our spectroscopic observation of topological edge states in Bi₄Br₄. I will show that the topological states, which typically can only be observed at temperatures around absolute zero, survive here at room temperature. I will also show how we probe the quantum transport response of this edge state using quantum interference. These observations mark the first steps in demonstrating the potential of topological materials for energy-saving applications. In the second part of my talk, I will discuss our discovery of a unique topological state in Ta₂Pd₃Te₅. Here, the Coulomb interactions pair fermions (electrons and holes) into bosons (excitons), leading to a superfluid condensate state in the bulk while hosting topological edge states on the boundary. Finally, I will touch upon how these discoveries suggest exciting possibilities. This includes new devices and experimental techniques to discover the fundamental physics of topological quantum matter, opening doors for more efficient room-temperature devices and quantum information technology. 

Rapid and efficient charge transfer following absorption of light is a process of intense interest from the
perspectives of both fundamental physics and optoelectronic applications. Among the exciting systems which
host charge transfer are photosynthetic reaction centers (RC), proteins packed with light-absorbing molecules
which yield a charge separated state with near unity quantum efficiency, and Transition Metal Dichalcogenide
Heterostructures (TMD HS), which host interlayer charge transfer to form spatially separated interlayer
excitons. Despite intense interest in understanding charge transfer in these systems, their complex electronic
structure and the ultrafast timescales of their dynamics have presented a significant challenge in clearly
resolving the underlying fundamental physics. Two-Dimensional Electronic Spectroscopy (2DES) is a nonlinear
optical spectroscopic technique with simultaneously high frequency and temporal resolution and is an ideal
tool for studying systems with complex electronic structure and femtosecond-timescale dynamics. In this talk I
will present on my work applying 2DES to resolving the excitonic structure of photosynthetic RCs as well as
resolving ultrafast interlayer exciton dynamics in TMD HS.