# CAMM & Condensed Matter Seminar

Seminar Time: Wednesdays, 10:20-11:10 AM

Location: IAMM 310 or 147

**Location:** IAMM 310

**Title:** Thermal Anyon Interferometry in Phonon-Coupled Kitaev Spin Liquids

**Abstract:** Recent theoretical studies inspired by experiments on the Kitaev magnet α-RuCl3 highlight the nontrivial impact of phonons on the thermal Hall conductivity of chiral topological phases. We introduce mixed mesoscopic-macroscopic devices that allow refined thermal-transport probes of non-Abelian spin liquids with Ising topological order. These devices feature a quantum-coherent region with quantized or negligible phonon conductance, flanked by macroscopic lobes that facilitate efficient thermalization between chiral Majorana edge modes and bulk phonons. We show that our devices enable (i) accurate determination of the quantized thermal Hall conductivity, (ii) identification of non-Abelian Ising anyons via the temperature dependence of the thermal conductance, and, most interestingly, (iii) single-anyon detection through heat-based anyon interferometry. Analogous results apply broadly to phonon-coupled chiral topological orders.

**Location:** IAMM 310

**Title:** Mechanism and Properties of Charge-4e Superconductivity

**Abstract:** Unlike in the BCS theory for regular (charge-2e) superconductivity, charge-4e superconductivity, driven by the condensation of four-fermion bound states, does not occur via a weak-coupling instability. Moreover, unlike charge-2e superconductors, a charge-4e superconductor is intrinsically interacting even at mean-field level, whose properties remain to be properly analyzed. In this talk, we first present a microscopic mechanism for charge-4e superconductivity. In a model with repulsive BCS coupling, the system exhibits strong fluctuations toward electron pairing with finite momenta, known as pair density waves (PDW). Upon lowering temperature, we show that the ground state is a spatially uniform charge-4e state formed by condensing pairs of the PDW bosons with d-wave pairing symmetry. In the second part, we present a solvable model describing a mean-field charge-4e superconductivity, and obtain its key properties such as gaplessness, superfluid density, and stability.

**Location:** Virtual seminar (https://tennessee.zoom.us/j/81386068237)

**Title:** Solving 2D Quantum Matter with Neural Quantum States

**Abstract:** Accessing theoretically the ground state of interacting quantum matter has remained a notorious challenge especially for complex two-dimensional systems. Recent developments have highlighted the potential of neural quantum states to solve the quantum many-body problem by encoding the quantum many-body wave function into artificial neural networks. So far, however, this method faces the critical limitation that the training of modern large-scale deep network architectures has not yet been possible, thereby failing to capitalize on the full power of artificial neural networks. Here, we introduce an optimization algorithm for neural quantum states, which allows to train unparalleled deep artificial neural networks yielding unprecedented accuracies for the ground states of large complex two-dimensional quantum spin models. We demonstrate the power of the formulated minimum-step stochastic reconfiguration (MinSR) method for the paradigmatic spin-1/2 Heisenberg J_1-J_2 models on the square lattice, yielding significantly better variational energies as compared to existing numerical results approaching different levels of machine precision on modern GPU and TPU hardware. We expect that the MinSR method provides the tool to solve the quantum many-body problem by means of deep neural quantum states on a new level with potential applications not only in quantum many-body physics but also condensed matter and quantum chemistry.

**Location:** IAMM 310

**Title:** A Pathway to Accelerate Physical and Material Discoveries through Aligned Autonomous Experiment Framework via Single, Multi-objective and Multi-fidelity Bayesian Optimization

**Abstract:** Material discoveries for improved societal, environmental, safety device applications etc. often require optimization of expensive experimental process for materials synthesis, characterization, and learning structure-property relationships over large parameter and function spaces where the exhaustive grid or random search are too data intensive. This resulted in strong interest towards active learning methods such as Bayesian optimization (BO) where the adaptive exploration occurs based on human learning (discovery) objective. However, classical BO are purely data-driven and does not guarantee to always provide the outcomes or experiments aligned to the domain scientist intended learning objectives. Whereas in the practical setting, the domain expert either poses partial prior knowledge of the systems or gain knowledge with on-the-fly continual learning from the experiments.

Therefore, I will showcase an approach of a human intervened (human in the loop) Bayesian optimization to improve AI-alignment in autonomous experiments through combined data, existing and real time domain knowledge driven approach. In this interactive BO, the human high-level decisions with minor intervention allow to better steer the experiments, and the ML policy low-level decisions allow to accelerate the human guided search. The approach is demonstrated via 1) autonomous material characterization in Scanning Probe Microscope for structure-(multi) property relationship learning, and 2) autonomous exploration in a multi-fidelity Ising model over ferromagnetic domain to learn phase transition. The proposed human in the loop BO highlights the best of both human operators and AI-driven experiments with appropriate alignment towards meaningful scientific discoveries in material research, with current challenges in algorithm development and potential opportunities to extend to considerably more complex discovery problems

**Location:**TBA

**Title:** Theory of Giant Phonon Magnetic Moment in Doped Dirac Semimetals

**Abstract:** Generally, the phonon magnetic moment is believed to be small compared to the electron's magnetic moment due to the large ion mass. However, several recent experiments have observed large phonon magnetic moments that are comparable to the electron magnetic moment in many materials, including band insulators, topological insulators, and Dirac semimetals. In this talk, I will present a new theoretical framework to calculate the phonon magnetic moment in doped Dirac semimetals, motivated by the recent experimental observation of giant phonon magnetic moment in these materials. Our theory is based on the emergent gauge approach, which can handle both insulators and semimetals, unlike previous theories that are only applicable to insulators. I will show that the phonon magnetic moment is linked to the Hall conductivity through the phonon Hall viscosity. I will apply our theory to 2D and 3D Dirac semimetals, such as graphene and Cd3As2, and obtain large phonon magnetic moments of the order of the electron Bohr magneton using electron-phonon couplings from first-principle calculations. I will also discuss the conditions for employing the emergent gauge approach. Our theory not only explains the recent experimental findings but also provides practical guidance for the dynamic generation of large magnetization in materials.

**Location:** IAMM 310 at **2 PM** (**Note this is different from the usual seminar time.**)

**Title:** Spectral Topology, Skin Effect and Impurity States in Non-Hermitian Bands

**Abstract:** The dynamics of a system having energy sources or drains can sometimes be approximated using a non-Hermitian Hamiltonian. The eigenvalue spectrum of a non-Hermitian lattice also fall into bands, not along the real axis, but on the complex plane. "Spectral topology" characterizes the shape of the shape of the spectrum on the complex plane, and is hence unique to non-Hermitian bands. Nontrivial spectral topology leads to physical phenomena such as the skin effect, the anomalous reflection, and characteristic features in impurity bound states. In this talk, I will introduce recent progress on the study of spectral topology in non-Hermitian bands.

Reference: [1] K. Zhang, Z. Yang, and C. Fang, Phys. Rev. Lett. 125, 126402 (2020)

[2] K. Zhang, Z. Yang*, and C. Fang, Nature Communications 13, 2496 (2022)

[3] K. Zhang, C. Fang*, and Z. Yang, Phys. Rev. Lett. 131, 036402 (2023)

[4] Zixi Fang, C. Fang*, and K. Zhang, Phys. Rev. B 108, 165132 (2023)

**Location:** IAMM 310

**Title:** Many-Body Edge Modes from Density-Dependent Hopping in 1D Bosonic Systems

**Abstract:** Increased control of quantum systems has led to many advancements in simulating and understanding many-body physics. An important ingredient for such simulations is controllable density-dependent hopping. Density-dependent hopping lies at the heart of several many-body phenomena, including fractionalization and formation of anyons. In this talk, I will discuss results on a chain of bosons experiencing a modified hopping proportional to the density difference between neighboring sites. Starting with few particle physics, I will demonstrate the existence of an SSH-like edge mode for two particles within the bulk spectrum that can best be understood as the results of an emergent chiral symmetry that is found explicitly when one introduces non-Hermiticity. As such, we see an example of how gauge fields lead to emergent topology. Moving towards many-body physics, we investigate localized modes appearing a low and high energies suggesting that the density-dependent hopping of this model can be understood as an interaction whose behavior is neither attractive nor repulsive a priori, but depends on the local density difference leading to the unusual feature of high and low energy localized states in the many-body regime.

**Location:** IAMM 310

**Title:** Topology, Spin, and Orbital in DNA-type Chiral Quantum Materials

**Abstract:** In chemistry and biochemistry, chirality represents the structural asymmetry characterized by non-superimposable mirror images for a material like DNA. In physics, however, chirality commonly refers to the spin-momentum locking of a particle or quasiparticle in the momentum space. While seemingly unrelated characters in different fields, the structural chirality leads to the electronic chirality featured by the orbital-momentum locking encoded in the wavefunction of chiral molecules or solids, i.e. the chirality information transfers from the atomic geometry to the electronic orbital. The electronic chirality provides deep insights into the chirality-induced spin selectivity (CISS), in which electrons exhibit salient spin polarization after going through a chiral material. It also gives rise to new phenomena in unusual light-matter interaction. These chirality-driven effects will generate broad impacts in fundamental science and technology applications in spintronics, optoelectronics, and biochemistry.

References: [1] Y Liu, J Xiao, J Koo, B Yan, Chirality-driven topological electronic structure of DNA-like materials, Nature Materials 20 (5), 638 (2021).

[2] Y. Adhikari, et al, Interplay of Structure Chirality, Electron Spin and Topological Orbital in Chiral Molecular Spin Valves, Nature Comm, 14, 5163 (2023).

[3] L. Wan, Y. Liu, M.J. Fuchter, B. Yan, Anomalous circularly polarized light emission in organic light-emitting diodes caused by orbital–momentum locking, Nature Photonics 17, 193 (2023).

[4] B Yan, Structural Chirality and Electronic Chirality in Quantum Materials (Review),arXiv:2312.03902

**Location:** IAMM 310

**Title:** Electronic Structure, Magnetic Interactions, and Charge Order in Low Valence Nickelates Probed by Resonant Inelastic X-ray Scattering

**Abstract:** After a 30-year quest, researchers recently succeeded in realizing superconductivity in low valence nickelates [1]. This ignited a vigorous debate regarding the essential electronic properties of these materials and their similarity to cuprates. However, challenges in preparing these materials have made them difficult to probe with many types of traditional spectroscopy. In this talk, I will describe how the flexibility of resonant inelastic x-ray scattering (RIXS) opens up important opportunities for probing bulk spin, charge, and orbital properties of small samples. We show how RIXS can unveil the strength of the magnetic interactions [2], the electronic character of the charge carriers [3], the states involved in charge order [4], and the presence of plasmon excitations [5] in the low valence nickelates.

References: [1] Danfeng Li, Kyuho Lee, Bai Yang Wang, Motoki Osada, Samuel Crossley, Hye Ryoung Lee, Yi Cui, Yasuyuki Hikita and Harold Y. Hwang, Nature 572, 624–627 (2019)

[2] Strong Superexchange in a d9-δ Nickelate Revealed by Resonant Inelastic X-Ray Scattering, J. Q. Lin, P. Villar Arribi, G. Fabbris, A. S. Botana, D. Meyers, H. Miao, Y. Shen, D. G. Mazzone, J. Feng, S. G. Chiuzbăian, A. Nag, A. C. Walters, M. Garcı́a-Fernández, Ke-Jin Zhou, J. Pelliciari, I. Jarrige, J. W. Freeland, Junjie Zhang, J. F. Mitchell, V. Bisogni, X. Liu, M. R. Norman, and M. P. M. Dean, Phys. Rev. Lett. 126, 087001 (2021)

[3] Role of Oxygen States in the Low Valence Nickelate La4Ni3O8, Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, I. Jarrige, Xi He, I. Bozovic, M. Mitrano, Junjie Zhang, J. F. Mitchell, A. S. Botana, V. Bisogni, M. R. Norman, S. Johnston, and M. P. M. Dean, Phys. Rev. X 12, 011055 (2022)

[4] Electronic character of charge order in square planar low valence nickelates, Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, M. Mitrano, W. He, Junjie Zhang, J. F. Mitchell, V. Bisogni, M. R. Norman, S. Johnston, and M. P. M. Dean, Phys. Rev. X 13, 011021 (2023)

[5] Y. Shen et al., in preparation (2024)

**Location:** IAMM 310 (in-person)

**Title:** High Throughput X-ray Imaging and Phase Retrieval

**Abstract:** X ray scattering, diffraction, coherent imaging, and related phase retrieval inverse problems are the main methods to solve protein structures using x-ray crystallography for drug discovery, to obtain the fastest images ever recorded at sub-optical resolution, or to achieve unprecedented resolutions with chemical specificity of microchip circuits, biological cells, nanomaterials for energy storage, photovoltaics, geological samples, or stardust particles brought back from space. Diffraction based x-ray instruments enable nanometer resolution imaging with chemical and magnetic contrast over large fields of view or volumes. I will present an overview of the experimental schemes, phase retrieval algorithms, computational methods developed for high through-put imaging applications using state of the art x-ray sources, detectors, and computing facilities

**Location: **IAMM 310

**Title:** Quest for Novel Quantum Materials and Devices

**Abstract:** Quantum materials host exotic states of matter with unique macroscopic phenomena, ranging from various correlated electron states to topological orders. The ability to create and manipulate their emergent properties with nanoscale precision is at the forefront of condensed matter research and underlies the future progress of new electronic and photonic technologies. In particular, 2D van der Waals (vdW) materials combined with complex transition-metal oxides exhibiting strong electron correlations open up exciting opportunities for designing new functional properties at their interface. In this talk, I will discuss a robust strategy to design novel photonic device platforms by integrating oxides into 2D materials using the notion of oxidation-activated charge transfer. Taking graphene as a model 2D system, I will describe applications of this strategy in controlling the propagation of polaritons—hybrid light-matter excitations with extreme light confinement—and in implementing low-loss nanostructured optical elements. I will further discuss future prospects of 2D/oxide heterostructures in creating new interfacial phenomena with potential next-generation device applications.

**Location:** IAMM 310

**Title:** Kinetic Magnetism in a Frustrated Fermi-Hubbard System

**Abstract:** Nagaoka famously proved that introducing a single itinerant charge to the half-filled Fermi-Hubbard model can transform a paramagnetic insulator into a ferromagnet through path interference. Such kinetic magnetism has recently been realized with strongly interacting fermions in a triangular optical lattice [1,2]. In this talk, I will give a quick summary of the experimental findings, including the emergence of Nagaoka polarons as extended ferromagnetic bubbles around particle dopants, and present theory results based on simulations of the model using the numerical linked-cluster expansions in support of these observations on both the square and triangular lattices. Time permitting, I will also discuss how ground state wavefunctions of quantum lattice models, including that for the Nagaoka phenomenon, may be represented and studied by recurrent neural networks.

[1] Lebrat et al., arXiv:2308.12269

[2] Prichard et al., arXiv:2308.12951

**Location: **IAMM 310

**Title:** Opportunities and Challenges of Bosonic Oscillators for Quantum Computation and Information Processing

**Abstract:**Quantum harmonic oscillators (bosonic modes) are promising quantum resources, owing to the infinite number of available states and their ubiquitous presence in nature. In this talk, I will present recent theoretical developments on leveraging bosonic oscillators for quantum computation and information processing, highlighting their unique role for both NISQ and fault-tolerant applications. In the first part, I will discuss generalization of quantum algorithmic primitives such as quantum signal processing and linear combination of unitaries from the discrete-variable (DV) to continuous-variable (CV) domain, and showcase their applications in quantum sensing, quantum Fourier transform, and Hamiltonian simulation. Building upon these, in the second part, I will present and analyze instruction set architectures for hybrid quantum processors consisting of both CV bosonic modes as well as more traditional DV qubits. I will conclude the talk with challenges and prospects of using hybrid CV-DV quantum systems to tackle problems across physical sciences and engineering.

**Location:** IAMM 310

**Title:** Machine Learning on Quantum Materials for Scattering and Spectroscopies

**Abstract:** Even with the rapid process, machine learning for quantum phenomena may still be at its infancy due to the data scarcity and out-of-distribution problem. In this seminar, we introduce our recent effort in applying and designing machine learning architectures for quantum phenomena. We start by introducing the detection of Majorana zero mode (MZM) from scanning tunneling spectroscopy, that how machine learning can distinguish the MZM from spurious signals. Then we introduce a generic way to boost graph neural networks termed virtual nodes, which enables a prediction of phonon bandstructures with comparable accuracy with MLIP but orders of magnitude faster. Finally, we will highlight our most recent effort that to generate potential magnetic materials on constrained lattices with certain geometry such as triangular, hexagonal and kagome lattices, based on designing a variation of division model. Millions of materials on these lattices are generated and high throughout DFT shows a significant portions are stable, at least to DFT level. The work may support the search of frustrated magnetic materials such as quantum spin liquids.