Introduction to CAMM seminar and PHYS 599 https://tennessee.zoom.us/j/87862823041
Title: Machine Learning Force-Field Model for Dynamical Modeling of Correlated Electron Systems
Abstract: In this talk, I will present our recent efforts on using machine learning (ML) methods to enable multi-scale dynamical modeling of functional electron materials, and in particular correlated electron systems. I first discuss the ML force field models for large-scale dynamical simulations on two canonical examples of correlated electron systems: the double-exchange and the Falicov-Kimball models. The central idea is to develop deep-learning neural-network models that can efficiently and accurately predict generalized forces required for dynamical evolutions based on local environment. The large-scale simulations enabled by the ML method also reveal new phase-ordering dynamics in these correlated electron systems that is beyond conventional empirical theories. We will also discuss preliminary results of similar ML models for the more difficult Hubbard-type models. In the second part, generalization of the ML framework to represent nonconservative forces of out-of-equilibrium systems is discussed. In particular, we present a novel ML structure for modeling spin transfer torques that play a crucial role in spintronics.
Title: Strain-Induced Magnetism and Superconductivity in Single-Crystalline Heusler Membranes
Abstract: Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. I will describe how the growth of thin films on graphene-terminated substrates enables synthesis of single crystalline, mechanically exfoliatable membranes [1,2]. Using rippled membranes of the Heusler compound GdPtSb, we demonstrate the first experimental example of flexomagnetism, that is, ferro/ferri-magnetism induced by strain gradients . I will also describe evidence of superconductivity induced in another Heusler membrane via strain. More broadly, Heusler membranes provide highly tunable platform for tuning ferroic order, topological states, and correlations .
- S. Manzo, et. al., Nature Commun., 13, 4014 (2022). https://doi.org/10.1038/s41467-022-31610-y
- D. Du et. al., Nano Lett. 22, 21, 8647 (2022). https://doi.org/10.1021/acs.nanolett.2c03187
- D. Du, et. al., Nature Commun., 12, 2494 (2021). https://doi.org/10.1038/s41467-021-22784-y
- D. Du, e. al. APL, 122, 170501 (2023). https://doi.org/10.1063/5.0146553
Title: Scattering Studies of Lattice and Spin Dynamics
Abstract: Many scientific and technical challenges require materials with structural, electronic, magnetic, and transport properties tailored to specific applications. Knowledge of lattice and spin excitations in materials is critical to understanding these physical properties. Scattering tools are valuable for measuring the lattice and spin excitations in functional materials, including spintronic materials, thermoelectrics, negative thermal expansion materials, fast ion conductors, and low-dimension materials, under various environments. Exotic and novel physics often arises from the interplay between different degrees of freedom, such as phonon-phonon, electron-phonon, and magnon-phonon interactions. With the help of first-principles calculation, scattering simulation, and advanced data science tools, we use neutron and X-ray scattering to study these fundamental interactions to better understand existing materials and provide insights into the engineering of future materials.
Title: Emergent Classicality from Information Bottleneck
Abstract: Our universe is quantum, but our everyday experience is classical. Where is the boundary between quantum and classical worlds? How does classical reality emerge in quantum many-body systems? Does the collapse of the quantum states involve intelligence? These are fundamental questions that have puzzled physicists and philosophers for centuries. The recent development of quantum information science and artificial intelligence offers new opportunities to investigate these old problems. In this talk, we present our preliminary research on using a transformer-based language model to process randomized measurement data collected from Schrödinger's cat quantum state. We show that the classical reality emerges in the language model due to the information bottleneck: although our training data contains the full quantum information of Schrödinger's cat, a weak language model can only learn the classical reality of the cat from the data. Our study opens up a new avenue for using the big data generated on noisy intermediate-scale quantum (NISQ) devices to train generative models for representation learning of quantum operators, which might be a step toward our ultimate goal of creating an artificial intelligence quantum physicist.
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.
Title: Topological Charge Pumping with Ultracold Atomic Gases
Abstract: Clarifying the evolution of topological states of matter with interactions is a core goal of condensed matter theory, ranging from the stability of topological insulators to the emergent regime of topological quantum matter. Ultracold quantum gases in optical lattices provide unique opportunities to realize topological band structures, yet face challenges concerning the loading of the bulk and the stability against heating in the interacting case inherent to Floquet engineering scheme. Topological charge pumps are conceptionally related to integer quantum Hall effects via dimensional reduction, with a different physical manifestation via the quantized charge. Their realization, notably, does not require fast driving and relies on established experimental loading schemes and therefore, this system provides unique opportunities to investigate the effect of interactions. I will report on three main results. First, we investigated the stability of charge pumping in the bosonic systems. Second, we characterized the breakdown of quantized pumping due to the addition of quenched disorder. Finally, we turn to interacting fermions and show that interactions can change the pumped charge per cycle from the value obtained for noninteracting systems.
- A. Hayward, C. Schweizer, M. Lohse, M. Aidelsburger, and F. Heidrich- Meisner Phys. Rev. B 98, 245148 (2018)
- A. L. C. Hayward, E. Bertok, U. Schneider, and F. Heidrich-Meisner Phys. Rev. A 103, 043310 (2021)
- E. Bertok, F. Heidrich-Meisner, and A. A. Aligia Phys. Rev. B 106, 045141 (2022)
Title: Exploring Quantum Many-body Physics with Artificial Intelligence
Abstract: 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 quantum chemistry, condensed matter physics and high energy physics. With recent advancement of machine learning, new methods have been proposed to enhance quantum many-body physics simulation. In this talk, I will overview the recent development of the field and discuss some of our progress. First, I will present a new class of wave functions for fermionic simulation, neural network backflow, which can study strongly correlated physics in lattice and continuum models. Next, I will talk about gauge symmetric neural network for investigating continuum quantum field theories as well as lattice gauge theories, such as 2+1D quantum electrodynamics with finite density dynamical fermions. Finally, I will discuss neural network representation for simulating non-equilibrium quantum dynamics in the context of quantum circuit and open quantum systems.
Title: Bond-Stretching Electron-Phonon Interactions in BaBiO3: a Hybrid Monte Carlo Study
Abstract: The relationship between electron–phonon (e-ph) interactions and charge-density-wave (CDW) order in the bismuthate family of high-temperature superconductors remains unresolved. We address this question using nonperturbative hybrid Monte Carlo calculations for the parent compound BaBiO3. Our model includes the Bi 6s and O 2p orbitals and coupling to the Bi-O bond-stretching branch of optical phonons via modulations of the Bi-O hopping integral. We simulate three-dimensional clusters of up to 4000 orbitals, with input model parameters taken from ab initio electronic structure calculations and a phonon energy of 60meV. Our results demonstrate that the coupling to the bond-stretching modes is sufficient to reproduce the CDW transition in this system, despite a relatively small dimensionless coupling. We also find that the transition deviates from the weak-coupling Peierls' picture. This work demonstrates that off-diagonal e-ph interactions in orbital space are vital in establishing the bismuthate phase diagram.
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.
Title: Control of Buried Bilayer Graphene – SiC Interface by Metal Intercalation and Local Electric Field
Abstract: Heterostructures consisting of vertically stacked two-dimensional materials have recently gained large attention due to their highly tunable electronic properties. Particularly, controlling the interlayer couplings can generate novel electronic and topological phases and its effective implementation is commonly done with a transverse electric field. Here, I will focus on a model system of epitaxially grown bilayer graphene (BLG) on a SiC(0001) surface of unprecedented quality over wafer-scale lengths. In the first part of my talk I will present a novel concept for the exceedingly precise manipulation of a buried interface, which offers functional long-term stability and chemical protection. By the use of a local electric field from the scanning tunnelling microscope, I will show that the BLG on SiC(0001) expresses a ferroelectric-like switching behaviour . The manipulation is based on the control of the atomically sharp graphene-SiC interface, vertically located about ~1nm below the top graphene layer. It is related to a highly reversible formation, and breaking, of C-Si covalent bonds at the interface, which is enhanced by the inherent interface strain. The resulting charge separation is used to pull, or push, the interface atoms depending on the polarity of the tip-sample bias voltage. The reported effect allows us to pattern, the otherwise chemically inert, epitaxial graphene with remarkable lateral precision: reaching single unit cells of the interface moiré lattice (~1.8 nm).
In the second part of my talk, I will present how to globally manipulate this interface via intercalation of gadolinium. The two specific interlayer locations of the metal atoms cause a highly asymmetric doping of the top BLG structure, which is equivalent to implementation of an exceptionally large transverse electric displacement field [2,3]. As a result, the top two graphene layers exhibit a significant difference in the on-site potential energy (~1 eV), which effectively breaks the interlayer coupling between them. Consequently, for energies close to the corresponding Dirac points, the BLG system behaves like two electronically isolated single graphene layers.
The work presents novel strategies for local and global modification of epitaxial graphene heterostructures by structural engineering of the buried interfaces.
- M. Kolmer, J. Hall, S. Chen, Y. Han, M.C. Tringides "Atomic-scale manipulation of buried graphene – SiC interface by local electric field", 21 July 2023, PREPRINT available at Research Square [https://doi.org/10.21203/rs.3.rs-3136595/v1]
- M. Kolmer, B. Schrunk, M. Hupalo, J. Hall, S. Chen, J. Zhang, C.-Z. Wang, A. Kaminski, M.C. Tringides, "Highly Asymmetric Graphene Layer Doping and Band Structure Manipulation in Rare Earth–Graphene Heterostructure by Targeted Bonding of the Intercalated Gadolinium", J. Phys. Chem. C. 126, 6863 (2022). https://doi.org/10.1021/acs.jpcc.2c01332.
- M. Kolmer, W. Ko, J. Hall, S. Chen, J. Zhang, H. Zhao, L. Ke, C.-Z. Wang, A.-P. Li, M.C. Tringides, "Breaking of Inversion Symmetry and Interlayer Electronic Coupling in Bilayer Graphene Heterostructure by Structural Implementation of High Electric Displacement Fields", J. Phys. Chem. Lett. 13, 11571 (2022). https://doi.org/10.1021/acs.jpclett.2c02407.
Title: Tuning Electronic Phase Transitions in Layered Quantum Materials
Abstract: The multifunctional behaviors of quantum materials are driven by the intricate interplay between charge, orbital and spin degrees of freedom. The quest to control the interplay of multiple degrees of freedom is indispensable in condensed matter physics, as it enables us to harness the multifunctionality of quantum devices on demand. In this talk, I will present how to employ an ionic gate to control the emerging physical phenomena in quantum devices, and how it offers new insights to the underlying physics. First, I will talk about the observation of a spontaneous anomalous Hall effect, a trademark of time-reversal symmetry breaking, in a layered polar semiconductor. Remarkably, the magnitude of anomalous Hall conductivity can be enhanced by tuning the carrier density, which sheds new light on the interplay of magnetic and ferroelectric-like responses . Then, I will present some recent results on the electric field control at the interfaces of ionic gated tellurium thin flake devices. The Rashba spin-orbit coupling coefficient is demonstrated to increase fourfold.
- S. Kim et al. arXiv:2307.03541
Title: New Quantum Algorithms for Old Challenges: from Quantum Simulation to Quantum Error Correction
Abstract: Harnessing the power of quantum computers and systems to solve important problems beyond the capability of classical computers is an outstanding challenge. In this talk, I will present our recent efforts on developing novel quantum algorithms to address longstanding challenges in quantum simulation, quantum error correction, and quantum chemistry. These progresses are made possible by leveraging the quantum embedding framework as well as a unified understanding for modern quantum algorithms via quantum signal processing. In addition to discrete-variable systems such as qubits, in the second part, I will present control protocols that exploit continuous-variable bosonic modes to perform universal qudit-based quantum computation. I will conclude the talk with a discussion on the prospects of using hybrid discrete-continuous-variable quantum systems for computation and information processing.
Title: Dynamics and Fusion of Majorana Zero Modes in Quantum Dot-based Interacting Kitaev Chains
Abstract: Majorana zero modes (MZMs) have generated considerable interest because of potential quantum information and quantum computation applications. Motivated by the recent realization of a minimal Kitaev chain in quantum dot systems , I will describe our computational results about the dynamics and fusion of Majorana zero-modes at or near the "sweet spot" t=Δ, where the fermionic hopping t and superconducting order parameter Δ are equal. We analyzed the dynamics and fusion of MZMs using recently developed time-dependent real-space local density-of-states methods . The movement of Majoranas and the detection of fusion channels are important for quantum information processing. I will also discuss our recent findings of exotic "multi-site" MZMs in Y-shape Kitaev wires, which is important for the potential braiding of Majoranas and for the performance of Y-junctions made from arrays of quantum dots . Finally, I will present results for "non-trivial" fusion in a 1D chains, i.e. mixing sectors of different parity, and results in Y-shape Kitaev wires at the sweet spot.
- Dvir, T., Wang, G., van Loo, N. et al. Realization of a minimal chain in coupled quantum dots. Nature 614, 445450 (2023).
- Pandey, B., Mohanta, N., Dagotto, E. Out-of-equilibrium Majorana zero modes in interacting Kitaev chains. Phys. Rev. B107, L060304 (2023).
- Pandey, B., Kaushal, N., Dagotto, E. Majorana zero modes in Y-shape interacting Kitaev wires. arXiv:2306.04081 (2023).