Condensed Matter Seminar
Seminar Time: Wednesdays, 10:20-11:10 AM
Location: IAMM 147 unless noted as Virtual
Zoom for Virtual Seminars: https://tennessee.zoom.us/j/95144505179
| Date | Speaker | Title | Host |
August 24 |
Seminar Introduction |
|
|
August 31 |
Hatem Barghathi |
Ruixing Zhang |
|
September 7 |
Peizhi Mai |
Steve Johnston |
|
September 14 |
Lingyuan Kong |
Ruixing Zhang |
|
September 21 |
Nitin Kaushal |
Elbio Dagotto |
|
September 28 |
Rubem Mondaini |
Steve Johnston |
|
October 5 |
Pouyan Ghaemi |
Ruixing Zhang |
|
October 12 |
Brenda Rubenstein |
Adrian Del Maestro |
|
October 19 |
Shenglong Xu |
Rainbows in 2D Quantum XY Model: Long-Range Entanglement and Many-Body Teleportation |
Ruixing Zhang |
October 26 |
Alex Frano |
New insights into the 2- and 3-Dimensional Structure of Charge Order in Superconducting Cuprates |
Jian Liu |
November 2 |
Hyungkook Choi |
Joon Sue Lee |
|
November 9 |
Yongtian Luo |
Maxim Lavrentovich |
|
November 16 |
Jiabin Yu |
Ruixing Zhang |
|
November 30 |
Hyunsoo Kim |
Joon Sue Lee |
Balls and Walls: A Compact Unary Coding for Bosonic States
Discrete lattice models play an essential role in the understanding of quantum phenomena, but their exact numerical solution is hindered by the exponentially growing size of the underlying Hilbert space. Such difficulty is more pronounced in the case of bosons due to the lack of any occupation restrictions as opposed to fermionic or even spin models. To ease some of the difficulties, we introduce a unary coding of bosonic occupation states based on the famous balls and walls counting for the number of configurations of N indistinguishable particles on L distinguishable sites. Each state is represented by an integer with a human-readable bit string that has a compositional structure allowing for the efficient application of operators that locally modify the number of bosons. By exploiting translational and inversion symmetries, we identify a speedup factor of order L over current methods when generating the basis states of bosonic lattice models. The unary coding is applied to a one-dimensional Bose-Hubbard Hamiltonian with up to L = N = 20, and the time needed to generate the ground-state block is reduced to a fraction of the diagonalization time. A widely adopted approximation in exact diagonalization as well as in the Density Matrix Renormalization Group is to restrict the bosonic occupation numbers to only a few bosons per lattice site. While the relative errors under this approximation in many observables including the energy or local particle number fluctuations could be negligible, we report that imposing such restrictions could have drastic effects on quantum information measures such as particle and accessible (symmetry resolved) entanglement entropies. Specifically, we show that for the ground state symmetry resolved entanglement, variational approaches restricting the local bosonic Hilbert space could result in an error that scales with system size.
Topological Mott insulator
While the recent advances in topology have led to a classification scheme for electronic bands described by the standard theory of metals, a similar scheme has not emerged for strongly correlated systems such as Mott insulators in which a partially filled band carries no current. By including interactions in the topologically non-trivial Haldane model, we show that a quarter-filled state emerges with a non-zero Chern number provided the interactions are sufficiently large. We establish this result first analytically by solving exactly a model in which interactions are local in momentum space. The exact same results obtain also for the Hubbard interaction, lending credence to the claim that both interactions lie in the same universality class. From the simulations with determinantal quantum Monte Carlo, we find that the spin structure at quarter filling is ferromagnetic for the topologically non-trivial case. We later generalize this study to quantum spin Hall system and obtain the quantum spin Hall Mott insulator.
Majorana Modes in Iron-Based Superconducting Vortex
Iron-based superconducting vortex is emerging as a promising platform for Majorana quasiparticle. After four-year intensive studies, substantial achievements have been made on several compounds, including Fe(Te,Se), (Li,Fe)OHFeSe, CaKFe4As4and LiFeAs. For example, the discovery of integer series of quantized bound states manifests nontrivial topology of vortex zero mode, promises a hope to the field of Majorana research. In this talk, I will depict the main profile of this emerging Majorana platform, on the aspects of materials, bound states, experimental configurations, and etc. Its advantages on physics study and the major controversies owing to the practical diversity will be discussed in short. The systematic investigations accomplished on this platform is promoting the entrance to a second-phase research for manipulating Majorana zero modes in an iron-based superconductor device.
Reference:
- Kong & Ding. arXiv: 2108.12850 (2021) [Review]
- Wang et al. Science 360, 182 (2018)
- Kong et al. Nat. Phys. 15, 1181 (2019)
- Kong et al. Nat. Commun. 12, 4146 (2021)
- Liu et al. arXiv: 2111.03786 (2021)
Magnetic Ground States of Honeycomb Lattice Wigner Crystals
In recent years, moiré materials constructed using two layers of transition metal dichalcogenides have been used to simulate the Hubbard model on triangular lattice procuring strongly correlated physics in half-filled (n=1) flat bands. Lattice Wigner crystal states, at other fractional fillings like n=2/3, 1/2, and 1/3, are also stabilized by long-range Coulomb interactions in these two-dimensional triangular moiré lattices. Recent ab-initio work on the Gamma-valley transition metal dichalcogenide homobilayers unveiled effective moiré honeycomb lattices near the Fermi level. We employ large-scale unrestricted Hartree-Fock techniques to unveil the magnetic phase diagrams of honeycomb lattice Wigner crystals. For the three lattice filling factors with the largest charge gaps, n = 2/3, 1/2, 1/3, the magnetic phase diagrams contain multiple phases, including ones with non-collinear and non-coplanar spin arrangements. We discuss magnetization evolution with the external magnetic field, which has potential as an experimental signature of these exotic spin states. Our theoretical results could potentially be validated in moiré materials formed from group VI transition metal dichalcogenide twisted homobilayers.
Overcoming Exponential Walls in Quantum Many-Body Systems
In this talk I plan to provide an overview of how limitations inherent to interacting quantum systems present as a roadblock to their study in classical computers. Nonetheless, ingenious methods can be used such that useful information can yet be obtained in the face of these impediments. In particular, I will discuss the emergence of the sign problem in quantum Monte Carlo simulations, and argue that besides being a mere nuisance, one can use it as a tool to investigate quantum criticality. Furthermore, I will show that using metrics associated to the Monte Carlo sampling, including the mean distance traveled in the hyper-dimensional space of configurations, constitute as powerful means to diagnostic the onset of ordered behavior.
Quantum Algorithm to Realize and Study Fractional Hall States and Their Dynamics on Near-Term Quantum Computers: A Tabletop Experiment on Quantum Gravity
Intermediate-scale quantum technologies provide unprecedented opportunities for scientific discoveries while posing the challenge of identifying important problems that can take advantage of them through algorithmic innovations. Fractional Hall systems which are one class of correlated electron systems with many interesting and puzzling properties. In this talk I present an efficient quantum algorithm to generate an equivalent many-body state to Laughlin's ν=1/3 fractional quantum Hall state on a digitized quantum computer. Our algorithm only uses quantum gates acting on neighboring qubits in a quasi-one-dimensional setting, and its circuit depth is linear in the number of qubits. I then present another quantum algorithm to generate and study out of equilibrium properties of fractional Hall state. Such features reveals novel geometric aspects of fractional Hall states which mimics gravitons.
References:
[1] PRX Quantum 1, 020309 (2020)
[2] Phys. Rev. Lett. 129, 056801 (2022)
Stochastic Electronic Structure Beyond Energies: Temperature, Magnetism, and Entanglement in Auxiliary Field Quantum Monte Carlo
Most many body methods for solving the Schrodinger Equation - perturbation theory, coupled cluster theory, Green’s function theories, etc. - are deterministic in nature. While deterministic methods can be highly accurate, many scale steeply with system size. In this talk, I will discuss a suite of new quantum Monte Carlo methods, Auxiliary Field Quantum Monte Carlo methods, that my group has recently developed. These techniques leverage stochasticity - randomness - to solve a variety of ground state and finite temperature problems in quantum mechanics difficult to approach using deterministic techniques. In particular, I will highlight our recent efforts to understand thermal matter, study magnetic 2D materials, and quantify entanglement. Our algorithms will ultimately enable the study of materials with chemical accuracy at a cost nearing that of Density Functional Theory.
Rainbows in 2D Quantum XY Model: Long-Range Entanglement and Many-Body Teleportation
Long-range entanglement is a valuable resource for quantum information applications. In this talk, I will present a simple protocol to engineer long-range entangled states, the rainbow states, in the quantum XY model by iterative measurements on two local qubits with feedback control. The long-range entangled rainbow state appears as the unique steady state of the quantum operations. I will show that the rainbow state can be used for quantum many-body teleportation, in which a single qubit state is teleported through the strongly-interacting quantum system as a result of thermalizing unitary dynamics and local measurements on a few qubits. The state engineering and teleportation protocols rely on a newly identified special scarred eigenstate in the XY model.
New insights into the 2- and 3-Dimensional Structure of Charge Order in Superconducting Cuprates
The superconducting copper oxides might appear interesting because of high-temperature superconductivity, but their real beauty is in the complex quantum phase space that a small set of interactions yields. Understanding these phases and their connection is among the key intellectual challenges of our time. In this talk, we discuss recent developments in our understanding of charge order and its connection with superconductivity. First, we survey some of our latest results pertaining to the in-plane structure of the charge order fluctuations, which is more intriguing than originally thought. Second, we will show a new way to produce 3-dimensional charge order by engineering the orbital character of the CuO2 wavefunction, which can offer a new way of studying the validity of single-band models used to describe 2-dimensional systems.
Mesoscopic Physics Research with Electronic Interferometry
The realization of electronic interferometers in the quantum Hall effect (QHE) regime opened a whole new field of quantum-optics like experiments, with electrons. This is thanks to the formation of current-carrying 1D chiral edge channels in which electrons propagate ballistically and coherence lengths up to several tens of microns. This has allowed us to observe interference of the electrons, setting the QHE as an advantageous playground for exploring fundamental quantum phenomena, such as coherence, entanglement and complementarity etc. In particular, much interests were drawn to interferometry in the fractional quantum Hall effect (FQHE) regime as they were proposed as most promising platforms to probe anyonic statistics of quasi-particles. In this talk, I will briefly review the basics of electronic interferometry in the quantum Hall regime and then discuss related research topics.
Pattern Formation and Deformations of Biophysical Systems at Small and Large Scales: from Lipid Bilayers to the Effect of Biological Flow Networks
Pattern formations are ubiquitous in soft matter and biological systems, arising from various physical mechanisms and often resulting in shape transformations. In this talk, I will present the theoretical and computational studies of surface patterns at different length scales, from the lateral phases and undulations of a lipid-bilayer vesicle (whose size is close to a single cell), to the shape morphing of a macroscopic thin sheet with embedded flow networks such as plant leaf or petal. Using continuum modeling, I studied the equilibrium phase patterns of a multicomponent lipid vesicle, including phase separation and modulated stripes. These phase phenomena are affected by the vesicular spherical geometry and finite size, related to phase behaviors of planar membrane through structure factor, and are shown to emerge from the composition-curvature coupling of a deformable surface. Based on a network model of flow transport system with fluid-storage capacitance that we recently developed, I simulated the differential swelling and buckling processes of a thin sheet controlled by flow networks, exploring the effects of venation architecture and hierarchies on the large-scale deformation dynamics by using a minimal model. These shape patterns reveal the underlying fluid distribution and mechanical changes, and may offer clues to the design and manufacture of thin materials whose shapes can be controlled by adjusting internal hydraulics.
Topological Lower Bound of Electron-Phonon Coupling
Electron-phonon coupling (EPC) is crucial for various quantum phases, especially superconductivity. Previous studies of the EPC did not reveal the influence of the electron band topology on the bulk EPC strength. We find that the electron band topology bounds the dimensionless EPC constant from below in graphene and MgB2. In particular, roughly half of the EPC strength in graphene comes from the electron band topology. Our results suggest that topologically nontrivial materials might be good candidates for large EPC.
Uncommon Unconventional Superconductivity
Since the surprising discovery of unconventional superconductivity in heavy fermion compound CeCu2Si2, the symmetry of pairing interaction has been considered the key measure of unconventional-ness. Whereas s-wave pairing interaction prevails in most superconductors, the unconventional p-wave and d-wave pairings are proposed in various superconductors. In this talk, I will introduce uncommon unconventional superconductors where detail band structure plays a central role. Measurements of the London penetration depth provide insight into the structure of the superconducting energy gap that shares the same symmetry of the pairing interaction. I will present unexpected temperature variations of the penetration depth in Yb-doped CeCoIn5 and YPtBi, which can be resolved by adopting concepts of composite pairing and high-spin superconductivity, respectively.