Condensed Matter Seminar
Seminar Time: Wednesdays, 10:2011:10 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 
TBA 
Adrian Del Maestro 
October 19 
Shenglong Xu 
TBA 
Ruixing Zhang 
October 26 
Alex Frano 
TBA 
Jian Liu 
November 2 
Hyungkook Choi 
TBA 
Joon Sue Lee 
November 9 
Yongtian Luo 
TBA 
Maxim Lavrentovich 
November 16 
Jiabin Yu 
TBA 
Ruixing Zhang 
November 30 
Hyunsoo Kim 
TBA 
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 humanreadable 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 onedimensional BoseHubbard Hamiltonian with up to L = N = 20, and the time needed to generate the groundstate 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 nontrivial Haldane model, we show that a quarterfilled state emerges with a nonzero 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 nontrivial case. We later generalize this study to quantum spin Hall system and obtain the quantum spin Hall Mott insulator.
Majorana Modes in IronBased Superconducting Vortex
Ironbased superconducting vortex is emerging as a promising platform for Majorana quasiparticle. After fouryear 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 secondphase research for manipulating Majorana zero modes in an ironbased 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 halffilled (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 longrange Coulomb interactions in these twodimensional triangular moiré lattices. Recent abinitio work on the Gammavalley transition metal dichalcogenide homobilayers unveiled effective moiré honeycomb lattices near the Fermi level. We employ largescale unrestricted HartreeFock 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 noncollinear and noncoplanar 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 ManyBody 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 hyperdimensional 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 NearTerm Quantum Computers: A Tabletop Experiment on Quantum Gravity
Intermediatescale 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 manybody 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 quasionedimensional 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)