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 
Adrian Del Maestro 

October 19 
Shenglong Xu 
Rainbows in 2D Quantum XY Model: LongRange Entanglement and ManyBody Teleportation 
Ruixing Zhang 
October 26 
Alex Frano 
New insights into the 2 and 3Dimensional 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 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)
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: LongRange Entanglement and ManyBody Teleportation
Longrange entanglement is a valuable resource for quantum information applications. In this talk, I will present a simple protocol to engineer longrange entangled states, the rainbow states, in the quantum XY model by iterative measurements on two local qubits with feedback control. The longrange 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 manybody teleportation, in which a single qubit state is teleported through the stronglyinteracting 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 3Dimensional Structure of Charge Order in Superconducting Cuprates
The superconducting copper oxides might appear interesting because of hightemperature 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 inplane structure of the charge order fluctuations, which is more intriguing than originally thought. Second, we will show a new way to produce 3dimensional charge order by engineering the orbital character of the CuO2 wavefunction, which can offer a new way of studying the validity of singleband models used to describe 2dimensional 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 quantumoptics like experiments, with electrons. This is thanks to the formation of currentcarrying 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 quasiparticles. 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 lipidbilayer 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 compositioncurvature coupling of a deformable surface. Based on a network model of flow transport system with fluidstorage 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 largescale 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 ElectronPhonon Coupling
Electronphonon 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 CeCu_{2}Si_{2}, the symmetry of pairing interaction has been considered the key measure of unconventionalness. Whereas swave pairing interaction prevails in most superconductors, the unconventional pwave and dwave 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 Ybdoped CeCoIn_{5} and YPtBi, which can be resolved by adopting concepts of composite pairing and highspin superconductivity, respectively.