CMTC Talks
CMTC talks are held in room 4402 Atlantic Building unless an alternate location is indicated
For all Zoom seminars email emartin3@umd.edu for details
Beginning Fall 2022, our seminar series will be renamed CMTC JLDS
Seminars and Symposia, to honor the late Janet Lynn Das Sarma, who dedicated
countless hours to the organization and administrative support of CMTC seminars
throughout her 18 year career with the CMTC. Our seminar series would not have been
possible without her tireless work.

Spring 2023

February 14, Tuesday, 11am
Ruixing Zhang (University of Tennessee), ATL 4402
Title: Vortex Majorana modes in trivial and topological superconductors
Abstract: In this talk, I will describe two distinct strategies for trapping Majorana zero modes (MZMs) with superconducting vortices. There exists a common belief that for s-wave superconductors, the existence of normal-state band topology, such as the topological Dirac surface state, is crucial for inducing vortex MZMs. We recently uncovered a striking example where nontrivial vortex Majorana physics arises in a trivial s-wave superconductor with a trivial normal state. This further leads to a prediction of highly unusual vortex topological behaviors in superconducting HgTe-class materials. For the second part, I will discuss new possibilities of vortex MZMs in topological superconductors (TSCs). Despite the well-known Read-Green paradigm of chiral TSCs, there have been surprisingly few research efforts to explore vortex response in non-chiral TSC systems. We recently found that 2D class-D Cn-protected higher-order TSCs will universally trap a pair of vortex MZMs as a topological response. The two MZMs living inside the same vortex carry distinct angular momentum and are hence protected by Cn. This new paradigm will shed new light on the experimental detection of crystalline topological physics in superconductors.
Host Erika Martin

Fall 2022

December 6, Tuesday, 11am
Maissam Barkeshli (University of Maryland), ATL 4402
Title: Discrete shift and quantized charge polarization: New topological invariants and quantized response of crystalline topological states
Abstract: I will describe our recent advances in understanding how to define and calculate invariants for topological phases of matter with crystalline symmetry. These invariants define a quantized contribution to fractional charge bound to lattice disclinations and dislocations. They also, as a dual response, determine the angular and linear momentum of magnetic flux in the ground state. I will show how our invariants give, for the first time since TKNN's seminal work in 1982 on quantized Hall conductance, a new class of topological invariants that can be defined throughout Hofstadter's famous butterfly.
Host Erika Martin

November 29, Tuesday, 11am
Michael Gullans (University of Maryland), ATL 4402
Title: Error Mitigation Thresholds in Noisy Quantum Circuits
Abstract: Noise in quantum devices can be corrected with quantum error correction or it can be mitigated via classical post-processing. The latter can be done with negligible overhead in the space-time volume of the quantum circuit, but will generally incur exponential overhead in sampling complexity. We use statistical-mechanical arguments to discuss the limits of error mitigation in quantum circuits. We show that noisy random quantum circuit models with imperfectly characterized noise remain robust to imperfections at a finite rate of disorder, before exhibiting a disorder-driven error mitigation threshold. Based on Imry-Ma arguments, we conjecture that this transition is in the same universality class as the classical random field Ising model in D+1 dimension for D>1 spatial dimensions of the qubits. Our results are based on a replica analysis of statistical mechanics models for noisy random circuits, as well as numerical simulations of error mitigated noisy quantum circuits. We discuss the implications of our results for quantum algorithms and tests of quantum computational advantage in near-term devices. Joint work with Pradeep Niroula and Sarang Gopalakrishnan.
Host Erika Martin

November 15, Tuesday, 11am
Jay Sau (University of Maryland), ATL 4402
Title: Subtleties in chiral anomalies and time crystals – a progress report
Abstract: I will discuss two topics that I have been collaborating on in CMTC. The first involves a careful study of an analog of the chiral anomaly in one dimension that was motivated by work from the Galitski group at Maryland which showed that the response of the chiral charge to electromagnetic fields could be affected by interactions. At the same time, the chiral anomaly, when it arises at boundaries of topological phases, is known to be associated with topological terms that cannot be renormalized. I will discuss explicit calculation of various response functions of one dimensional interacting systems that show that while interactions can affect the current response, which is the variable used in relativistic as well as topological field theories to define the chiral charge, it does not affect the momentum response. The momentum response has been proposed in condensed matter anomalies by Lieb/Schulz/Mattis. The anomalous current response is shown to be related to a conserved chiral charge in a weakly gapped Sine-Gordon model. In a second part of my talk, I will discuss work in progress towards a nearly analytical model for a classical time-crystal that builds on the period doubling transition of a driven Harmonic oscillator. We will analyze a limit of a weakly non-linear damped version of the system where the spontaneous breaking of time-translation symmetry can be described as a conventional symmetry breaking transition using a Landau free-energy. Both topics are works in progress, and the talk is intended to motivate discussion and collaboration.
Host Erika Martin

November 1, Tuesday, 11am
Victor Albert (NIST), ATL 4402
Title: Spin chains, defects, and quantum wires for the quantum-double edge
Abstract: Non-Abelian defects that bind Majorana or parafermion zero modes are prominent in several topological quantum computation schemes. Underpinning their established understanding is the quantum Ising spin chain, which can be recast as a fermionic model or viewed as a standalone effective theory for the surface-code edge -- both of which harbor non-Abelian defects. We generalize these notions by deriving an effective Ising-like spin chain describing the edge of quantum-double topological order. Relating Majorana and parafermion modes to anyonic strings, we introduce quantum-double generalizations of non-Abelian defects. We develop a way to embed finite-group valued qunits into those valued in continuous groups. Using this embedding, we provide a continuum description of the spin chain and recast its non-interacting part as a quantum wire via addition of a Wess-Zumino-Novikov-Witten term and non-Abelian bosonization.
Host Erika Martin

October 4, Tuesday, 11am
Yahui Zhang (Johns Hopkins University), ATL 4402
Title: Exotic exciton phases and quantum spin liquids in quantum Hall bilayer and moire bilayer
Abstract: In this talk I will discuss some exotic phases of excitons beyond the conventional exciton condensation phase. (1) In the first part, I will consider a coulomb coupled quantum Hall bilayer at filling (1/3,-1/3). (Equivalently (1/3,2/3)) and then tune d/l_b. d is inter-layer distance and l_B is magnetic length. I propose that there is a continuous quantum phase transition between an exciton condensation phase at small d and the decoupled Laughlin state at large d. The transition is driven by condensation of a fractional exciton formed by anyone with 1/3 and -1/3 charges in the two layers. This critical point is in the universality class of XY*. (2) In the second part, I consider a moire bilayer and use the layer pseudo-spin to simulate spin physics. “Magnon” of this pseudo spin is just the inter-layer exciton discussed in the first part. Similar to quantum Hall bilayer, conductivity of this pseudo-spin can be electrically measured through the counter-flow techniques. Thus one may obtain smoking gun evidences of certain quantum spin liquids. I will provide some examples of quantum spin liquids which may be realized in this system, including chiral spin liquid, Z2 spin liquid and spinon Fermi surface.
Hosts Erika Martin and Maya Matava

September 20, Tuesday, 11am
Sankar Das Sarma (University of Maryland), ATL 4402
Title: The 'simplest' quantum electronic phases of real matter and the phase transitions among them
Abstract:I will discuss in great depth the density-tuned metal-insulator transitions in 2D semiconductors, discussing in the process three important phases of quantum matter: metal, localized insulator, and Wigner crystal.
Hosts Erika Martin and Maya Matava

Summer 2022

June 14, Tuesday
Graduate Student Symposium , ATL 4402
11:15 AM: Ali Lavasani
11:30 AM: Yi-Hua Lai
11:45 AM: Subhayan Sahu

June 14, Tuesday, 10am
Xiao-Gang Wen (MIT), Zoom
Title: Classify phases and continues phase transitions from categorical symmetry and its condensable algebras
Abstract: We study possible phases and possible continuous phase transitions in systems with a given finite symmetry. We use the corresponding categorical symmetry and its condensable algebras to classify the possible gapped phases and possible gapless critical points, as well as determine the CFT of the critical points in 1+1D.
Host Yu-An Chen
Contact emartin3@umd.edu or mmatava@umd.edu for Zoom details

Spring 2022

May 17, Tuesday, 11am
Anton Kapustin (Caltech), Zoom
Title: Piezosuperconductivity: the interaction between elastic and superfluid degrees of freedom in P and T breaking superconductors.
Abstract: When parity and time-reversal are broken, superfluid and elastic degrees of freedom can interact in novel ways. This includes the much-discussed Lifshitz coupling which is often interpreted as the net Cooper-pair momentum. I will discuss the physical effects arising from this and other couplings. One such effect is a fractional magnetic flux trapped at dislocations. Another one is the strain-generated Josephson current.
Host Jiabin Yu
Contact mmatava@umd.edu for Zoom details

May 10, Tuesday, 11am
Dr. Shinsei Ryu (Princeton University), ATL 4402
Title: Multipartite correlations in topological liquids
Abstract: I will discuss entanglement quantities in two-dimensional topologically-ordered phases that can potentially capture correlations beyond what bipartite entanglement entropy can. Specifically, I will present the calculations of the reflected entropy and entanglement negativity for topological ground states when we consider two spatial sub regions. I will also discuss applications of these ideas to one-dimensional quantum lattice many-body systems.
Host Ryohei Kobayashi

May 3, Tuesday, 11am
Chaoxing Liu (Pennsylvania State University), ATL 4402
Title: Pseudo-Gauge Field in Dirac/Weyl Materials
Abstract: In solid materials, electrons are usually described by the non-relativistic Schrodinger equation since electron velocity is much slower than the speed of light. However, the relativistic Dirac/Weyl equation can emerge as a low-energy effective theory for electrons in certain materials. These systems are dubbed “Dirac/Weyl materials” and provide a tunable platform to test quantum relativistic phenomena in table-top experiments. Owing to the linear-in- momentum form, a variety of physical fields, e.g. strain and phonons, can couple to Dirac/Weyl quasi-particles in a similar form as the minimal gauge coupling. These physical fields thus are dubbed the “pseudo-gauge field”, which provides a useful theoretical concept to understand or predict a variety of physical phenomena beyond the electromagnetic response in Dirac/Weyl materials. In this talk, I will focus on the physical phenomena related to the pseudo-gauge field created by strain and phonons. I will first discuss the Berry curvature contribution to the piezo-electric response, which can be understood as the Hall current response driven by strain-induced pseudo-electric field [1]. Our theory predicts a jump of piezo-electric coefficients across a topological phase transition in 2D Dirac materials. Then I will show that electron-phonon interaction in 2D Dirac materials also takes a gauge coupling form, and consequently, electron Berry curvature can appear in the effective action of phonon dynamics [2]. This leads to a “helical texture” of phonon angular momentum in the momentum space, from which a heat current can drive a total phonon angular momentum. Finally, I will show phonons can also induce a gravitational torsion field for the Kramers-Weyl fermions in chiral crystals and discuss the possibility of probing the Nieh-Yan anomaly through thermal transport measurement.
Host Jiabin Yu

CMTC Graduate Student Symposium
Tuesday, April 19
12:00 PM: Welcome, Prof. Das Sarma
12:10 PM: DinhDuy Vu
12:20 PM: Haining Pan
12:30 PM: Yuxuan Zhang
12:40 PM: Donovan Buterakos
12:50 PM: Wrap-Up
1:00 PM: Lunch

April 5, Tuesday, 11am
Prof. Kin Fai Mak (University of Maryland), Zoom Seminar
Title: Kane-Mele-Hubbard physics in semiconductor moiré materials
Abstract: Semiconductor moiré materials provide a physical realization of the Kane-Mele-Hubbard model for studies of the combined effects of non-trivial band topology and strong electronic correlations. In this talk, I will discuss the rich electronic phase diagram of the Kane-Mele-Hubbard model realized in AB-stacked MoTe2/WSe2 moiré bilayers. In particular, I will discuss the emergence of the quantum spin Hall and the quantum anomalous Hall effects, the realization of the Haldane model, the nature of the Chern insulators, and, if time permits, a metamagnetic quantum phase transition between different intervalley coherent states.
Host Jiabin Yu
Contact emartin3@umd.edu or mmatava@umd.edu for Zoom details

March 29, Tuesday, 2pm
Andrey Gromov (Brown University), ATL4402
Title: Applied Fractons
Abstract: Fractons are a class of quasiparticles that cannot freely propagate through space. They were first introduced in a model of quantum (almost) self-correcting memory. Later it became clear that fractons, as well as, adjacent ideas such as tensor gauge theories and multipole or subsystem conservation laws provide a language to describe some known and some new phenomena. In this talk I will explain what fractons are, what kind of systems are known to support them and what kind of problems they will help to elucidate in the future.

March 29, Tuesday, 11am
Dr. B. Andrei Bernevig (Princeton University), Zoom Seminar
Title: An Exact Map Between the TBG (and multilayers) and Topological Heavy Fermions
Abstract: Magic-angle (?=1.05°) twisted bilayer graphene (MATBG) has shown two seemingly contradictory characters: the localization and quantum-dot-like behavior in STM experiments, and delocalization in transport experiments. We construct a model, which naturally captures the two aspects, from the Bistritzer- MacDonald (BM) model in a first principle spirit. A set of local flat-band orbitals (f) centered at the AA-stacking regions are responsible to the localization. A set of extended topological conduction bands (c), which are at small energetic separation from the local orbitals, are responsible to the delocalization and transport. The topological flat bands of the BM model appear as a result of the hybridization of f- and c-electrons. This model then provides a new perspective for the strong correlation physics, which is now described as strongly correlated f-electrons coupled to nearly free topological semimetallic c-electrons - we hence name our model as the topological heavy fermion model. Using this model, we obtain the U(4) and U(4)×U(4) symmetries as well as the correlated insulator phases and their energies. Simple rules for the ground states and their Chern numbers are derived. Moreover, features such as the large dispersion of the charge ±1 excitations and the minima of the charge gap at the G point can now, for the first time, be understood both qualitatively and quantitatively in a simple physical picture. Our mapping opens the prospect of using heavy-fermion physics machinery to the superconducting physics of MATBG. All the model’s parameters are analytically derived
Host Jiabin Yu
Contact emartin3@umd.edu or mmatava@umd.edu for Zoom details

February 4, Friday, 11am
Daniel Bulmash (University of Maryland), Zoom Seminar
Title: Anomalies in (2+1)D symmetry-enriched topological phases
Abstract: Topological phases of matter, like fractional quantum Hall systems, can host anyon excitations with fractional electric charge. More generally, when topological phases with anyons have global symmetries, the anyons can carry fractional quantum numbers under those symmetries. Remarkably, some choices of fractional quantum numbers are anomalous, that is, they are physically allowed to exist, but only when the (2+1)D system lives on the surface of a bulk (3+1)D symmetry-protected topological phase like a topological insulator. Given abstract algebraic data specifying these quantum numbers, I will explain how to determine whether it is anomalous and, if so, how to describe the required bulk theory.
Host Jiabin Yu
Contact emartin3@umd.edu or mmatava@umd.edu for Zoom details

Fall 2021

December 17, Friday, 11am
Andy Lucas (University of Colorado Boulder), Seminar
Title: Fingerprints of quantum criticality in locally resolved transport
Abstract: New experimental methods such as nitrogen vacancy center magnetometry allow for the imaging of local transport phenomena well below the micron length scale. I will describe how these methods might be used to experimentally reveal quantum critical dynamics which is invisible in conventional bulk transport measurements. Using a holographic system as a toy model, I will describe what happens as current is pushed through a geometric constriction in both hydrodynamic and quantum critical transport regimes, both in charge neutral and non-zero density limits. Remarkably, our model does seem to quantitatively capture locally imaged transport in charge neutral graphene at high temperatures, albeit in a largely ohmic transport regime with only weak “quantum critical” corrections.
Host Yu-An Chen and Jiabin Yu

December 16, Thursday, 4pm
Kostyantyn Kechedzhi (Google), Seminar
Title: Many-body Physics with NISQ superconducting processors
Abstract: Recent demonstration of the first beyond classical quantum computation with a programmable superconducting quantum processor [1] opens the path to discovery of new quantum physics phenomena using these hardware systems. Gate model quantum computers used in [1] realize complex multi-qubit evolution in terms of discrete gates, elementary one and two qubit unitary operations, practically realized by a local time-dependent control Hamiltonian. In this talk we overview the recent many-body physics experiments implemented on these processors. We will specifically focus on the theory and experimental data describing quantum circuit kinetics in such systems, which answers the following question: how does a system initialized in a product state generate highly entangled states and in the case of a non-integrable system approach universal random matrix statistics? We characterize circuit kinetics using time-dependent evolution of out-of-time-order correlators (OTOC) and their ensemble variance. We demonstrate that dynamics of an ensemble average OTOC can be mapped onto a classical dynamical process akin to a population dynamics in biology. OTOC variance on the other hand is subject to a “sign problem” and therefore evades efficient classical description. We use ensemble average OTOC to verify hardware output, whereas OTOC variance provides a signature of operator entanglement generated in the system [2].
[1] Arute et. al. Nature, Vol 574, 505 (2019)
[2] Mi et. al. Science 2021 (10.1126/science.abg5029) arXiv:2101.08870
Host Yu-An Chen and Jiabin Yu

December 16, Thursday, 11am
Ryan Barnett (Imperial College London), Seminar
Title: Polarons and Topological Effects in Ultracold Atomic Gases
Abstract: Ultracold atomic gases have proven to provide valuable platforms to simulate quantum systems arising in disparate areas of physics. Polarons are well-studied quasiparticles in solid-state systems that describe an electron dressed by lattice distortions. The so-called Frohlich model is the typical starting point for theoretically describing such systems. More recently, polarons arising in Bose-Einstein condensates have been the focus of much attention, both theoretical and experimental. For such systems, the Bogoliubov phonons play the role of the lattice vibrations of a solid. Furthermore, by using a Feshbach resonance the strong and weak coupling regimes are readily accessed in experiments. In this talk, I will describe a recent framework developed to understand Bose-polarons in ultracold atomic gases. This framework incorporates the back action of the impurity on the BEC at the mean field level and forms a natural starting point for incorporating quantum fluctuations. Time permitting, I also hope to discuss separate but related work describing topological band systems which can arise in ultracold atomic gases. In particular, I will discuss so-called local topological markers which can naturally be used to characterise topological band systems that do not have translational invariance.
Phys. Rev. Research 2, 033142 (2020)
Phys. Rev. B 103, 155134 (2021)
Host Yu-An Chen and Jiabin Yu

CMTC Symposium
November 30
10:00 AM: Sankar Das Sarma
10:10 AM: Maissam Barkeshli
10:20 AM: Jay Sau
10:30 AM: Robert Throckmorton
10:40 AM: Seongjin Ahn
10:50 AM: Danny Bulmash
11:00 AM: Yu-An Chen
11:10 AM: Yang-Zhi Chou
11:20 AM: Seth Davis
11:30 AM: Sheng-Jie Huang
11:40 AM: Ryohei Kobayashi
11:50 AM: Yunxiang Liao
12:00 PM: Lunch break
1:00 PM: Jiabin Yu
1:10 PM: Tamoghna Barik
1:20 PM: Donovan Buterakos
1:30 PM: Nathan Foulk
1:40 PM: Navya Gupta
1:50 PM: Jeet Shah
2:00 PM: Yi-Hua Lai
2:10 PM: Naren Manjunath
2:20 PM: Haining Pan
2:30 PM: Subhayan Sahu
2:40 PM: Stuart Thomas
2:50 PM: DinhDuy Vu
3:00 PM: Shuyang Wang
3:10 PM: Yuxuan Zhang
3:20 PM: Huan-Kuang Wu
3:30 PM: Ming Xie
3:40 PM: Gautam Nambiar

November 9, Tuesday, 11am
Kyle Kawagoe (University of Chicago), Informal Zoom seminar
Title: A Bulk-Boundary Correspondence for 2D Fermionic Symmetry Protected Topological Phases
Abstract: A universal property of symmetry protected topological (SPT) phases is that they have low energy boundary modes that are protected under the symmetry. This fact inspires an important problem in the theory of SPT phases: How does one identify a bulk SPT phase given a low energy theory of its boundary? This question is particularly challenging in the case of interacting SPT phases where band theory approaches are inapplicable. In this talk, we present a general method for solving this problem in the case of (2+1) D interacting fermionic systems with internal (non-spatial) symmetries.
Host Yu-An Chen
Email emartin3@umd.edu for Zoom details

October 26, Tuesday, 4pm
Charles Kane (University of Pennsylvania), Prange Prize Lecture
Title: The Emergence of Topological Quantum Matter
Location: University of Maryland, College Park, John S. Toll Physics Building, Room 1412
Abstract:Matter can arrange itself in the most ingenious ways. In addition to the solid, liquid and gas phases that are familiar in classical physics, electronic phases of matter with both useful and exotic properties are made possible by quantum mechanics. In the last century, the thorough understanding of the simplest quantum electronic phase - the electrical insulator - enabled the development of the semiconductor technology that is ubiquitous in today's information age. In the present century, new "topological" electronic phases are being discovered that allow the seemingly impossible to occur: indivisible objects, like an electron or a quantum bit of information, can be split into two, allowing mysterious features of quantum mechanics to be harnessed for future technologies. Our understanding of topological phases builds on deep ideas in mathematics. We will try to convey that they are as beautiful as they are fundamental.

October 25, Monday, 11am
Charles Kane (University of Pennsylvania), Prange Prize seminar
Title: Quantized Nonlinear Response in Ballistic Metals
Location: University of Maryland, College Park, Atlantic Building, Room 2400
Abstract:A dramatic consequence of the role of topology in the structure of quantum matter is the existence of topological invariants that are reflected in quantized response functions. In this talk we will discuss a new variant on this theme. We introduce a non-linear frequency dependent D+1 terminal conductance that characterizes a D dimensional Fermi gas, generalizing the Landauer conductance in D = 1. For a ballistic conductor we show that this conductance is quantized and probes the Euler characteristic of the Fermi sea. We critically address the roles of electrical contacts and of Fermi liquid interactions, and we propose experiments on 2D Dirac materials such as graphene using a triple point contact geometry.
Email mmatava@umd.edu for Zoom details

Summer 2021

July 20, Tuesday, 11am
Yi-Hsien Du (University of Chicago), Informal Zoom seminar
Title: Volume-preserving diffeomorphism as higher-rank gauge symmetry
Abstract: Higher-rank gauge theories have been drawing attention in condensed matter physics in recent years. The physical motivation of such theories is thought to be associated with a new class of topological matter so-called "fractons," quasiparticles with restricted mobility. We demonstrate a nonlinear version of the higher rank gauge symmetry in 2+1D and 3+1D with volume-preserving diffeomorphism as the symmetry group. We show that various condensed matter systems, including fractional quantum Hall effect and ferromagnetism, possess this symmetry, which exhibits fractonic behavior of the excitations in these systems.
Host Yu-An Chen
Email rcawthor@umd.edu for Zoom details

CMTC Mini-Symposium
July 8 & 9

Thursday, July 8
10:30 AM: Jay Deep Sau "Search for Majorana modes in iron superconductors"
11:30 PM: Lunch break
12:30 PM: Robert Throckmorton, "Studying many-body localization in exchange-coupled electron spin qubits using spin-spin correlations"
12:40 PM: Sheng-Jie Huang, "Quantum many-body topology of quasicrystals"
12:50 PM: Ali Lavasani "An order parameter for k-local non-trivial states"
1:00 PM: Subhayan Sahu
1:10 PM: Jiabin Yu
1:20 PM: Huan-Kuang Wu
10 min break
1:40 PM: Haining Pan, "SPT entanglement in fermionic topological phases"
1:50 PM: Yi-Hua Lai
2:00 PM: Donovan Buterakos, "Spin-valley entanglement in Si quantum dots"
2:10 PM: Richard Barney

Friday, July 9
10:00 AM: Ruixing Zhang, "Competing vortex topologies in iron-based superconductors"
10:10 AM: Shao-Kai Jian
10:20 AM: Yu-An Chen "Higher up products on hypercubes"
10:30 AM: Danny Bulmash
10:40 AM: DinhDuy Vu
10:50 AM: Seongjin Ahn
11:00 AM: Lunch break
12:00 PM: Naren Manjunath "The torsion vector: a new form of symmetry fractionalization"
12:10 PM: Yang-Zhi Chou "Interacting helical edge states: Coulomb drag and band manipulation"
12:20 PM: Alireza Parhizkar
12:30 PM: Shankar Balasubramanian

Janet Das Sarma Conference: "Twisted: Physics of 2D Twisted Moire Systems"
June 14, Monday, 10am - 6pm ET
10:00 AM: Pablo Jarillo-Herrero, "Moire Magic 3.0"
11:00 AM: Ashvin Vishwanath, "Moire magic near charge neutrality: From sign-free numerics to fractional vortices"
12:00 PM: Andrea Young, "Easy as ABC"
1:00 PM: Michael Zaletel, "DMRG evidence for skyrmion-mediated superconductivity"
2:00 PM: Oskar Vafek, "Correlations and topology in the magic angle twisted bilayer graphene"
3:00 PM: Cory Dean, "TBA"
4:00 PM: Kin Fai Mak, "Semiconductor moire 2.0"
5:00 PM: Sankar Das Sarma, "Superconductivity, magnetism, strange metals, and correlated insulators in twisted 2D moire systems"
Host Prof. Jay D. Sau, Yang-Zhi Chou, Danny Bulmash, Ruixing Zhang, Jiabin Yu
Email rcawthor@umd.edu for Zoom details

Spring 2021

Janet Das Sarma Conference: "Quantum"
May 17, Monday, 10am - 6pm
10:00 AM: Immanuel Bloch, "Realising and probing quantum matter using large scale quantum simulations"
11:00 AM: Ignacio Cirac, "Simulations with analog and digital quantum computers"
12:00 PM: Lieven Vandersypen, "Analog quantum simulating of Fermi-Hubbard physics using quantum dot arrays"
1:00 PM: Andrew Childs, "Efficient quantum algorithm for dissipative nonlinear differential equations"
2:00 PM: Markus Greiner, "Tentative: Simulating strongly correlated quantum materials using cold fermions in the laboratory"
3:00 PM: Maissam Barkeshli, "Measurement-induced topological entanglement transitions in random quantum circuits"
4:00 PM: Edwin Barnes, "Quantum error mitigation at the level of hardware control: from spin echo to geometric space curves"
5:00 PM: Garnet Chan, "Quantum advantage and quantum chemistry"
Host Prof. Jay D. Sau, Yang-Zhi Chou, Danny Bulmash, Ruixing Zhang
Email rcawthor@umd.edu for Zoom details

May 11, Tuesday, 11am
Yizhi You (Princeton), Informal Zoom seminar
Title: Fracton critical point and Topological phase transition beyond renormalization
Abstract: The theory of quantum phase transitions separating different phases with distinct symmetry patterns at zero temperature is one of the foundations of modern quantum many-body physics. In this talk, I will demonstrate that the existence of a 2D topological phase transition between a higher-order topological insulator (HOTI) and a trivial Mott insulator with the same symmetry eludes this paradigm. A significant new element of our phase transition theory is that the infrared (IR) effective theory is controlled by short wave-length fluctuations so the critical phenomenon is beyond the renormalization perspective.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

May 4, Tuesday, 1pm
Xie Chen (Caltech), Zoom seminar
Title: Fracton and Chern-Simons Theory
Abstract: Fracton order describes the peculiar phenomena that point excitations in certain strongly interacting systems either cannot move at all or are only allowed to move in a lower dimensional sub-manifold. It has recently been discovered in various lattice models, tensor gauge theories, etc. In this talk, we discuss how another powerful field theory framework -- the 2+1D Chern-Simons (CS) gauge theory -- can be used to provide new insights and explore new possibilities in 3+1D fracton order. 2+1D U(1) gauge theories with a CS term provide a simple and complete characterization of 2+1D Abelian topological orders. To study 3+1D fracton order, we extend the theory by taking the number of component gauge fields to be infinity. In the simplest case of infinite-component CS gauge theory, different components do not couple to each other and the theory describes a decoupled stack of 2+1D fractional Quantum Hall systems with quasi-particles moving only in 2D planes -- hence a fractonic system. More interestingly, we find that when the component gauge fields do couple through the CS term, more varieties of fractonic orders are possible. For example, they may describe foliated fractonic systems which extends the framework found in exactly solvable models. At the same time, we find examples which lie beyond the foliation framework, characterized by 2D excitations of infinite order and braiding statistics that are not strictly local.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

April 20, Tuesday, 11am
Patrick Lee (MIT), Zoom seminar
Title: A tale of two TMD insulators: TaSe2 and WTe2.
Abstract: I shall discuss two monolayer transition metal dichalcogenides (TMD) that have unusual properties and are insulating for very different reasons. TaSe2 is a cluster Mott insulator that is a promising candidate for a quantum spin liquid with spinon Fermi surface. I shall discuss the formation of a Kondo resonance between a magnetic impurity and the spinon Fermi surface as well as super modulations and their consequences for STM tunneling. Monolayer WTe2 is a topological quantum spin Hall insulator but why it is insulating is not fully understood. Recently sharp quantum oscillation peaks in the conductivity have been discovered in the insulator that resemble quantum oscillations in metals. While there are some controversies about whether this is intrinsic, I shall discuss a proposed explanation in terms of its being an excitonic insulator.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

Janet Das Sarma Conference: "Searching for Topological Majorana Zero Modes"
April 12, Monday, 10am - 6pm
10:00 AM: Sankar Das Sarma, "Searching for Topological Majorana Zero Modes"
11:00 AM: Tadashi Machida, "Searching for Majorana quasiparticle in Iron-based superconductors"
12:00 PM: Erik Bakkers, "Reducing disorder in Semiconductor/superconductor nanowires"
1:00 PM: Jesper Nygard, "Hybrid nanowires, dots and bound states towards topological devices"
2:00 PM: Roland Wiesendanger, "Emergent Majorana zero modes in disorder-free bottom-up fabricated atomic spin chains proximity-coupled to clean superconducting Re and Nb substrates"
3:00 PM: Georgios Katsaros, "InAs/Al full-shell nanowires and proximitized Ge hole gases: Suitable platforms for the realization of Majorana zero modes?"
4:00 PM: Peter Liljeroth, "Topological superconductivity in designer van der Waals heterostructures"
5:00 PM: Tudor Stanescu, "How far away is the Majorana zero mode? Landmarks in the desert of disorder"
Host Prof. Jay D. Sau, Yang-Zhi Chou, Danny Bulmash, Ruixing Zhang
Email rcawthor@umd.edu for Zoom details

March 26, Friday, 11am
Ruixing Zhang (UMD), Practice Job Talk
Title: Hierarchy of Topology in Quantum Materials
Abstract: Topological insulators are distinguished from their conventional counterparts by the presence of gapless surface states, which originate from their nontrivial bulk-state topologies. Recently, it has been realized that even a surface-gapped insulator can be topological in a higher-order manner when harboring robust gapless modes on the boundary with codimension two or higher (i.e., inter-surface hinges or corners). In this talk, I will discuss how such exotic higher-order topological phases will naturally arise in two distinct classes of realistic quantum materials. I will first describe our proposal on the emergent hinge and corner localized Majorana modes in certain iron-based superconductors and discuss the corresponding experimental evidence that has been observed. Furthermore, I will introduce (MnBi2Te4)(Bi2Te3)n, a newly discovered family of magnetic topological materials, as a highly tunable platform for achieving various higher-order topological insulating phases with chiral electron hinge modes. In particular, I will demonstrate how higher-order topology in this class of materials can acquire a new surface Mobius twist with an applied in-plane magnetic field.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

March 23, Tuesday, 11am
Sid Parameswaran (Oxford University), Zoom seminar
Title: Topology and broken symmetry of excitons in two-dimensional materials
Abstract: I will discuss recent theoretical work on exciton physics in two-dimensional materials. First, I will focus on the neutral particle-hole pair excitations of correlated orbital Chern insulators recently detected in twisted bilayer graphene, whose approximately flat conduction and valence bands have equal and opposite non-zero Chern number. Using a mix of exactly solvable models and microscopic nuumerical simulations, I will argue that these features can combine to endow the exciton bands with nontrivial Berry curvature [1]. I will then argue, more speculatively, that under certain circumstances this could stabilize a series of excitonic fractional quantum Hall states descending from the parent integer OCI state [2]. I will then change gears to focus on work motivated by recent experiments on monolayer WTe2. I willl describe the zero-field excitonic insulating phase thought to be present in this system, and clarify the role played by band topology in the competition between distinct broken-symmetry exciton orders [3]. I will also comment on the concomitant observation of quantum oscillations coincident with insulating behaviour when the same system is placed in a transverse magnetic field.
[1] YH Kwan, Y Hu, SH Simon, SP, arXiv: 2003.11560, to appear in PRL.
[2] YH Kwan, Y Hu, SH Simon, SP, arXiv: 2003.11559.
[3] YH Kwan, T Devakul, S Sondhi, SP, arxiv:2012.5255 (also 2101.05294).
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

March 9, Tuesday, 11am
Yu-Ping Lin (Ph.D. candidate at CU Boulder), Informal Zoom seminar
Title: Novel phases at Van Hove singularities in graphene moire systems
Abstract: I will present the weak-coupling renormalization group analyses at the Van Hove singularities in graphene moire systems, where various novel correlated phases can develop. Motivated by the structures of low-energy flat bands on the moire superlattices, I will focus on the two-orbital hexagonal lattice models at the Van Hove doping. First, I will show that a chiral 'high-Tc'-like phase diagram is hosted by an SU(4) symmetric model with conventional Van Hove singularity. A d-wave staggered-flux Chern insulator occurs at the Van Hove singularity, which is flanked by the d-wave chiral superconducting domes on both sides of doping. Upon valley splitting which breaks the SU(4) symmetry, the phase diagram turns into a competition between chiral superconductivity and various spin and/or valley density waves. Second, I will demonstrate how various polarized ordered phases may occur when the Van Hove singularity becomes 'high-order'. The potential correlated phases include the p-wave chiral and helical and d-wave chiral superconductivities, s-wave ferromagnetism, as well as f-wave and p-wave polar valley-polarized orders.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

February 24, Wednesday, 11am
Yunxiang Liao (CMTC), Zoom seminar
Title: Emergence of many-body quantum chaos via spontaneous breaking of unitarity
Abstract: Despite numerous efforts, there lacks a microscopic understanding of the emergence of irreversible statistics mechanics behavior in isolated many-body quantum systems evolving under the reversible unitary dynamics. The fundamental question of thermalization is intertwined with many-body quantum chaos, and in particular its connection with random matrix theory. It has been found that many-body level statistics of noninteracting fermions populating single-particle levels of a Gaussian unitary ensemble exhibits rich structure, which stems from single-particle quantum chaos and is reflected by the exponential ramp in the spectral form factor (SFF). We generalize this study to interacting theory and evaluate the SFF for both cases of zero and positive infinitesimal inverse temperature \beta using the field integral formulation. The SFF for \beta=0 in the regime of interest is dominated by contribution from the fluctuations around the standard saddle point. The associated soft modes responsible for the exponential ramp of the noninteracting theory acquire a mass in the presence of interactions, resulting in the suppression of the exponential ramp -- a necessary prerequisite for the emergence of random matrix structure in many-body spectrum. By contrast, the SFF for positive infinitesimal \beta is instead governed by fluctuations around a series of nonstandard saddle points in addition to the standard one. Furthermore, for a particular nonstandard saddle point, the presence of interactions does not lead to a mass suppressing the exponential ramp. This work suggests that the emergence of many-body quantum chaos might appear as spontaneous symmetry breaking of unitarity and consequently time-reversal invariance.
Host Dr. Jay Deep Sau and Dr. Victor Galitski
Email rcawthor@umd.edu for Zoom details

February 9, Tuesday, 11am
Kun Yang (Florida State University), Zoom seminar
Title: Interplay of Topology and Geometry in Fractional Quantum Hall Liquids
Abstract: Fractional Quantum Hall Liquids (FQHL) are the ultimate strongly correlated electron systems, and the birth place of topological phase of matter. Early theoretical work has emphasized the universal or topological aspects of quantum Hall physics. More recently it has become increasingly clear that there is very interesting bulk dynamics in FQHL, associated with an internal geometrical degree of freedom, or metric. The appropriate quantum theory of this internal dynamics is thus expected to take the form of a “quantum gravity”, whose elementary excitations are spin-2 gravitons. After briefly reviewing the topological aspect of FQHL, I will discuss in this talk how to probe the presence of this internal geometrical degree of freedom experimentally in the static limit, and detect the graviton excitation in spectroscopic measurements, in particular how to reveal its chirality. Comparison will be made with recent experimental and numerical work, and discussions on future experimental probe of the graviton chirality as well as its significance will be presented.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

Fall 2020

December 15, Tuesday, 11am
Pablo Jarillo-Herrero (MIT), Zoom seminar
Title: Moire Magic 3.0
Abstract: Moire superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moire systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. In this talk I will present a new moire superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging at two carriers per moire unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is di cult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large T_BKT/T_F ratios in excess of 0.1. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moire superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

November 30, Monday, 11am
Xiao-Gang Wen (MIT), Zoom seminar
Title: Categorical symmetry and non-invertible gravitational anomaly: Understanding strongly correlated gapless systems.
Abstract: Using some 1d examples, we show the emergence of symmetry and dual symmetry in gapless states, which may be a general feature for all gapless state. The symmetry and dual symmetry together is called categorical symmetry which is same as non-invertible gravitational anomaly. We will use this point of view to study some new strongly correlated gapless states.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

November 17, Tuesday, 11am
Cory Dean (Columbia University), Zoom seminar
Title: Correlated states in transition metal dichalcogenides
Abstract: Quantum Hall bilayers, in which two parallel quantum wells are separated by an insulating tunnel barrier, and subjected to a transverse magnetic field, provide a rich platform to study interaction-driven correlated phenomenon. The transverse magnetic field transforms the energy spectrum of the 2D electrons into a series of discrete Landau levels, where kinetic energy is quenched and Coulomb interactions become the dominant energy scale. In addition, coupling between the layers can be precisely tuned through variations in the effective interlayer separation. This provides unique experimental access to correlated ground states with tunable degrees of freedom. In this talk I will discuss several novel phenomena that can be realized graphene quantum Hall bilayers, consisting of monolayer graphene separated by an interlayer BN barrier. I will focus my discussion on recent studies of exciton condensates in these systems, including potential identification of an exciton liquid-to-exciton solid crossover, as well as the first demonstration of correlated interlayer states in a three-layer system. Time permitting I will also present recent evidence for the appearance of novel multi-component fractional quantum hall states in graphene bilayers, including potential evidence of a new non-abelian state. The experimental opportunities in graphene quantum Hall bilayers and its relation to twisted bilayer graphene at zero magnetic field will be discussed.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

November 10, Tuesday, 10am
Ryohei Kobayashi (Graduate Student, ISSP University of Tokyo), Informal Zoom seminar
Title: Interacting fermionic topological phases with time reversal symmetry
Abstract: In this talk, we discuss a recipe to produce a lattice construction of fermionic topological phases of matter on unoriented spacetime, which plays a crucial role to study topological phases or anomalies based on the time reversal symmetry. As an application, we construct a gapped boundary for a large class of fermionic SPT phases protected by finite onsite symmetry, based on our path integral description in the presence of boundaries. We will also formulate a local path integral for the (1+1)d topological superconductor in class BDI classified by Z8, and discuss its application to the problem of finding non-local order parameter for the Z8 classification. If time permits, we also refer to an exactly solvable Hamiltonian model for (3+1)d topological superconductor in class DIII, which covers the Z8 subgroup of the Z16 classification. The talk will be based on arXiv:1905.05902, 1905.05391, 1911.00653, 2006.00159
Host Yu-An Chen
Email rcawthor@umd.edu for Zoom details

October 27, Tuesday, 11am
Leo Radzihovsky (University of Colorado Boulder), Zoom seminar
Title: Quantum smectic gauge theory
Abstract:We present a gauge theory formulation of a two-dimensional quantum smectic and its relatives, motivated by their realizations in correlated quantum matter. The description gives a unified treatment of phonons and topological defects, respectively encoded in a pair of coupled gauge fields and corresponding charges. The charges exhibit subdimensional constrained quantum dynamics and anomalously slow highly anisotropic diffusion of disclinations inside a smectic. This approach gives a transparent description of a multi-stage quantum melting transition of a two-dimensional commensurate crystal (through an incommensurate crystal - a supersolid) into a quantum smectic, that subsequently melts into a quantum nematic and isotropic superfluids, all in terms of a sequence of Higgs transitions.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

Symposium, October 19-23, 2:30-4pm PDF Schedule
Monday, October 19:
2:30pm - Yu-An Chen, "The duality between fermionic supercohomology SPT and bosonic 2-group SPT phases in (3+1)D"
3:00pm - Jay Sau, "On the theoretical ability to identify topological wires based on transport measurements of three terminal and Coulomb blockaded devices"
3:30pm - Student introductions
4:00pm - Discussion

Tuesday, October 20:
2:30pm - Sheng-Jie Huang, "Faithful derivation of symmetry indicators: A case study for topological superconductors with time-reversal and inversion symmetries"
3:00pm - Colin Rylands, "A Photon Mediated Peierls Transition"
3:30pm - Yang-Zhi Chou, "Hofstadter butterfly and Floquet topological insulators in minimally twisted bilayer graphene"
4:00pm - Discussion

Wednesday, October 21:
2:30pm - Robert Throckmorton, "Fidelity of a sequence of SWAP operations on a spin chain"
3:00pm - Victor Yakovenko, joint paper with Lance Boyer "How to measure Hall conductivity in a superconductor"
3:30pm - Ruixing Zhang, "Theory of Anomalous Floquet Higher-Order Topology: Classification, Characterization, and Bulk-Boundary Correspondence"
4:00pm - Discussion

Thursday, October 22:
2:30pm - Danny Bulmash, "Topological Defect Networks - A Framework for Fractons"
3:00pm - Shao-Kai Jian, "Dynamics of Renyi entropy in coupled Brownian SYK model"
3:30pm - Seongjin Ahn, "Fermi surface topology and quasiparticle properties in an anisotropic electron gas"
4:00pm - Discussion

Friday, October 23:
2:30pm - Jiabin Yu, "Dynamical Symmetry Indicators For Periodically Driven Crystals"
3:00pm - Christopher White, "How hard is it to prepare a Haar-random state?"
3:30pm - Yunxiang Liao, "Many-body level statistics of single-particle quantum chaos"
4:00pm - Discussion
Email rcawthor@umd.edu for Zoom details

October 6, Tuesday, 11am
Steve Kivelson (Stanford University), Zoom seminar
Title: The quantum superconductor to metal transition
Abstract: Theoretical considerations concerning the nature of the quantum superconductor to metal transition, especially where the superconducting order is inhomogeneous, are discussed. Conditions under which mean-field theory gives a good description of the transition are distinguished from those in which fluctuation superconductivity is expected to be important in a significant range of parameters.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

September 29, Tuesday, 11 am
Subir Sachdev (Harvard University), Zoom seminar
Title: The strange quantum physics of the high temperature superconductors
Abstract: Numerous experiments have explored the phases of the cuprate high temperature superconductors with increasing doping density p from the antiferromagnetic insulator. There is now strong evidence that the small p region is a novel phase of matter, often called the pseudogap metal, separated from conventional Fermi liquid at larger p by a quantum phase transition. Symmetry-breaking orders play a spectator role, at best, at this quantum phase transition. I will describe trial wavefunctions across this metal-metal transition employing hidden layers of ancilla qubits. Quantum fluctuations are described by a gauge theory of ghost fermions that carry neither spin nor charge. I will also describe a separate approach to this transition in a t-J model with random exchange interactions in the limit of large dimensions. This approach leads to a partly solvable SYK-like critical theory of holons and spinons, and a linear in temperature resistivity from time reparameterization fluctuations. Near criticality, both approaches have in common emergent fractionalized excitations, and a significantly larger entropy than naively expected.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

September 22, Tuesday, 11 am
David Huse (Princeton), Zoom seminar
Title: Many-body-localization to thermalization phase transition
Abstract: I will present our current understanding of this novel phase transition as obtained from a strong-randomness real-space renormalization group (RG) approach. This is for the case of isolated one-dimensional quantum many-body systems with quenched randomness and short-range interactions. The RG flow is qualitatively like the famous Kosterlitz-Thouless RG, but is different in some important features, thus making a new universality class of phase transition. Time permitting, I will then try to set this result in the context of the much larger set of transitions and crossovers found in other cases of this transition, such as varying the dimension of space, random vs nonrandom, and changing the range of the interactions.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

September 8, Tuesday, 11 am
Andrea Young (UC Santa Barbara), Zoom seminar
Title: Orbital magnetism and an isospin Pomeranchuk effect in twisted bilayer graphene.
Abstract: Moire flat bands host a wide range of correlated states at low temperatures. My talk will focus on the role of orbital magnetism in these systems, in which the electron system spontaneously polarizes into one or more spin- and valley-isospin flavors. In the first part of the talk, I will focus on the observation of quantized anomalous Hall effects in a variety of moire systems. In this spectacular manifestation of orbital magnetism, the ground state at certain integer filling factors is spontaneously polarized into a single valley-projected moire miniband, leading to robust magnetic hysteresis and a quantized Hall effect at zero magnetic field. We observe a variety of novel phenomena in this regime, including ultra-low power current induced switching and gate-tuned reversal of magnetic order that can be tied to the magnetization of the topological edge states. In the second part of the talk, I will discuss the more general role orbital magnetism plays in the phase diagram of these materials at high temperatures. In particular, we find that even when the ground state is isospin unpolarized, a finite polarization can obtain at high temperatures. We ascribe this effect--observed generically in twisted bilayer graphene--to an isospin analogue of the Pomeranchuk effect of 3He, in which the high entropy associated with isospin excitations of the orbital magnets favors fluctuating magnetic order at high temperatures.
Serlin et al., arXiv:1907.00261
Polshyn et al., arXiv:2004.11353
Tschirhart et al., arXiv:2006.08053
Saito et al., arXiv:2008.10830
Host Yang-Zhi Chou

Summer 2020

August 18, Tuesday, 11 am
Igor Mazin (George Mason University), Zoom seminar
Title: Ising superconductivity in NbSe2 monolayers
Abstract: Recent studies on superconductivity in monolayer NbSe2 have demonstrated a giant anisotropy in the superconducting critical field. This phenomenon was quite well understood in terms of the so-called “Ising superconductivity”, where the spins of Cooper pairs are strictly aligned with one particular crystallographic direction. Besides the (formally infinite) critical field anisotropy Ising superconductors (IS) have demonstrated a number of unusual and seeming exotic phenomena. IS is sometimes misleading perceived as an esoteric subset of the theory of superconductivity, which is hard to explain and even harder to understand for outsiders. In the first part of my talk I will debunk this notion and demonstrate that the physics of IS is exceedingly simple and hardly requires any formulaics to be grasped. In the second part I will make, in terms of DFT calculations, a quantitative connection with the specific material in which most of the IS studies are being performed, monolayer NbSe2; in particular, I will show that, contrary to a common misconception, NbSe2 is close to a magnetic instability and this fact cannot be ignored when discussing IS. In the third part I will discuss to what extent the existing models allow for a sizeable singlet-triplet mixing (NbSe2 had been till recently believed to be, for all intents and purposes, a singlet superconductor, but that is not necessarily the case). Up to now the talk will be based on our paper with Darshana Wickramaratne (NRL) and Daniel Agterberg (UWi), to be published in PRX. If time allows, I will also say a few words about the new experiments from the Fai Mak group in Cornell and present some results from our work in progress with Darshana and Maxim Khodas (Hebrew U) striving to explain his observations microscopically.
Host Victor Yakovenko
1. ArXiv:2005.05497

August 11, Tuesday, 11 am
Dmitry Green (AppliedTQC), Zoom seminar, Joint CMTC-QuICS seminar
A superconducting circuit realization of combinatorial gauge symmetry
We propose an integrated superconducting circuit design in combination with a general symmetry principle, combinatorial gauge symmetry, to build artificial quantum spin liquids that serve as foundation for the construction of topological qubits. The superconducting wire arrays exhibit rich features. In the classical limit of large capacitances its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a soup of loops of all sizes associated to Z_2 topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding Z_2 quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain Z_2 quantum topological order with a gap of the order of the Josephson coupling in the array.
Host Prof. Victor Galitski
Email rcawthor@umd.edu for Zoom details

August 4, Tuesday, 11 am
Masaki Oshikawa (University of Tokyo), Zoom seminar
Non-Fermi Liquids in 2d Conducting Networks
We investigate 2-dimensional periodic superstructures consisting of 1-dimensional conducting segments. Such structures naturally appear in twisted transition metal dichalcogenides, some charge-density-wave materials, and a marginally twisted bilayer graphene, in which intriguing non-Fermi liquid transports have been experimentally observed. We model such a system as a network of Tomonaga-Luttinger Liquids, and theoretically derive a variety of non-Fermi liquid behaviors, based on a Renormalization-Group analysis of the junctions of Tomonaga-Luttinger Liquids. In particular, a continuously varying resistivity exponent appears naturally in the 2-dimensional network through the continuously varying Luttinger parameter of the constituent Tomonaga-Luttinger Liquid.
Host Yang-Zhi Chou
Email rcawthor@umd.edu for Zoom details

July 28, Tuesday, 10 am
Eun-Gook Moon (KAIST), Zoom seminar
Instability of j=3/2 Bogoliubov Fermi surfaces
Exotic quantum phases including topological states and non-Fermi liquids may be realized by quantum states with total angular momentum j=3/2, as manifested in HgTe and pyrochlore iridates. Recently, an exotic superconducting state with non-zero density of states of zero energy Bogoliubov quasiparticles, Bogoliubov Fermi-surface (BG-FS), was also proposed in a centrosymmetric j=3/2 system, protected by a Z2 topological invariant. Here, we consider interaction effects of a centrosymmetric BG-FS and demonstrate its instability by using mean-field and renormalization group analysis. The Bardeen-Cooper-Schrieffer (BCS) type logarithmical enhancement is shown in fluctuation channels associated with inversion symmetry. Thus, we claim that the inversion symmetry instability is an intrinsic characteristic of a BG-FS under generic attractive interactions between Bogoliubov quasiparticles. In drastic contrast to the standard BCS superconductivity, a Fermi-surface may generically survive even with the instability. We propose the experimental setup, a second harmonic generation experiment with a strain gradient, to detect the instability. Possible applications to iron based superconductors and heavy fermion systems including FeSe are also discussed.
Host Danny Bulmash
Email rcawthor@umd.edu for Zoom details

July 14, Tuesday, 11 am
Jennifer Cano (Stony Brook/Flatiron Institute), Zoom seminar
Lattice dislocations as a probe of higher order topological insulators
Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode to a lattice dislocation. I will show that higher-order topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations in the absence of weak topological indices. When this occurs, the helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can measurably affect the expected conductivity in these materials. I will also discuss our prediction of HOTIs in antiperovskites with spin-orbit coupling.
1. Phys. Rev. Lett. 123, 266802 (2019) (arXiv:1809.03518)
2. Phys. Rev. B 101, 245110 (2020) (arXiv:2002.02969)
Host Danny Bulmash

Spring 2020

January 16, Thursday, 2 pm
Fengcheng Wu (CMTC), practice talk for a job interview
Symmetry, Topology, and Many-Body Interactions in Moiré Systems
Van der Waals bilayers with small differences in the lattice constants or orientations of the individual layers have long-period Moiré patterns, which provide vast new opportunities to control material properties. In this talk, I will present our work on Moiré pattern physics that arise from an interplay of symmetry, topology and many-body interactions. First, I will describe our theoretical proposal of using twisted bilayer transition metal dichalcogenides as quantum simulators of Hubbard model [1, 2], and discuss recent experimental realizations. Then I will focus on twisted bilayer graphene (TBG) and show how the interplay between many-body interactions and Bloch band symmetry of TBG can lead to unconventional superconductivity [3, 4]. Finally, I will discuss quantum anomalous Hall insulators in TBG and demonstrate their stability against spin/valley magnon excitations [5]. I will describe the effects of quantum geometry on spin stiffness and show that Berry curvature of Moiré bands helps to stiffen spin magnons.
1. F. Wu, T. Lovorn, E. Tutuc, A. H. MacDonald, Phys. Rev. Lett. 121, 026402 (2018).
2. F. Wu, T. Lovorn, E. Tutuc, I. Martin, A. H. MacDonald, Phys. Rev. Lett. 122, 086402 (2019).
3. F. Wu, A. H. MacDonald, and I. Martin, Phys. Rev. Lett. 121, 257001 (2018).
4. F. Wu, E. Hwang, and S. Das Sarma, Phys. Rev. B 99, 165112 (2019).
5. F. Wu, S. Das Sarma, arXiv:1908.05417 (2019).

February 11, Tuesday, 2:30 pm
Eugene Demler (Harvard), in conjunction with his JQI seminar
Nonlinear optics with collective excitations and photoinduced superconductivity
This talk will review the recent progress in theoretical modeling of nonequilibrium dynamics of electron-phonon systems. There will be an emphasis on understanding experimental observations of photoinduced superconductivity.
Host Jay Sau; last update 2020-1-26 by Victor Yakovenko

March 10, Tuesday, 2 pm
Ronny Thomale (Theoretische Physik I, Universitat Wurzburg), see also his JQI and QMC seminars
The Quest of the Kagome Hubbard Model
Since its (re-)discovery in the second half of the 20th century, the lattice of corner-sharing triangles called kagome has become one of the key domains featuring paradigmatic models for exotic quantum electronic states of matter. Depending on the filling, the Hubbard model on the kagome lattice exhibits several fascinating phases subject to contemporary research in condensed matter physics, ranging from topological spin liquids over correlated Dirac metals and unconventional superconductivity to spin-type and charge-type Peierls phases as well as turbulent hydrodynamic flow. I will discuss recent progress in theory to understand such scenarios of correlated electron systems on the kagome lattice.
Host Jay Sau; last update 2020-3-4 by Victor Yakovenko

March 11, Wednesday, 11 am
Gregory Bentsen (Princeton University), QuICS seminar at CMTC location
Tunable geometry and fast scrambling in nonlocal spin networks
The past decade has seen a dramatic increase in the degree, quality, and sophistication of control over quantum-mechanical interactions available between artificial degrees of freedom in a variety of experimental platforms. The geometrical structure of these interactions, however, remains largely constrained by the underlying rectilinear geometry of three-dimensional Euclidean space. At the same time, there has been growing interest in exploring many-body dynamics in systems, such as the SYK model and tensor network models, for which the interaction structure bears little or no resemblance to Euclidean space. Inspired by these complementary developments, here we study a tunable, nonlocal spin network that can be engineered using cold atoms coupled to an optical cavity. The network exhibits two distinct notions of emergent geometry -- linear and treelike -- that can be accessed using a single tunable parameter. In either of these two extreme limits, we find a succinct description of the resulting dynamics in terms of two distinct metrics on the network, encoding a notion of either linear or treelike distance between spins. Moreover, at the crossover between these two regimes, the spin network becomes highly connected and exhibits signatures of fast scrambling. These observations highlight the essential role played by the geometry of the interaction structure in determining a system's dynamics, and raise prospects for novel studies of nonlocal and highly chaotic quantum dynamics in near-term experiments.
Host Brain Swingle; last update 2020-3-6 by Victor Yakovenko

CMTC Seminar Committee as of 2020-1-19:
Jay Sau, Maissam Barkeshli, Brian Swingle, Yi-Ting Hsu, and Danny Bulmash

For the earlier CMTC talks, see this page