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CMTC Talks
CMTC talks are held in room 4402 Atlantic Building unless an alternate location is indicated
For all Zoom seminars email rcawthor@umd.edu for details

Fall 2020

December 8, Tuesday, 11am
Pablo Jarillo-Herrero (MIT), Zoom seminar
Title: TBA
Abstract: TBA
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.
References:
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