6/2/2021 Quantum Matter Seminar
2021-06-02 10:30 - 12:00 (America/New_York)

6/9/2021 Quantum Matter Seminar
2021-06-09 10:30 - 12:00

6/10/2021 Quantum Matter Seminar
2021-06-10 10:30 - 12:00

6/16/2021 Quantum Matter Seminar
2021-06-16 10:30 - 12:00

6/17/2021 Quantum Matter Seminar
2021-06-17 10:30 - 12:00

7/7/2021 Quantum Matter Seminar
2021-07-07 10:30 - 12:00 | Virtual

7/8/2021 Quantum Matter Seminar
2021-07-08 20:00 - 21:30

7/14/2021 Quantum Matter Seminar
2021-07-14 11:30 - 12:00

Hybrid Fracton Orders
2021-07-15 10:30 - 12:30 | Virtual
Nathanan Tantivasadakarn (Harvard)
Video
Title: Hybrid Fracton Orders
Abstract: I will introduce a family of gapped quantum phases that exhibit the phenomenology of both conventional three-dimensional topological orders and fracton orders called “Hybrid Fracton Orders”. First, I will present the simplest example of such an order: the “Hybrid X-cube” model, where excitations can be labeled identically to those of the Z2 toric code tensored with the Z2 X-cube model, but exhibit fusion and braiding properties between the two sets of excitations. Next, I will provide a general construction of hybrid fracton orders which inputs a finite group G and an abelian normal subgroup N and produces an exactly solvable model. Such order can host non-abelian fracton excitations when G is non-abelian. Furthermore, the mobilities of a general excitation is dictated by the choice of N, from which by varying, one can view as “interpolating” between a pure 3D topological order and a pure fracton order.
Based on 2102.09555 and 2106.03842

7/21/2021 Quantum Matter Seminar
2021-07-21 10:30 - 12:00

7/22/2021 Quantum Matter Seminar
2021-07-22 08:00 - 09:30

The nu=5/2 enigma: Recent insights from theory and experiment
2021-07-29 10:30 - 12:00 | Virtual
peaker: Ady Stern & David Mross (Weizmann)
Speaker: Ady Stern & David Mross (Weizmann
Title: The nu=5/2 enigma: Recent insights from theory and experiment
Abstract: Non-Abelian phases of matter have long inspired quantum physicists across various disciplines. The strongest experimental evidence of such a phase arises in quantum Hall systems at the filling factor 5/2 but conflicts with decades of numerical works. We will briefly introduce the 5/2 plateau and explain some of the key obstacles to identifying its topological order. We will then describe recent experimental and theoretical progress, including a proposal for resolving the 5/2 enigma based on electrical conductance measurements.

Naturalness and muon anomalous magnetic moment
2021-09-01 18:31 - 19:31 | Virtual
Title: Naturalness and muon anomalous magnetic moment
Abstract: We study a model for explaining the apparent deviation of the muon anomalous magnetic moment, (g-2), from the Standard Model expectation. There are no new scalars and hence no new hierarchy puzzles beyond those associated with the Standard model Higgs; the only new particles that are relevant for (g-2) are vector-like singlet and doublet leptons. Interestingly, this simple model provides a calculable example violating the Wilsonian notion of naturalness: despite the absence of any symmetries prohibiting its generation, the coefficient of the naively leading dimension-six operator for (g−2) vanishes at one-loop. While effective field theorists interpret this either as a surprising UV cancellation of power divergences, or as a delicate cancellation between matching UV and calculable IR corrections to (g−2) from parametrically separated scales, there is a simple explanation in the full theory: the loop integrand is a total derivative of a function vanishing in both the deep UV and IR. The leading contribution to (g−2) arises from dimension-eight operators, and thus the required masses of new fermions are lower than naively expected, with a sizable portion of parameter space already covered by direct searches at the LHC. All of the the viable parameter can be probed by the LHC and planned future colliders.

Exotic quantum matter: From lattice gauge theory to hyperbolic lattices
2021-09-02 18:34 - 19:34 | Virtual
Title: Exotic quantum matter: From lattice gauge theory to hyperbolic lattices
Abstract: This talk, in two parts, will discuss two (unrelated) instances of exotic quantum matter. In the first part, I will discuss quantum critical points describing possible transitions out of the Dirac spin liquid, towards either symmetry-breaking phases or topologically ordered spin liquids. I will also comment on the role of instanton zero modes for symmetry breaking in parton gauge theories. In the second part, I will propose an extension of Bloch band theory to hyperbolic lattices, such as those recently realized in circuit QED experiments, based on ideas from algebraic geometry and Riemann surface theory.

Cornering the universal shape of fluctuations and entanglement
2021-09-08 18:37 - 19:37 | Virtual
Title: Cornering the universal shape of fluctuations and entanglement
Abstract: Understanding the fluctuations of observables is one of the main goals in physics. We investigate such fluctuations when a subregion of the full system can be observed, focusing on geometries with corners. We report that the dependence on the opening angle is super-universal: up to a numerical prefactor, this function does not depend on anything, provided the system under study is uniform, isotropic, and correlations do not decay too slowly. The prefactor contains important physical information: we show in particular that it gives access to the long-wavelength limit of the structure factor. We illustrate our findings with several examples: classical fluids, fractional quantum Hall (FQH) states, scale invariant quantum critical theories, and metals. Finally, we discuss connections with the entanglement entropy, including new results for Laughlin FQH states.
Ref: arXiv:2102.06223

Quantum gravity from quantum matter
2021-09-09 18:38 - 19:38 | Virtual
Title: Quantum gravity from quantum matter
Abstract: We present a model of quantum gravity in which dimension, topology and geometry of spacetime are collective dynamical variables that describe the pattern of entanglement of underlying quantum matter. As spacetimes with arbitrary dimensions can emerge, the gauge symmetry is generalized to a group that includes diffeomorphisms in general dimensions. The gauge symmetry obeys a first-class constraint operator algebra, and is reduced to a generalized hypersurface deformation algebra in states that exhibit classical spacetimes. In the semi-classical limit, we find a saddle-point solution that describes a series of (3+1)-dimensional de Sitter-like spacetimes with the Lorentzian signature bridged by Euclidean spaces in between.

More Exact Results in Gauge Theories: Confinement and Chiral Symmetry Breaking
2021-09-10 18:44 - 19:44 | Virtual
Title: More Exact Results in Gauge Theories: Confinement and Chiral Symmetry Breaking
Abstract: In this follow-up to Hitoshi Murayama’s talk “Some Exact Results in QCD-like and Chiral Gauge Theories”, I present a detailed analysis of the phases of $SO(N_c)$ gauge theory.
Starting with supersymmetric $SO(N_c)$ with $N_F$ flavors, we extrapolate to the non-supersymmetric limit using anomaly-mediated supersymmetry breaking (AMSB). Interestingly, the abelian Coulomb and free magnetic phases do not survive supersymmetry breaking and collapse to a confining phase. This provided one of the first demonstrations of true confinement with chiral symmetry breaking in a non-SUSY theory.

Three-particle mechanism for pairing and superconductivity
2021-09-15 18:46 - 19:46 | Virtual
Title: Three-particle mechanism for pairing and superconductivity
Abstract: I will present a new mechanism and an exact theory of electron pairing due to repulsive interaction in doped insulators. When the kinetic energy is small, the dynamics of adjacent electrons on the lattice is strongly correlated. By developing a controlled kinetic energy expansion, I will show that two doped charges can attract and form a bound state, despite and because of the underlying repulsion. This attraction by repulsion is enabled by the virtual excitation of a third electron in the filled band. This three-particle pairing mechanism leads to a variety of novel phenomena at finite doping, including spin-triplet superconductivity, pair density wave, BCS-BEC crossover and Feshbach resonance involving “trimers”. Possible realizations in moire materials, ZrNCl and WTe2 will be discussed.
[1] V. Crepel and L. Fu, Science Advances 7, eabh2233 (2021)
[2] V. Crepel and L. Fu, arXiv:2103.12060
[3] K. Slagle and L. Fu, Phys. Rev. B 102, 235423 (2020)

The Hilbert Space of large N Chern-Simons matter theories
2021-09-16 18:47 - 19:47 | Virtual
Title: The Hilbert Space of large N Chern-Simons matter theories
Abstract: We demonstrate that all known formulae for the thermal partition function for large N Chern Simons matter theory admit a simple Hilbert Space interpretation. In each case this quantity equals the partition function of an associated ungauged large $N$ matter theory with a particular local Lagrangian with one additional element: the Fock Space of this associated theory is projected down to the subspace of its WZW singlets. This projection, in particular, implies the previously encountered `Bosonic Exclusion Principle’, namely that no single particle state can be occupied by more than $k_B$ particles ($k_B$ is the Chern Simons level). Unlike its Gauss Law counterpart, the WZW constraint does not trivialize in the large volume limit. However thermodynamics does simplify in this limit; the final partition function reduces to a product of partition functions associated with each single particle state. These individual single particle state partition functions are a one parameter generalizations of their free boson and free fermion counterparts, and reduce to the later at extreme values of the ‘t Hooft coupling. At generic values of the rank and the level the occupation statistics of each energy level is given by a $q$ deformation of the usual free formulae of Bose and Fermi statistics.

Strong Coupling Theory of Magic-Angle Graphene: A Pedagogical Introduction
2021-09-17 18:54 - 19:54 | Virtual
Title: Strong Coupling Theory of Magic-Angle Graphene: A Pedagogical Introduction
Abstract: In this talk, I will review a recently developed strong coupling theory of magic-angle twisted bilayer graphene. An advantage of this approach is that a single formulation can capture both the insulating and superconducting states, and with a few simplifying assumptions, can be treated analytically. I begin by reviewing the electronic structure of magic angle graphene’s flat bands, in a limit that exposes their peculiar band topology and geometry. I will show how similarities between the flat bands and the lowest Landau level can provide valuable insights into the effect of interactions and form the basis for an analytic treatment of the problem. At integer fillings, this approach points to flavor ordered insulators, which can be captured by a sigma-model in its ordered phase. Remarkably, topological textures of the sigma model carry electric charge which enables the same theory to describe the doped phases away from integer filling. I will show how this approach can lead to superconductivity on disordering the sigma model, and estimate the Tc for the superconductor. I will highlight the important role played by an effective super-exchange coupling both in pairing and in setting the effective mass of Cooper pairs. At the end, I will show how this theory provides criteria to predict which multilayer graphene stacks are expected to superconduct including the recently discovered alternating twist trilayer platform.

Symmetry types in QFT and the CRT theorem
2021-09-22 11:33 - 13:33 | Virtual
Title: Symmetry types in QFT and the CRT theorem
Abstract: I will discuss ideas around symmetry and Wick rotation contained in joint work with Mike Hopkins (https://arxiv.org/abs/1604.06527). This includes general symmetry types for relativistic field theories and their Wick rotation. I will then indicate how the basic CRT theorem works for general symmetry types, focusing on the case of the pin groups. In particular, I expand on a subtlety first flagged by Greaves-Thomas.

Applications of instantons, sphalerons and instanton-dyons in QCD
2021-09-23 11:47 - 13:47 | Virtual
Title: Applications of instantons, sphalerons and instanton-dyons in QCD
Abstract: I start with a general map of gauge topology, including monopoles, instantons and instanton-dyons. Then comes reminder of the “topological landscape”, the minimal energy gauge field configurations, as a function of Chern-Simons number Ncs and r.m.s. size. It includes “valleys” at integer Ncs separated by mountain ridges. The meaning of instantons, instanton-antiinstanton “streamlines” or thimbles, and sphalerons are reminded, together with some proposal to produce sphalerons at LHC and RHIC.
Applications of instanton ensembles, as a model of QCD vacuum, are mostly related to their fermionic zero modes and t’Hooft effective Lagrangian, which explains explicit and spontaneous breaking of chiral symmetries. Recent applications are related with hadronic wave functions, at rest and in the light front (LFWFs). Two application would be spin-dependent forces and the so called “flavor asymmetry of antiquark sea” of the nucleons. At temperatures comparable to deconfinement transition, instantons get split into constituents called instanton-dyons. Studies of their ensemble explains both deconfinement and chiral transitions, in ordinary and deformed QCD.

Oscillations in the thermal conductivity of a spin liquid*
2021-09-29 11:48 - 13:48 | Virtual
Title: Oscillations in the thermal conductivity of a spin liquid*
Abstract: The layered honeycomb magnet alpha-RuCl3 orders below 7 K in a zigzag phase in zero field. An in-plane magnetic field H||a suppresses the zigzag order at 7 Tesla, leaving a spin-disordered phase widely believed to be a quantum spin liquid (QSL) that extends to ~12 T. We have observed oscillations in the longitudinal thermal conductivity Kxx vs. H from 0.4 to 4 K. The oscillations are periodic in 1/H (with a break-in-slope at 7 T). The amplitude function is maximal in the QSL phase (7 –11.5 T). I will describe a benchmark for crystalline disorder, the reproducibility and intrinsic nature of the oscillations, and discuss implications for the QSL state. I will also show detailed data on the thermal Hall conductivity Kxy measured from 0.4 K to 10 K and comment on recent half-quantization results.
*Czajka et al., Nature Physics 17, 915 (2021).
Collaborators: Czajka, Gao, Hirschberger, Lampen Kelley, Banerjee, Yan, Mandrus and Nagler.

Line defects in CFTs: Renormalization group flows and semiclassical limits
2021-10-06 11:49 - 13:49 | Virtual
Title: Line defects in CFTs: Renormalization group flows and semiclassical limits
Abstract: I will discuss line defects in d-dimensional Conformal Field Theories (CFTs). In the first part of the talk, I will argue that the ambient CFT places nontrivial constraints on Renormalization Group (RG) flows on such line defects. I will show that the flow on line defects is consequently irreversible and furthermore a canonical decreasing entropy function exists. This construction generalizes the g theorem to line defects in arbitrary dimensions. In the second part of the talk, I will present some applications. In particular, I will discuss impurities with large isospin S for some O(3) symmetric theories in the epsilon expansion. For sufficiently large S diagrammatic perturbation theory breaks down, and these are studied in a semiclassical expansion at fixed epsilon S.

A tour of categorical symmetry
2021-10-07 11:54 - 13:54 | Virtual
Title: A tour of categorical symmetry
Abstract: I will discuss some perspectives on symmetry coming from the study of topological defects in quantum field theory. I will argue that we should take topological defects themselves to define the symmetries of QFT. This gives us a view of the “category of QFTs”. I will describe some examples of these “categorical symmetries”, their applications, and some open problems.

UV/IR and Effective Field Theory
2021-10-07 20:30 - 22:00
Speaker: Nima Arkani-Hamed (IAS Princeton)
Title: UV/IR and Effective Field Theory

Electric-magnetic duality and the Geometric Langlands duality
2021-10-21 15:30 - 17:00 | Virtual
Title: Electric-magnetic duality and the Geometric Langlands duality
Abstract: I will give a pedagogical review of the connection between electric-magnetic duality and the Geometric Langlands duality.

Anomaly resolution via decomposition
2021-10-29 12:00 - 13:00 | Virtual
Speaker: Eric Sharpe (Virginia Tech)
Title: Anomaly resolution via decomposition
Abstract: In this talk we will discuss a method of anomaly resolution due to Wang-Wen-Witten in the special case of (1+1) dimensional theories. Briefly, for our purposes, Wang-Wen-Witten argued that an ill-defined anomalous orbifold [X/G] could be resolved by extending G to a larger group and adding suitable phases. We analyze this process from the perspective of decomposition, a property of (1+1)-dimensional theories with “one-form symmetries” first described in 2006. Examples of such theories include orbifolds with trivially-acting subgroups, of which the extensions of [X/G] are examples. After a review of decomposition, we will see that decomposition implies that in (1+1) dimensions, the Wang-Wen-Witten procedure results in orbifolds that are equivalent to disjoint unions of orbifolds of X by explicitly nonanomalous subgroups of G.

Integrability and chaos of 1+1d chiral edge states
2021-10-29 12:00 - 13:00
Speaker: Biao Lian (Princeton)
Title: Integrability and chaos of 1+1d chiral edge states
Abstract: I will talk about the integrability and chaos of 1+1d interacting chiral edge states, which may arise on the edge of 2+1d topological phases. We show that integrable chiral Luttinger liquid is not always a good low energy description of the edge states, and marginal interactions can significantly affect their spectrum and integrability. We first study N identical chiral Majorana fermion modes with random 4-fermion interactions, where we show that the system undergoes a transition from integrable to quantum chaotic as N increases. The large N limit defines a chiral SYK model where the Lyapunov exponent in the out-of-time-ordered correlation can be solved analytically. I will also present a chiral SY model consisting of N interacting SU(M)_1 WZW models, which host anyons and exhibits similar quantum chaos for Abelian anyons. Lastly, I will talk about the analytical and numerical study of the 4/3 FQH edge theory, which shows unusual behavior in its integrability.

Non-Invertible Duality Defects in 3+1 Dimensions
2021-11-03 14:00 - 15:30 | Virtual
Speaker: Clay Cordova (U Chicago)
Title: Non-Invertible Duality Defects in 3+1 Dimensions
Abstract: For any quantum system invariant under gauging a higher-form global symmetry, we construct a non-invertible topological defect by gauging in only half of spacetime. This generalizes the Kramers-Wannier duality line in 1+1 dimensions to higher spacetime dimensions. We focus on the case of a one-form symmetry in 3+1 dimensions and determine the fusion rule. From modular invariance and a direct analysis of one-form symmetry-protected topological phases, we show that the existence of certain kinds of duality defects is intrinsically incompatible with a trivially gapped phase. By further assuming time-reversal symmetry, we find that the presence of certain duality defects implies that the low-energy phase has to be gapless unless the one-form symmetry is spontaneously broken. We give an explicit realization of this duality defect in the free Maxwell theory where the duality defect is realized by a Chern-Simons coupling between the gauge fields from the two sides.
