Quantum Matter
Frontier Seminars
Organizers:
Anton Burkov, Tim Hsieh, Yong-Baek Kim, Sung-Sik Lee, Chong Wang
Schedule (Winter/Spring 2022)
Monday, February 7, 2022
12:00pm (EST)
Speaker: Jennifer Cano (Stony Brook University)
Title: Topological Twistronics
Abstract: Twisting van der Waals heterostructures to induce correlated many-body states provides a novel tuning mechanism in solid-state physics, launching the field of “twistronics.” In this talk, we apply twistronics to renormalize the Dirac cone on the surface of a 3D topological insulator. Our goal is to realize tunable interacting topological phases. To achieve this goal, we consider two different platforms: 1) twisted heterostructures in 2D and 3D; and 2) the effect of a moire superlattice potential. In all cases, we consider the renormalization of the non-interacting band structure, specifically, whether magic angles are possible and where van Hove singularities appear. Finally, we show that interactions drive a magnetic instability on the surface of a TI in the presence of a superlattice potential, resulting in a novel gapless skyrmion lattice.
Note: For this talk only, please use the link provided in the Perimeter Institute seminar page.
Chair: Chong Wang
Monday, February 14, 2022
12:00pm (EST)
Speaker: Romain Vasseur (University of Massachusetts, Amherst)
Abstract: Monitored quantum circuits (MRCs) exhibit a measurement-induced phase transition between area-law and volume-law entanglement scaling. In this talk, I will argue that MRCs with a conserved charge additionally exhibit two distinct volume-law entangled phases that cannot be characterized by equilibrium notions of symmetry-breaking or topological order, but rather by the non-equilibrium dynamics and steady-state distribution of charge fluctuations. These include a charge-fuzzy phase in which charge information is rapidly scrambled leading to slowly decaying spatial fluctuations of charge in the steady state, and a charge-sharp phase in which measurements collapse quantum fluctuations of charge without destroying the volume-law entanglement of neutral degrees of freedom. I will present some statistical mechanics and effective field theory approaches to such charge-sharpening transitions.
Chair: Tim Hsieh
Monday, February 28, 2022
12:00pm (EST)
Speaker: Philip Kim (Harvard University)
Title: Toward realization of novel superconductivity based on twisted van der Waals Josephson junction in Cuprates (talk link)
Abstract: Twisted interfaces between stacked van der Waals Cuprate crystals enable tunable Josephson coupling, utilizing anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate high-temperature Josephson junctions with an atomically sharp twisted interface between Bi_2Sr_2CaCu_2O_{8+x} crystals. The critical current density J_c sensitively depends on the twist angle. While near 0 degree twist, J_c nearly matches that of intrinsic junctions, it is suppressed almost 2-orders of magnitude but remained finite near 45 degree. J_c also exhibits non-monotonic behavior versus temperature due to competition between two supercurrent contributions from nodal and anti-nodal regions of the Fermi surface. Near 45 degree twist angle, we observe two-period Fraunhofer interference patterns and fractional Shapiro steps at half integer values, a signature of co-tunneling Cooper pairs necessary for high temperature topological superconductivity.
Chair: Yong-Baek Kim
Monday, March 7, 2022
12:00pm (EST)
Speaker: Giulia Semeghini (Harvard University)
Title: New frontiers in quantum simulation and computation with neutral atom arrays
Abstract: Learning how to create, study, and manipulate highly entangled states of matter is key to understanding exotic phenomena in condensed matter and high energy physics, as well as to the development of useful quantum computers. In this talk, I will discuss recent experiments where we demonstrated the realization of a quantum spin liquid phase using Rydberg atoms on frustrated lattices and a new architecture based on the coherent transport of entangled atoms through a 2D array. Combining these results with novel technical tools on atom array platforms could open a broad range of possibilities for the exploration of entangled matter, with powerful applications in quantum simulation and information.
Chair: Anton Burkov
Monday, March 21, 2022
12:00pm (EDT)
Speaker: Cenke Xu (UC Santa Barbara)
Title: Quantum States of Matter with Fractal Symmetries: Theory and realization with Rydberg atoms
Abstract: In recent years, generalizations of the notion of symmetry have significantly broadened our view on states of matter. We will discuss some recent progress of understanding and realizing the "fractal symmetry", where the symmetric charge i.e. the generator of the symmetry is defined on a fractal subset of the system with a noninteger or more generally irrational Hausdorff dimension. We will introduce a series of models with exotic fractal symmetries, which can in general be deduced from a "Pascal Triangle" (also called Yang Hui Triangle in ancient China) symmetry. We will discuss their various features including quantum phase transitions. We will also discuss the potential realization of these phases and phase transitions in experimental systems, such as the highly tunable platform of Rydberg atoms.
Chair: Sung-Sik Lee
Monday, March 28, 2022
12:00pm (EDT)
Speaker: Andrei Bernevig (Princeton University)
Title: An Exact Map Between the TBG (and multilayers) and Topological Heavy Fermions
Abstract: Magic-angle (θ=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 Γ 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.
Chair: Yong-Baek Kim
Monday, May 2, 2022
12:00pm (EDT)
Speaker: Michael Hermele (University of Colorado at Boulder)
Title: Subsystem symmetry fractionalization in two dimensions
Abstract: Unlike conventional global symmetries that act on all of space, subsystem symmetries act only on rigid subspaces such as lines, planes or fractal regions. Such symmetries are of interest for their close connection to fracton physics and in their own right. In this talk, I discuss phenomena that can occur in two-dimensional systems with subsystem symmetry and anyon excitations, where the symmetry fractionalizes on anyon excitations. This turns out to be manifest in the mobility of the fractionalized excitations under dynamics that respect the symmetry. Such symmetry fractionalization was previously thought to be impossible, and indeed there are a number of differences from conventional symmetry fractionalization. I will present a general framework to describe subsystem symmetry fractionalization in two dimensions, and give a number of simple examples of this phenomenon in commuting Pauli Hamiltonians.
This talk is based on arXiv:2203.13244, work in collaboration with David Stephen, Arpit Dua, José Garre-Rubio and Dominic Williamson.
Chair: Sung-Sik Lee
Schedule (Fall 2021)
Monday, September 27, 2021
12:00pm (EDT)
Speaker: Jason Alicea (Caltech)
Abstract: Twisted bilayer graphene (TBG) realizes an exquisitely tunable, strongly interacting system featuring superconductivity and various correlated insulating states. In this talk I will introduce gate-defined wires in TBG as an enticing platform for Majorana-based fault-tolerant qubits. Our proposal notably relies on “internally” generated superconductivity in TBG – as opposed to “external” superconducting proximity effects commonly employed in Majorana devices – and may operate even at zero magnetic field. I will also describe how electrical measurements of gate-defined wires can reveal the nature of correlated insulators and shed light on the Cooper-pairing mechanism in TBG.
Chair: Anton Burkov
Monday, October 4, 2021
12:00pm (EDT)
Speaker: Yin-Chen He (Perimeter Institute)
Abstract: Critical states of matter are a class of highly entangled quantum matter with various interesting properties and form important bases for emergence of a variety of novel quantum phases. Such states pose serious challenges for the community due to their strongly interacting nature. In this talk, I will discuss our recent progress on tackling critical quantum matter using the method of conformal bootstrap. I will start with introducing several representative examples of critical quantum matter, including the familiar deconfined quantum phase transition, U(1) Dirac spin liquid phase, and the newly proposed Stiefel liquid phase. Next I will focus on the SU(N) deconfined phase transition (i.e. scalar QED), and demonstrate that they can be solved by conformal bootstrap, namely we have obtained their bootstrap kinks and islands.
Chair: Chong Wang
Monday, October 18, 2021
12:00pm (EDT)
Speaker: Matthew P. A. Fisher (UC Santa Barbara)
Abstract: Traditionally, quantum many-body theory has focussed on ground states and equilibrium properties of spatially extended systems, such as electrons and spins in crystalline solids. In recent years “noisy intermediate scale quantum computers” (NISQ) have emerged, providing new opportunities for controllable non-equilibrium many-body systems. In such dynamical quantum systems the inexorable growth of non-local quantum entanglement is expected, but monitoring (by making projective measurements) can compete against entanglement growth. In this talk I will overview theoretical work exploring the behavior of “monitored” quantum circuits, which can exhibit a novel quantum dynamical phase transition between a weak measurement phase and a quantum Zeno phase, the former which we characterize in detail. Accessing such physics in the lab is challenged by the need for post-selection, which might be circumnavigated by decoding using active error correction, conditioned on the measurement outcomes, as will be described in systems with Z2 symmetry.
Chair: Tim Hsieh
Monday, October 25, 2021
12:00pm (EDT)
Speaker: Peter Armitage (Johns Hopkins University)
Abstract: Kitaev quantum spin liquids (QSLs) are exotic states of matter that are predicted to host Majorana fermions and gauge flux excitations. However, so far all known Kitaev QSL candidates are known to have appreciable non-Kitaev interactions that pushes these systems far from the QSL regime. Co based magnets have been proposed to be perhaps a more ideal platform for realizing Kitaev QSLs. In this talk I will show evidence for a Kitaev interactions in both the quasi-one-dimensional ferromagnet CoNb2O6 as well as the hexagonal magnet BaCo2(AsO4). Although it is usually believed to be the best material realization of a 1D Ising chain, by combining terahertz spectroscopy and calculations we have shown that CoNb2O6 is well described by a model with bond-dependent interactions. We call this model the ‘twisted Kitaev chain’, as these interactions are similar to those of the honeycomb Kitaev spin liquid. The ferromagnetic ground state of CoNb2O6 arises from the compromise between two axes. Owing to this frustration, even at zero field domain walls have quantum motion, which is described by the celebrated Su–Schrieffer–Heeger model of polyacetylene and shows rich behavior as a function of field. Most recently, we have shown also that the honeycomb cobalt-based Kitaev QSL candidate, BaCo2(AsO4)2, has dominant Kitaev interactions. Due to only small non-Kitaev terms a magnetic continuum consistent with Majorana fermions and the existence of a Kitaev QSL can be induced by a small out-of-plane-magnetic field. Our results demonstrate BaCo2(AsO4)2 as a far more ideal version of Kitaev QSL compared with other candidates.
C. M Morris et al. "Hierarchy of bound states in the one-dimensional ferromagnetic Ising chain CoNb2O6 investigated by high-resolution time-domain terahertz spectroscopy.” Phys. Rev. Lett. 112.13 137403 (2014).
C.M. Morris et al. “Duality and domain wall dynamics in a twisted Kitaev chain”, Nature Physics
volume 17, pages 832–836 (2021).
X. Zhang, et al., "In- and out-of-plane field induced quantum spin-liquid states in a more ideal Kitaev material: BaCo2(AsO4)”, https://arxiv.org/abs/2106.13418
Chair: Yong-Baek Kim
Monday, November 1, 2021
12:00pm (EDT)
Speaker: Hitesh Changlani (NHMFL, Florida State University)
Abstract: Quantum spin liquids (QSL) are enigmatic phases of matter characterized by the absence of symmetry breaking and the presence of fractionalized quasiparticles. While theories for QSLs are now in abundance, tracking them down in real materials has turned out to be remarkably tricky. I will focus on two sets of studies on QSLs in three dimensional pyrochlore systems, which have proven to be particularly promising. In the first work, we analyze the newly discovered spin-1 pyrochlore compound NaCaNi2F7 whose properties we find to be described by a nearly idealized Heisenberg Hamiltonian [1]. We study its dynamical structure factor using molecular dynamics simulations, stochastic dynamical theory, and linear spin wave theory, all of which reproduce remarkably well the momentum dependence of the experimental inelastic neutron scattering intensity as well as its energy dependence (with the exception of the lowest energies) [2]. We apply many of the lessons learnt to Ce2Zr2O7 which has been recently shown to exhibit strong signatures of QSL behavior in neutron scattering experiments. Its magnetic properties emerge from interacting cerium ions, whose ground state doublet (with J = 5/2,m_J = ±3/2) arises from strong spin orbit coupling and crystal field effects. With the help of finite temperature Lanczos calculations, we determine the low energy effective spin-1/2 Hamiltonian parameters using which we reproduce all the prominent features of the dynamical spin structure factor. These parameters suggest the realization of a U(1) π-flux QSL phase [3] and they allow us to make predictions for responses in an applied magnetic field that highlight the important role played by octupoles in the disappearance of spectral weight.
*Supported by FSU and NHMFL, funded by NSF/DMR-1644779 and the State of Florida, and NSF DMR-2046570
[1] K. W. Plumb, H. J. Changlani, A. Scheie, S. Zhang, J. W. Krizan, J. A. Rodriguez-Rivera, Yiming Qiu, B. Winn, R. J. Cava & C. L. Broholm, Nature Physics 15, 54–59 (2019)
[2] S. Zhang, H. J. Changlani, K. W. Plumb, O. Tchernyshyov, and R. Moessner, Phys. Rev. Lett. 122, 167203 (2019)
[3] A.Bhardwaj, S.Zhang, H.Yan, R. Moessner, A. H. Nevidomskyy, H. J. Changlani, arXiv:2108.01096 (2021), under review.
Chair: Yong-Baek Kim
Monday, November 8, 2021
12:00pm (EST)
Speaker: Subir Sachdev (Harvard)
Title: Planckian Metals (talk link)
Abstract:
Many modern materials feature a “Planckian metal”: a phase of electronic quantum matter without quasiparticle excitations, and relaxation in a time of order Planck's constant divided by the absolute temperature. I will review recent progress in understanding such metals using insights from the Sachdev-Ye-Kitaev model of many-particle quantum dynamics. I will also note connections to progress in understanding the quantum nature of black holes.
Chair: Sung-Sik Lee
Monday, November 15, 2021
12:00pm (EST)
Speaker: Meng Cheng (Yale University)
Abstract: 2+1d U(1) Chern-Simons (CS) gauge theories provide a simple and complete description of all 2+1d Abelian topological phases. In this talk, I will discuss how the theoretical framework can be used to explore 3+1d quantum phases of matter. The 3+1d generalization involves an infinite number of 2+1d U(1) gauge fields, which can be thought of as living in different spatial layers, coupled through CS terms. When the theory is fully gapped, it describes a new kind of fracton topological order, where all excitations are restricted to move in planes. We will discuss how these new examples compare with existing constructions of fracton order. Interestingly, for certain couplings the theory becomes gapless. I will discuss our current understanding of the nature of the gapless theories, with the help of a microscopic realization with coupled wire constructions. I will argue that these models realize stable gapless phases, including in particular new types of compressible quantum liquids in (3+1)d.
Chair: Chong Wang
Monday, November 22, 2021
12:00pm (EST)
Speaker: Arun Paramekanti (University of Toronto)
Abstract: Traditionally, magnetism in solids deals with ordering patterns of the electron magnetic dipole moment, as probed, for instance, via neutron diffraction. However, f-electron heavy fermion systems are well-known candidates for more complex forms of symmetry breaking, involving higher-order magnetic or electric multipoles. In this talk, I will discuss our recent theoretical proposal for Ising octupolar order in d-orbital systems, which appears to explain a wide range of experiments in certain 5d transition metal oxides with spin-orbit coupling. The proposed Ising ferro-octupolar order is shown to be linked to a type of orbital loop-current order. Deviations from cubic symmetry, via strain or surfaces, induces a transverse field on the octupolar order which can lead to surface quantum phase transitions, or transitions in thin films or in strained 3D crystals. We propose further experimental tests of our proposal.
Chair: Anton Burkov
Monday, December 6, 2021
12:00pm (EST)
Speaker: Andrew Potter (University of British Columbia)
Title: Realizing a dynamical topological phase without symmetry protection in trapped ions
Abstract: In thermal equilibrium, 1d bosonic systems (e.g. spin- or qubit- chains) cannot support intrinsically topological phases without symmetry protection. For example, the edge states of the Haldane spin chain are fragile to magnetic fields, in contrast to the absolutely stable Majorana edge states of a topological superconducting wire of fermionic electrons. This fragility is a serious drawback to harnessing topological edge states as protected quantum memories in existing AMO and qubit platforms for quantum simulation and information processing. In this talk, I will present evidence for a non-equilibrium topological phase of quasiperiodically-driven trapped ion chains, that exhibits topological edge states that are protected purely by emergent dynamical symmetries that cannot be broken by microscopic perturbations. This represents both the first experimental realization of a non-equilibrium quantum phase, and the first example of a 1d bosonic topological phase that does not rely on symmetry-protection.
Chair: Tim Hsieh
Monday, December 13, 2021
12:00pm (EST)
Speaker: Sri Raghu (Stanford University)
Title: From Anderson insulators to Random singlets: DMRG studies of 1d systems
Abstract: Local moment formation is a ubiquitous phenomenon in disordered semiconductors, alloys and related systems. They likely play an important role in universal behavior near metal-insulator transitions. Much work has been done in this area starting from the metallic side. We discuss local moment behavior in disordered insulators. Given the lack of analytic tools, we must resort to numerics. Using DMRG, we study disorder and interaction effects in a 1D Hubbard model where all single particle states are localized. For both random fermion hoppings and random chemical potentials, and over some range of half-filling, we find exponential charge and single fermion correlations but power law spin correlations indicative of the random singlet phase. The numerical results can be understood qualitatively by appealing to the U/t >> 1 limit of the Hubbard chain where a remarkably simple picture emerges.
Chair: Anton Burkov
Schedule (Winter/Spring 2021)
Monday, January 25, 2021
12:30pm (EST)
Speaker: Cenke Xu (UC Santa Barbara)
Abstract: Recent years new concepts of symmetries have been developed such as higher form symmetries, and categorical symmetries. The higher form symmetries can be either explicit in a Hamiltonian, or inexplicit as a dual of an ordinary symmetry. The behavior of higher form symmetries are easy to evaluate in phases with gaps. But at quantum criticalities their behaviors are more nontrivial. We evaluate the behaviors of higher form symmetries (either explicit or inexplicit) at various quantum critical points, and demonstrate that for many quantum critical points a universal logarithmic contribution arises, which is analogous to the quantum entanglement entropy. This logarithmic contribution is related to the universal conductance at the quantum critical points, and in some cases can be computed exactly using duality between CFTs developed in last few years. We also evaluate the behavior of categorical symmetries for more exotic cases with subsystem symmetries.
Chair: Anton Burkov
Monday, February 1, 2021
12:30pm (EST)
Speaker: Taylor Hughes (UIUC)
Abstract: In this talk I will present a general framework to distinguish different classes of charge insulators based on whether or not they insulate or conduct higher multipole moments (dipole, quadrupole, etc.). This formalism applies to generic many-body systems that support multipolar conservation laws. Applications of this work provide a key link between recently discovered higher order topological phases and fracton phases of matter.
Chair: Anton Burkov
Monday, February 8, 2021
12:30pm (EST)
Speaker: Oskar Vafek (Magnet Lab, Florida State Univ.)
Abstract: When the twist angle of a bilayer graphene is near the ``magic'' value, there are four narrow bands near the neutrality point, each two-fold spin degenerate. These bands are separated from the rest of the bands by energy gaps. In the first part of the talk, the topology of the narrow bands will be discussed, as well as the associated obstructions --or lack there of -- to construction of a complete localized basis [1,3].
In the second part of the talk, I will present a two stage renormalization group treatment [4] which connects the continuum Hamiltonian at length scales shorter than the moire superlattice period to the Hamiltonian for the active narrow bands only, which is valid at distances much longer than the moire period. Via a progressive numerical elimination of remote bands the relative strength of the one-particle-like dispersion and the interactions within the active narrow band Hamiltonian will be determined, thus quantifying the residual correlations and justifying the strong coupling approach in the final step.
In the last part of the talk, the states favored by electron-electron Coulomb interactions within the narrow bands will be discussed. Analytical and DMRG results based on 2D localized Wannier states [2,5], 1D localized hybrid Wannier states [3] and Bloch states [3,4] will be compared. Topological and symmetry constraints on the spectra of charged and neutral excitation[4] for various ground states, as well as non-Abelian braiding of Dirac nodes[3] , will also be presented.
[1] Jian Kang and Oskar Vafek, Phys. Rev. X 8, 031088 (2018).
[2] Jian Kang and Oskar Vafek, Phys. Rev. Lett. 122, 246401 (2019)
[3] Jian Kang and Oskar Vafek, Phys. Rev. B 102, 035161 (2020)
[4] Oskar Vafek and Jian Kang Phys. Rev. Lett. 125, 257602 (2020)
[5] Bin-Bin Chen, Yuan Da Liao, Ziyu Chen, Oskar Vafek, Jian Kang, Wei Li, Zi Yang Meng arXiv:2011.07602
Chair: Sung-Sik Lee
Monday, February 15, 2021
12:30pm (EST)
Speaker: Aditi Mitra (New York University)
Abstract: In this talk I will begin by introducing symmetry protected topological (SPT) Floquet systems in 1D. I will describe the topological invariants that characterize these systems, and highlight their differences from SPT phases arising in static systems. I will also discuss how the entanglement properties of a many-particle wavefunction depend on these topological invariants. I will then show that the edge modes encountered in free fermion SPTs are remarkably robust to adding interactions, even in disorder-free systems where generic bulk quantities can heat to infinite temperatures due to the periodic driving. This robustness of the edge modes to heating can be understood in the language of strong modes for free fermion SPTs, and almost strong modes for interacting SPTs. I will then outline a tunneling calculation for extracting the long lifetimes of these edge modes by mapping the Heisenberg time-evolution of the edge operator to dynamics of a single particle in Krylov space.
Chair: Sung-Sik Lee
Monday, February 22, 2021
12:30pm (EST)
Speaker: Ashvin Vishwanath (Harvard University)
Abstract: Despite decades of theoretical work, the physical realization of topological order, outside of the fractional quantum Hall effect, has proved to be an elusive goal. Even the simplest example of a time-reversal symmetric topological order, as encountered in the paradigmatic toric code, awaits experimental realization. Key challenges include the lack of physically realistic models in these phases, and of ways to probe their defining properties. I will discuss a simple `Rydberg blockade' model, and describe numerical results that point to (i) a ground state with toric code topological order that could potentially be realized in experiment and (ii) ``smoking gun'' signatures of the phase which be accessed using a dynamic protocol. I will also briefly discuss how a topological qubit can be constructed in this platform by tuning boundaries as well as implications for constructing fault-tolerant quantum memories. Time permitting, a different platform for realizing exotic phases, magic-angle graphene and the special features of its band structure will be described, which make it a prime candidate for realizing fractional quantum Hall topological order even in the absence of a magnetic field.
References: arXiv:2011.12310. and arXiv:1912.09634
Chair: Yong Baek Kim
Monday, March 1, 2021
12:30pm (EST)
Speaker: Xie Chen (Caltech)
Abstract: Fracton models are characterized by an exponentially increasing ground state degeneracy and point excitations with constrained motion. In this talk, I will focus on a prototypical 3D fracton model -- the X-cube model -- and discuss how its ground state degeneracy can be understood from a foliation structure in the model. In particular, we show that there are hidden 2D topological layers in the 3D bulk. To calculate the ground state degeneracy, we can remove the layers until a minimal structure is reached. The ground state degeneracy comes from the combination of the degeneracy of the foliation layers and that associated with the minimal structure. We discuss explicitly how this works for X-cube model with periodic boundary condition, open boundary condition, and even in the presence of screw dislocation defects.
Chair: Tim Hsieh
Monday, March 8, 2021
12:30pm (EST)
Speaker: Xiao-Gang Wen (MIT)
Abstract: Recently, the notion of symmetry has been extended from 0-symmetry described by group to higher symmetry described by higher group. In this talk, we show that the notion of symmetry can be generalized even further to "algebraic higher symmetry". Then we will describe an even more general point of view of symmetry, which puts the (generalized) symmetry charges and topological excitations at equal footing: symmetry can be viewed gravitational anomaly, or symmetry can be viewed as shadow topological order in one higher dimension.
Chair: Yong Baek Kim
Monday, March 22, 2021
12:30pm (EDT)
Speaker: Vedika Khemani (Stanford University)
Abstract: The extension of many-body quantum dynamics to the non-unitary domain has led to a series of exciting developments, including new out-of-equilibrium entanglement phases and phase transitions. We show how a duality transformation between space and time on one hand, and unitarity and non-unitarity on the other, can be used to realize steady state phases of non-unitary dynamics that exhibit a rich variety of behavior in their entanglement scaling with subsystem size --- from logarithmic to extensive to fractal. We show how these outcomes in non-unitary circuits (that are ``spacetime-dual" to unitary circuits) relate to the growth of entanglement in time in the corresponding unitary circuits, and how they differ, through an exact mapping to a problem of unitary evolution with boundary decoherence, in which information gets ``radiated away'' from one edge of the system. In spacetime-duals of chaotic unitary circuits, this mapping allows us to uncover a non-thermal volume-law entangled phase with a logarithmic correction to the entropy distinct from other known examples. Most notably, we also find novel steady state phases with fractal entanglement scaling, $S(\ell) \sim \ell^{\alpha}$ with tunable $0 < \alpha < 1$ for subsystems of size $\ell$ in one dimension. These fractally entangled states add a qualitatively new entry to the families of many-body quantum states that have been studied as energy eigenstates or dynamical steady states, whose entropy almost always displays either area-law, volume-law or logarithmic scaling. We also present an experimental protocol for preparing these novel steady states with only a very limited amount of postselection via a type of ``teleportation" between spacelike and timelike slices of quantum circuits.
Chair: Tim Hsieh
Monday, March 29, 2021
12:30pm (EDT)
Speaker: Chong Wang (Perimeter Institute)
Abstract: We propose a new type of critical quantum liquids, dubbed Stiefel liquids, based on 2+1 dimensional Wess-Zumino-Witten models on target space SO(N)/SO(4). We show that the well known deconfined quantum critical point and U(1) Dirac spin liquid are unified as two special examples of Stiefel liquids, with N = 5 and N = 6, respectively. Furthermore, we conjecture that Stiefel liquids with N > 6 are non-Lagrangian, in the sense that the theories do not (at least not easily) admit any weakly-coupled UV completion. Such non-Lagrangian states are beyond the paradigm of parton gauge theory familiar in the study of exotic quantum liquids in condensed matter physics. The intrinsic absence of mean-field construction also makes it difficult to decide whether a non-Lagrangian state can emerge from a specific UV system (such as a lattice spin system). For this purpose we hypothesize that a quantum state is emergible from a lattice system if its quantum anomalies match with the constraints from the (generalized) Lieb-Schultz-Mattis theorems. Based on this hypothesis, we find that some of the non-Lagrangian Stiefel liquids can indeed be realized in frustrated quantum spin systems, for example, on triangular or Kagome lattice, through the intertwinement between non-coplanar magnetic orders and valence-bond-solid orders.
Chair: Sung-Sik Lee
Monday, April 5, 2021
12:30pm (EDT)
Speaker: Fiona Burnell (University of Minnesota)
Abstract: We know that different systems with the same unbroken global symmetry can nevertheless be in distinct phases of matter. These different "symmetry-protected topological" phases are characterized by protected (gapless) surface states. After reviewing this physics in interacting systems with global symmetries, I will describe how a different class of symmetries known as subsystem symmetries, which are neither local nor global, can also lead to protected gapless boundaries. I will discuss how some of these subsystem-symmetry protected phases are related (though not equivalent) to interacting higher-order topological insulators, with protected gapless modes along corners or hinges in higher dimensional systems.
Chair: Anton Burkov
Monday, April 19, 2021
12:30pm (EDT)
Speaker: Liang Fu (MIT)
Abstract: I will present a new mechanism for superconductivity from strong electron-electron repulsion in multi-band systems. When the kinetic energy is small, the dynamics of nearby electrons on the lattice is strongly correlated. I will introduce a controlled expansion in the kinetic term to demonstrate pairing induced by correlated tunneling process involving a third electron in the occupied band. This mechanism can also be viewed as the real space picture of exciton-mediated pairing. Possible realization of this idea leading to strong-coupling, spin-triplet superconductivity in WTe2 and magic-angle graphene will be discussed.
Based on works with Valentine Crepel and Kevin Slagle:
[1] V. Crepel and L. Fu, arXiv:2012.08528
[2] V. Crepel and L. Fu, arXiv:2103.12060
[3] K. Slagle and L. Fu, Phys. Rev. B 102, 235423 (2020)
Chair: Sung-Sik Lee
Monday, April 26, 2021
12:30pm (EDT)
Speaker: Leon Balents (KITP, UCSB)
Abstract: Recently, a new class of kagomé metals, with chemical formula AV3Sb5 , where A = K, Rb, or Cs, have emerged as an exciting realization of quasi-2D correlated metals with hexagonal symmetry. These materials have been shown to display several electronic orders setting in through thermodynamic phase transitions: multi-component (“3Q”) hexagonal charge density wave (CDW) order below a Tc≈90K, and superconductivity with critical temperature of 2.5K or smaller, and some indications of nematicity and one-dimensional charge order in the normal and superconducting states. Other experiments show a strong anomalous Hall effect, suggesting possible topological physics. I will discuss a theory of these phenomena based in part on strong interactions between electrons at saddle points, as well as ideas related to different competing density wave orders.
Chair: Yong Baek Kim
Monday, May 3, 2021
12:30pm (EDT)
Speaker: Chandra Varma (UC Berkeley)
Abstract: I will talk on experiments and their interpretation done with Professor Lei Shu and her collaborators at Fudan University, Shanghai, and some tentative theory for the observations. Thermodynamic and magnetic relaxation measurements in zero and finite magnetic field have been performed in two related almost triangular lattices of S=1/2 spins. One of these compounds is the purest of any of the potential spin-liquid compounds investigated so far. All its measured properties are extra-ordinary and characterized simply by just one parameter, the exchange energy obtained from susceptibility measurements. There are also colossal ultra-low energy singlet excitations. This may be the first characterization of the intrinsic properties of a class of spin-liquids. An ansatz in which the excitations are calculated from a state of singlet-dimers interacting with excitations from other such singlets can be expressed in terms of Majoranas and gives properties similar to those observed.
Chair: Tim Hsieh
Schedule (Fall 2020)
Monday, September 28, 2020
12:30pm (EDT)
Speaker: T. Senthil (MIT)
Abstract: The interplay between topology, strong correlations, and kinetic energy presents a new challenge for the theory of quantum matter. In this talk I will describe some recent progress on understanding a simple class of problems where these effects can all be analytically handled. I will first present results on a microscopic lowest Landau theory of the composite fermi liquid state of bosons at filling 1. Building on work from the 1990s I will derive an effective field theory for this system that takes the form of a non-commutative field theory. I will show that an approximate mapping of this theory to a commutative field theory yield the familiar Halperin-Lee-Read action but with parameters correctly described by the interaction strength. I will describe the effect of a finite bandwidth introduced to the Landau Level and describe the evolution between the composite Fermi liquid and a boson superfluid. Time permitting, I will describe some generalizations that will include the evolution between a Quantum Anomalous Hall state and a Landau Fermi liquid that may be experimentally accessible in moire graphene systems.
Chair: Yong-Baek Kim
Monday, October 5, 2020
12:30pm (EDT)
Speaker: Dam Thanh Son (University of Chicago)
Abstract: We construct a minimal theory describing the optical activity of a thin sheet of a twisted material, the simplest example of which is twisted bilayer graphene. We introduce the notion of "twisted electrical conductivity" which parametrizes the parity-odd response of a thin film to a perpendicularly falling electromagnetic wave with wavelength larger than the thickness of the sheet, and relate the chiral response to this kinetic coefficient. We show that the low-frequency Faraday rotation angle has different behaviors in different phases: \omega^2 for insulators and \omega^0 for superconductors. In both cases the frequency dependence of the Faraday rotation angle can be obtained from a simple power counting in an effective field theory. In the metallic state, the twisted conductivity is proportional to the "magnetic helicity" (scalar product of the velocity and the magnetic moment) of the quasiparticle, averaged around the Fermi surface. Many aspects of the theory are general and applicable to strongly correlated phases.
Reference: Dung X. Nguyen and DTS, arXiv:2008.02812.
Chair: Sung-Sik Lee
Monday, October 19, 2020
12:30pm (EDT)
Speaker: Shinsei Ryu (Princeton)
Title: The quasi-particle picture and its breakdown after local quenches in conformal field theories (talk link)
Abstract: We discuss the dynamics of (Rényi) mutual information, logarithmic negativity, and (Rényi) reflected entropy after exciting the ground state by a local operator in (1+1)d conformal field theories. In particular, we contrast "integrable" and "chaotic" conformal field theories, by looking at the quasi-particle picture and its possible breakdown. In comparing the calculations in the two classes of theories, we are able to identify the dynamical mechanism for the breakdown of the quasi-particle picture in 2D conformal field theories. Intriguingly, we also find preliminary evidence that our general lessons apply to quantum systems considerably distinct from conformal field theories, such as integrable and chaotic spin chains, suggesting a universality of entanglement dynamics in non-equilibrium systems.
Chair: Anton Burkov
Monday, October 26, 2020
12:30pm (EDT)
Speaker: Dmitry Abanin (University of Geneva)
Abstract: The remarkable experimental advances made it possible to create highly tunable quantum systems of ultracold atoms and trapped ions. These platforms proved to be uniquely suited for probing non-equilibrium behavior of interacting quantum systems. From statistical mechanics, we expect that a non-equilibrium system will thermalize, settling to a state of thermodynamic equilibrium. Surprisingly, there are classes of systems which do not follow this expectation. I will give examples of systems which avoid thermalization, thanks to disorder-induced localization and quantum scarring. While thermalization leads to “scrambling” of quantum information, its absence may protect local quantum coherence. This enables non-equilibrium states of matter not envisioned within the framework of statistical mechanics. I will highlight the recent theoretical insights into the remarkable physical properties of such states, based on the underlying patterns of quantum entanglement. I will finally describe a possible theoretical route towards developing a classification of dynamical universality classes in many-body systems.
Chair: Anton Burkov
Monday, November 2, 2020
12:30pm (EST)
Speaker: Erez Berg (Weizmann Institute of Science)
Abstract: Flat bands in Moire superlattices are emerging as a fascinating new playground for correlated electron physics. I will present the results of several studies inspired by these developments. First, I will address the question of whether superconductivity is possible even in the limit of a perfectly flat band. Then, I will discuss transport properties of a spin-polarized superconductor in the limit of zero spin-orbit coupling, where the topological structure of the order parameter space allows for a new dissipation mechanism not known from conventional superconductors. If time allows, I will also discuss the interpretation of new measurements of the electronic compressibility in twisted bilayer graphene, indicating a cascade of symmetry-breaking transitions as a function of the density of carriers in the system.
References:
https://arxiv.org/abs/2006.10073
https://arxiv.org/abs/1912.08848
https://arxiv.org/abs/1912.06150