Registration and coffee Room: 122:026

Planckian Bound on the Decay of Simple Operators

• Andrew Lucas (Stanford)

Room: 122:026 I propose a generic Planckian bound on the lifetime of "simple" operators. This is related -- but not identical -- to the Planckian bound on quantum many-body chaos. This proposal reconciles the intuition that quantum dynamics (and chaos) is bounded by the Planckian time scale with the behavior of free theories, disordered metals in the low temperature limit, and disorder-driven metal-insulator transitions.

Space-Time in the SYK Model

• Kenta Suzuki (Ecole Polytechnique)

Room: 122:026

Higher-form global symmetry in Theory of elasticity and holographic model with transverse sound

• Nick Poovuttikl (Iceland)

Room: 122:026

Fine Grained Chaos in AdS2 and AdS3

• Moshe Rozali (UBC)

Room: 122:026

Slow relaxation and diffusion in holographic quantum critical phases

• Richard Davison (Harvard)

Room: 122:026 In many-body quantum systems with strong interactions, the timescale governing transport is typically expected to be short and set by the inverse temperature. For example, in many holographic quantum critical phases of matter the thermal diffusivity (in appropriate units) is set by the inverse temperature. However, there are a simple class of such phases where this is not the case. I will discuss these phases and show that they have a collective excitation whose lifetime is parametrically longer than the inverse temperature. The lifetime of this excitation is enhanced due to its sensitivity to an irrelevant deformation, and I will show that it is this lifetime which ultimately governs the thermal diffusivity in these phases.

Reception Room: 122:026

Origin of macroscopic Thouless (diffusion) time from the underlying quantum mechanics

• Anatoly Dymarsky (Kentucky)

Room: 122:026

Coffe break Room: 122:026

What is a transport coefficient?

• Pavel Kovtun (University of Victoria)

Room: 122:026

A theory of reparameterizations for AdS3 gravity

• Kristan Jensen (San Francisco State University)

Room: 122:026

Locality Bound for Dissipative Quantum Transport

• Xizhi Han (Stanford)

Room: 122:026

Holographic Strange Insulators from Pinning of Spontaneous Superstructures

• Alexander Krikun (Leiden University)

Room: 122:026 When one considers the interplay between spontaneous and explicit symmetry breaking in finite charge density systems, one can observe the metal-insulator phase transition when the spontaneous superstructure is formed on top of the explicitly broken state. This goes under the name of "pinning of charge density wave" and eventually can lead to the formation of a Mott insulator, when the pinning gets strong. I will discuss the examples of this phenomenon in holographic models with periodic inhomogeneous and helical homogeneous lattices. The insulating state formed in these holographic models turns out to be quite interesting: firstly it is gapless and inherits the scaling features of the near horizon geometry, secondly, it demonstrates strong suppression of conductivity even for weak explicit translational symmetry breaking, which goes beyond the usual picture of the conductivity being governed by the momentum relaxation rate.

Holographic Fermions in Striped Superconductors

• Sera Cremonini (Lehigh University)

Room: 122:026 In this talk I will discuss a holographic model of a striped superconductor, in which a U(1) symmetry and translational invariance are broken spontaneously at the same time. This construction provides a concrete example of intertwined orders in holography, and realizes certain key features of pair density wave order. I will also examine the behavior of a probe fermion in this background and the formation and evolution of a Fermi surface in the corresponding PDW phase, in the presence of an explicit UV lattice. The structure of the Fermi surface is very sensitive to the strength of the lattice. In particular, strong lattice effects lead to a broadening of the spectral weight peaks and a gradual disappearance of the Fermi surface along the symmetry breaking direction. The resulting Fermi surface appears to consist of detached segments reminiscent of Fermi arcs.

Registration and coffee Room: 122:026

Bad metallic transport in a modified Hubbard model

• Connie Mousatov (Stanford)

Room: 122:026 Strongly correlated metals often display anomalous transport, including T-linear resistivity above the Mott-Ioffe-Regel limit. We introduce a tractable microscopic model for bad metals, by supplementing the well-known Hubbard model --- with hopping t and on-site repulsion U --- with a `screened Coulomb' interaction between charge densities that decays exponentially with spatial separation. This interaction lifts the extensive degeneracy in the spectrum of the t=0 Hubbard model, allowing us to fully characterize the small t electric, thermal and thermoelectric transport in our strongly correlated model. Throughout the phase diagram we observe T-linear resistivity above the Mott-Ioffe-Regel limit, together with strong violation of the Weidemann-Franz law and a large thermopower that can undergo sign change.

Many body chaos in QED3

• Julia Steinberg (Harvard)

Room: 122:026

Quantum Chaos, hydrodynamics and black hole scrambling

• Koenraad Schalm (Leiden University)

Room: FA32 (Albanova)

Quantum chaos in a Rydberg atom system

• Yochai Werman (U. C. Berkeley)

Room: FB42 (Albanova)

Reception Room: 122:026

Observation of Quantum Bounds in Spin Diffusivity

• Joseph Thywissen (University of Toronto)

Room: FB42 (Albanova)

Thermal Transport Beyond the Mott-Ioffe-Regel Limit

• Aharon Kapitulnik (Stanford)

Room: FB42 (Albanova) The Boltzmann-Drude picture of electronic transport ceases to be valid when the electron mean-free-path becomes shorter than the Fermi wavelength. The Mott-Ioffe-Regel limit is defined where the two lengths are equal, and beyond it metallic transport becomes incoherent. Most experimental works have focused on the electronic transport, with the host crystal providing a phonon background. Here we report thermal diffusivity measurements that, together with the resistivity, give a full account of transport of charge and entropy. Utilizing a high-resolution photothermal apparatus, we study the thermal diffusivity of several cuprate systems in the regime where the quasiparticle picture fails for both electron and phonons. The inverse diffusivity at high temperature can be fitted with a temperature-linear form, where the slope term is set by a unique velocity and a Planckian relaxation time ~h/kBT, The constant term represents a quantum-diffusion constant separating incoherent transport from a regime with well-defined quasiparticles.

Hydrodynamic electron flows and Hall viscosity

• Thomas Scaffidi (U. C. Berkeley)

Room: FB42 (Albanova) In metallic systems of small enough size and sufficiently strong momentum-conserving scattering, the viscosity of the electron gas can become the dominant process governing transport. In this regime, momentum is a long-lived quantity whose evolution is described by an emergent hydrodynamical theory. Furthermore, breaking time-reversal symmetry leads to the appearance of an odd component to the viscosity called the Hall viscosity, which has attracted considerable attention recently due to its quantized nature in gapped systems but still eludes experimental confirmation. Based on microscopic calculations, we discuss how to measure the effects of both the even and odd components of the viscosity using hydrodynamic electronic transport in mesoscopic samples under applied magnetic fields.

Experimental Searches for Electron Hydrodynamics

• Andrew Mackenzie (Max Planck Institute for Chemical Physics of Solids)

Room: FB42 (Albanova)

Nernst effect near a Superconductor-Insulator transition

• Efrat Shimshoni (Bar-Ilan University)

Room: FB42 (Albanova)

Pole-skipping

• Saso Grozdanov (MIT)

Room: FB42 (Albanova)

Coffe break Room: FB42 (Albanova)

Hydrodynamics with fluctuating broken translations

• Anna Karlsson (Institute for Advanced Study)

Room: FB42 (Albanova)

Topological semimetals from holography

• Ya-Wen Sun (University of Chinese Academy of Sciences)

Room: FB42 (Albanova)

Registration and coffee Room: FB42 (Albanova)

Introduction

• Sean Hartnoll (Stanford)

Room: FB42 (Albanova)

Bad Metallic Transport in a Cold Atom Fermi-Hubbard System

• Peter Schauss (Princeton University)

Room: FB42 (Albanova) Probing the charge transport properties of quantum materials can reveal their unique microscopic properties. Weakly interacting systems such as Fermi liquids are well described by semiclassical Boltzmann transport, but strong interactions blur the particle-like behavior of charge carriers causing this picture to break down. Transport in strongly interacting quantum systems is poorly understood, but exhibits interesting phenomenology in many real materials. In our work, we experimentally study charge conductivity in the Fermi-Hubbard model. Using a quantum gas microscope, we impose a density modulation on a uniform system of ultracold 6Li in a 2D optical lattice and observe this modulation decay due to charge diffusion. We find that the decay can be described by a hydrodynamic model and extract the momentum relaxation rate and diffusion constant for a range of temperatures. We determine the conductivity from the diffusion constant using the Nernst-Einstein relation. We observe that the resistivity scales linearly with temperature and shows no sign of saturation for temperatures ranging from near the super-exchange energy scale to the bandwidth. These anomalous behaviors are characteristic of bad metals.

A quantum hydrodynamical description for chaos

• Michael Blake (MIT)

Room: FB42 (Albanova) Rapid progress in calculating out-of-time ordered correlation functions in holographic theories and solvable models like the Sachdev-Ye-Kitaev model has led to new characterisations of chaos in quantum many-body systems. However, a unified description of this behaviour is so far lacking. Here I will discuss a recent proposal for an effective field theory description of chaos in terms of `quantum hydrodynamics'. In this approach the scrambling of quantum information (i.e. chaos) arises from interactions between generic operators and hydrodynamic fluctuations.

Hall number and other anomalies in the strange metal phase of overdoped cuprates

• Nigel Hussey (High Field Magnet Laboratory, Nijmegen)

Room: 122:026

Spectral form factor in an exactly solvable model of many body quantum chaos in 1+1 D

• Tomaz Prosen (University of Ljubljana)

Room: FB42 (Albanova)

Reception Room: 122:026

Phonons' Poiseuille flow

• Kamran Behnia (ESPCI, France)

Room: 122:026

Operator spreading and the emergence of dissipation in unitary dynamics with conservation laws

• Vedika Khemani (Harvard)

Room: 122:026 I will discuss the “scrambling” of local quantum information in chaotic quantum many-body systems in the presence of a locally conserved quantity like charge or energy that moves diffusively. The interplay between conservation laws and scrambling sheds light on the mechanism by which unitary quantum dynamics, which is reversible, gives rise to diffusive hydrodynamics, which is a dissipative process. Our results are obtained by examining the dynamics of operator spreading under unitary time evolution in a random quantum circuit model that is constrained to have a conservation law. We find that a generic spreading operator spreads ballistically with a front that moves at a “butterfly speed”, but develops a power law “tail” behind its leading ballistic front due to the slow dynamics of the conserved component of the operator. This structure implies that the out-of-time-order commutator (OTOC) between two initially spatially separated operators grows sharply upon the arrival of the ballistic front but, in contrast to systems with no conservation laws, it develops a diffusive tail and approaches its asymptotic late-time value only as a power of time instead of exponentially. I will also present these results within an effective hydrodynamic description which contains multiple coupled diffusion equations.

Bounds on transport and thermalization from positivity

• Luca Delacrétaz (Stanford)

Room: 122:026

Strange metals from local quantum chaos

• John Mcgreevy (UCSD)

Room: 122:026

Towards a comprehensive test of holographic principles in cuprates

• Erik Van Heumen (van der Waals-Zeeman institute, Amsterdam)

Room: 122:026

Registration and coffee Room: 122:026

Quantum fluctuations in real and complex Sachdev-Ye-Kitaev models

• Dmitry Bagrets (Institute for Theoretical Physics, University of Cologne)

Room: 122:026

Applications of higher form symmetries

• Nabil Iqbal (Durham University)

Room: 122:026

Solvable models of correlated metals with interactions and disorder, and their transport properties

• Aavishkar Patel (Harvard)

Room: 122:026 Despite much theoretical effort, there is no complete theory of the “strange” metal phase of the high temperature superconductors, and its linear-in-temperature resistivity. This phase is believed to be a strongly-interacting metallic phase of matter without fermionic quasiparticles, and is virtually impossible to model accurately using traditional perturbative field-theoretic techniques. Recently, progress has been made using large-N techniques based on the solvable Sachdev-Ye-Kitaev (SYK) model, which do not involve expanding about any weakly-coupled limit. I will describe constructions of solvable models of strange metals based on SYK-like large-N limits, which can reproduce some of the experimentally observed features of strange metals and adjoining phases. These models, and further extensions, could possibly pave the way to developing a controlled theoretical understanding of the essential building blocks of the electronic state in correlated-electron superconductors near optimal doping.

Towards bad metallic transport from holographic failed insulators

• Daniel Arean (IFT, Madrid)

Room: 122:026

Reception Room: 122:026

Interpolating between strong and weak coupling in thermal QFTs with gravity duals

• Andrei Starinets (University of Oxford)

Room: 122:026

Electrical transport near Ising-nematic QCP

• Xiaoyu Wang (James Frank Institute, University of Chicago)

Room: 122:026 An electronic nematic order spontaneously breaks the rotation symmetry of many body system, making various physical properties anisotropic. It has been observed in various systems, in particular the cuprate and iron-based high temperature superconductors. In the vicinity of a nematic quantum critical point — achieved by tuning some external parameter such as pressure or doping — the physics is described by that of low-frequency long-wavelength order parameter fluctuations coupled to a Fermi surface. However, due to the momentum-conserving nature of the induced electron-electron interaction, the temperature dependence of the resistivity near an Ising nematic QCP remains unclear. In this talk, we shed light on the problem by incorporating disorder and Umklapp process into the low-energy theory. Our work can be viewed as solving an extended Boltzmann equation, with a collision integral that accounts for complicated multi-particle scattering processes important near the QCP.

Entanglement entropy in generalised Quantum Lifshitz models

• Valentina Giangreco Puletti (University of Iceland)

Room: 122:026 n the initial part of the talk I will review the generalised quantum Lifshitz models, that is free field theories with Lifshitz scaling symmetry which extend the (2+1)-dimensional quantum Lifshitz model to higher dimension. As in the lower dimensional case the ground state wave functional is invariant under spatial conformal symmetries, whenever the dynamical critical exponent is equal to the number of spatial dimensions. Thanks to this feature, they provide us with simple models for non-relativistic critical theories, suitable to investigate the scaling properties of entanglement entropy. I will then outline the computation of the sub-leading universal terms in the entanglement entropy for spherical entangling hyper-surfaces in these theories, and discuss the results.

Holographic Abrikosov lattice

• Christiana Pantelidou (Durham University)

Room: 122:026

Instabilities of 2d quantum critical metals in the Nf->0 limit

• Petter Saterskog (Nordita)

Room: 122:026 We study a Fermi surface coupled to fluctuations of a critical order parameter in 1+2 dimensions. The limit of vanishing fermion flavor number gives a controlled way of studying this strongly coupled theory. In this talk I show how to calculate charge, spin and pairing susceptibilities in this limit and find that the critical fluctuations induce charge/spin density wave order or pairing for come ranges of parameters. We show that adding some of the Nf corrections does not change the range of parameters where these instabilities show up.

Holographic tensor models

• Nicolas Delporte (LPT Orsay)

Room: 122:026

Transport and Black Hole Horizons

• Aristomenis Donos (Durham University)

Room: 122:026

Holographic Plasmons

• Ulf Gran (Chalmers)

Room: 122:026 Plasmons are ubiquitous in all conducting compounds. We show how to model plasmons holographically, relevant for systems without quasiparticles, e.g. the ‘strange metal’ phase of high temperature superconductors. By introducing holographic electromagnetism we derive a novel set of of boundary conditions which the plasmon modes, and in fact all physical modes, must satisfy. We present the results for both bulk and 2DEG plasmons, and highlight that holographic bulk plasmons exhibit an exotic dispersion in a part of the parameter space not accessible by other methods.

Need for sound speed and holography

• Niko Jokela (University of Helsinki)

Room: 122:026 Several works in holography has lent support on the conjecture that there could be a universal speed limit for the sound set by the conformal value. Further support comes from kinetic theory and QCD. Yet, there are many instances violating this bound. I furthermore present several holographic models that break the speed barrier and discuss their common denominator.