Sašo Grozdanov

Absence of Disorder-Driven Metal-Insulator Transitions in Simple Holographic Models.

We study electrical transport in a strongly coupled strange metal in two spatial dimensions at finite temperature and charge density, holographically dual to the Einstein-Maxwell theory in an asymptotically four-dimensional anti-de Sitter space spacetime, with arbitrary spatial inhomogeneity, up to mild assumptions including emergent isotropy. In condensed matter, these are candidate models for exotic strange metals without long-lived quasiparticles. We prove that the electrical conductivity is bounded from below by a universal minimal conductance: the quantum critical conductivity of a clean, charge-neutral plasma. Beyond nonperturbatively justifying mean-field approximations to disorder, our work demonstrates the practicality of new hydrodynamic insight into holographic transport.

From strong to weak coupling in holographic models of thermalization

A bstractWe investigate the analytic structure of thermal energy-momentum tensor correlators at large but finite coupling in quantum field theories with gravity duals. We compute corrections to the quasinormal spectra of black branes due to the presence of higher derivative R2 and R4 terms in the action, focusing on the dual to N=4

Viscosity and dissipative hydrodynamics from effective field theory

\mathcal{N}=4

Second-order transport, quasinormal modes and zero-viscosity limit in the Gauss-Bonnet holographic fluid

SYM theory and Gauss-Bonnet gravity. We observe the appearance of new poles in the complex frequency plane at finite coupling. The new poles interfere with hydrodynamic poles of the correlators leading to the breakdown of hydrodynamic description at a coupling-dependent critical value of the wave-vector. The dependence of the critical wave vector on the coupling implies that the range of validity of the hydrodynamic description increases monotonically with the coupling. The behavior of the quasinormal spectrum at large but finite coupling may be contrasted with the known properties of the hierarchy of relaxation times determined by the spectrum of a linearized kinetic operator at weak coupling. We find that the ratio of a transport coefficient such as viscosity to the relaxation time determined by the fundamental non-hydrodynamic quasinormal frequency changes rapidly in the vicinity of infinite coupling but flattens out for weaker coupling, suggesting an extrapolation from strong coupling to the kinetic theory result. We note that the behavior of the quasinormal spectrum is qualitatively different depending on whether the ratio of shear viscosity to entropy density is greater or less than the universal, infinite coupling value of ℏ/4πkB . In the former case, the density of poles increases, indicating a formation of branch cuts in the weak coupling limit, and the spectral function shows the appearance of narrow peaks. We also discuss the relation of the viscosity-entropy ratio to conjectured bounds on relaxation time in quantum systems.

Generalized global symmetries and dissipative magnetohydrodynamics.

We study thermal transport in strongly disordered, strongly interacting quantum field theories without quasiparticles using gauge-gravity duality. We analyze linear perturbations of black holes with broken translational symmetry in Einstein-Maxwell-dilaton theories of gravity. Using general geometric arguments in the bulk, we derive bounds on thermal conductivity for the dual disordered field theories in one and two spatial dimensions. In the latter case, the thermal conductivity is always non-zero at finite temperature, so long as the dilaton potential is bounded from below. Hence, generic holographic models make non-trivial predictions about the thermal conductivity in a strongly disordered, strongly coupled metal in two spatial dimensions.

Dynamics of the electric current in an ideal electron gas: A sound mode inside the quasiparticles

Strasbourg University, CNRS-IPHC, 23 rue du Loess, BP28 67037 Strasbourg Cedex 2, France(Dated: September 23, 2013)Using the action principle for open systems inspired by Schwinger’s Closed-Time-Path method,we derive the energy-momentum balance equation for a dissipative fluid with energy loss from aneffective Goldstone action. Near hydrodynamical equilibrium, we construct the first-order dissipativestress-energy tensor and derive the Navier-Stokesequations. Shear viscosity is shown to vanish, whilebulk viscosity and thermodynamical quantities are determined by the form of the effective action.

Coupling constant corrections in a holographic model of heavy ion collisions

A bstractWe compute the ’t Hooft coupling correction to the infinite coupling expression for the second order transport coefficient λ2 in N=4