On universality of charge transport in AdS/CFT

TL;DR

This paper develops a holographic approach to charge transport in strongly coupled two-layer systems, revealing universal conductivity properties in conformal field theories with gravity duals, unaffected by translation symmetry breaking.

Contribution

It introduces a universal dc conductivity in holographic two-layer systems and connects thermoelectric and heat conductivities to electrical conductivity via Ward identities.

Findings

Universal dc conductivity in holographic models

No Drude peak in finite-frequency conductivity

Universality persists under weak translation symmetry breaking

Abstract

We develop the holographic formulation of transport in strongly coupled two-layer systems. We identify a dc conductivity, sigma^dc, that is finite even in a translationally invariant setup, and universal for CFTs with a gravity dual. The thermoelectric conductivity and heat conductivity are fully determined by the electrical conductivity matrix, as a consequence of Ward identities. We use the memory-matrix approach for double-layer systems, together with Ward identities, to show that sigma^dc - extended to finite frequency - has no Drude peak and, similarly, that its universal value is unaffected if translation invariance is softly broken.