On universality of charge transport in AdS/CFT

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.