Beyond η/ s = 1/4π

We study several aspects of charged dilaton black holes with planar symmetry in (d+2)-dimensional spacetime, generalizing the four-dimensional results investigated in arXiv:0911.3586 [hep-th]. We revisit the exact solutions with both zero and finite temperature and discuss the thermodynamics of the near-extremal black holes. We calculate the AC conductivity in the zero-temperature background by solving the corresponding Schrödinger equation and find that the AC conductivity behaves like ω δ , where the exponent δ is determined by the dilaton coupling α and the spacetime dimension parameter d. Moreover, we also study the Gauss-Bonnet corrections to η/s in a five-dimensional finite-temperature background.

Section: Gauss-bonnet Corrections To η/S At Finite Temperaturementioning

confidence: 54%

“…By evaluating the conserved flux at the horizon, we can easily check that 33) and thus, combining with the result (4.31), the real part of the conductivity is…”

Section: Conductivitymentioning

confidence: 88%

Holography of charged dilaton black holes in general dimensions

Chen

Pang

2010

“…It was also initially suggested that this value is a bound, and the ratio can never become smaller. We now know that this is not true [4][5][6][7], see also [8,9], but in all controlled counter-examples the bound is violated at best by a numerical factor, and not in a parametric manner. Attempts to produce bigger violations lead to physically unacceptable situations, e.g., to causality violations, for example, see [10,11].…”

Section: Jhep10(2015)028mentioning

confidence: 96%

The shear viscosity in anisotropic phases

Jain

Samanta

Trivedi

2015

103

We construct anisotropic black brane solutions and analyse the behaviour of some of their metric perturbations. These solutions correspond to field theory duals in which rotational symmetry is broken due an externally applied, spatially constant, force. We find, in several examples, that when the anisotropy is sufficiently big compared to the temperature, some components of the viscosity tensor can become very small in units of the entropy density, parametrically violating the KSS bound. We obtain an expression relating these components of the viscosity, in units of the entropy density, to a ratio of metric components at the horizon of the black brane. This relation is generally valid, as long as the forcing function is translationally invariant, and it directly connects the parametric violation of the bound to the anisotropy in the metric at the horizon. Our results suggest the possibility that such small components of the viscosity tensor might also arise in anisotropic strongly coupled fluids found in nature.

“…If the latter conditions are not met (1.1) may fail. For instance, once the 't Hooft coupling of the gauge theory or the rank of its gauge group are finite, the ratio (1.1) is corrected [22][23][24][25][26][27][28][29][30][31][32][33][34]. More recently, (1.1) has been shown to break down in non isotropic configurations which are described by two derivative theories of gravity [35][36][37][38][39][40][41].…”

Section: Introductionmentioning

confidence: 99%

A modulated shear to entropy ratio

Ovdat

Yarom

2014

We study correlation functions in an equilibrated spatially modulated phase of Einstein-Maxwell two-derivative gravity. We find that the ratio of the appropriate low frequency limit of the stress-stress two point function to the entropy density is modulated. The conductivity, the stress-current and current-stress correlation functions are also modulated. At temperatures close to the phase transition we obtain analytic expressions for some of the correlation functions.