AdS/CFT and Landau Fermi liquids

We study the field theory dual to a charged gravitational background in which the low temperature entropy scales linearly with the temperature. We exhibit the existence of a sound mode which is described by hydrodynamics, even at energies much larger than the temperature, and explain how this, and other properties of the field theory, are consistent with those of a (3+1)-dimensional Landau Fermi liquid, finely tuned to the Pomeranchuk critical point. We also discuss how one could engineer a higher-derivative gravitational Lagrangian which reproduces the correct low temperature behavior of shear viscosity in a generic Landau Fermi liquid.Comment: harvmac, 35 pages, 2 figures. v2: minor changes and references adde

On Non-Critical Superstring/Black Hole Transition

An interesting case of string/black hole transition occurs in two-dimensional non-critical string theory dressed with a compact CFT. In these models the high energy densities of states of perturbative strings and black holes have the same leading behavior when the Hawking temperature of the black hole is equal to the Hagedorn temperature of perturbative strings. We compare the first subleading terms in the black hole and closed string entropies in this setting and argue that the entropy interpolates between these expressions as the energy is varied. We compute the subleading correction to the black hole entropy for a specific simple model.Comment: 10 pages; harvma

The Ground Ring of N=2 Minimal String Theory

We study the \NN=2 string theory or the \NN=4 topological string on the deformed CHS background. That is, we consider the \NN=2 minimal model coupled to the \NN=2 Liouville theory. This model describes holographically the topological sector of Little String Theory. We use degenerate vectors of the respective \NN=2 Verma modules to find the set of BRST cohomologies at ghost number zero--the ground ring, and exhibit its structure. Physical operators at ghost number one constitute a module of the ground ring, so the latter can be used to constrain the S-matrix of the theory. We also comment on the inequivalence of BRST cohomologies of the \NN=2 string theory in different pictures.Comment: 25 pages, latex, small correction

Penrose limit and string quantization in AdS_3 \times S^3

We consider corrections to the Penrose limit of AdS_3 \times S^3 with NS-NS flux which are due to the terms next to leading order in inverse radius expansion. The worldsheet theory of a lightcone string is interacting due to the presence of quartic terms in the action. Perturbative corrections to the spectrum are shown to agree with the results from the exact quantization in AdS_3 \times S^3.Comment: 18 pages v2: typos fixed, reference added, to appear in JHE

Hydrodynamics from the Dp-brane

We complete the computation of viscous transport coefficients in the near horizon geometries that arise from a stack of black Dp-branes for p=2,...,6 in the decoupling limit. The main new result is the obtention of the bulk viscosity which, for all p, is found to be related to the speed of sound by the simple relation \zeta/\eta = -2(v_s^2-1/p). For completeness the shear viscosity is rederived from gravitational perturbations in the shear and scalar channels. We comment on technical issues like the counterterms needed, or the possible dependence on the conformal frame.Comment: 15 page

The Viscosity Bound Conjecture and Hydrodynamics of M2-Brane Theory at Finite Chemical Potential

Kovtun, Son and Starinets have conjectured that the viscosity to entropy density ratio $\eta/s$ is always bounded from below by a universal multiple of $\hbar$ i.e., $\hbar/(4\pi k_{B})$ for all forms of matter. Mysteriously, the proposed viscosity bound appears to be saturated in all computations done whenever a supergravity dual is available. We consider the near horizon limit of a stack of M2-branes in the grand canonical ensemble at finite R-charge densities, corresponding to non-zero angular momentum in the bulk. The corresponding four-dimensional R-charged black hole in Anti-de Sitter space provides a holographic dual in which various transport coefficients can be calculated. We find that the shear viscosity increases as soon as a background R-charge density is turned on. We numerically compute the few first corrections to the shear viscosity to entropy density ratio $\eta/s$ and surprisingly discover that up to fourth order all corrections originating from a non-zero chemical potential vanish, leaving the bound saturated. This is a sharp signal in favor of the saturation of the viscosity bound for event horizons even in the presence of some finite background field strength. We discuss implications of this observation for the conjectured bound.Comment: LaTeX, 26+1 Pages, 4 Figures, Version 2: references adde

Type IIB tensionless superstrings in a pp-wave background

We solve the tensionless string in a constant plane wave background and obtain a hugely degenerate spectrum. This is the case for a large class of plane wave backgrounds. We show that the solution can also be derived as a consistent limit of the quantized tensile theory of IIB strings in a pp-wave. This is in contrast to the situation for several other backgrounds.Comment: 1+17 pages, LaTeX, minor corrections, added new reference