Charged magnetic brane solutions in AdS5 and the fate of the third law of thermodynamics

We construct asymptotically AdS(5) solutions to 5-dimensional Einstein-Maxwell theory with Chern-Simons term which are dual to 4-dimensional gauge theories, including N = 4 SYM theory, in the presence of a constant background magnetic field B and a uniform electric charge density rho. For the solutions corresponding to supersymmetric gauge theories, we find numerically that a small magnetic field causes a drastic decrease in the entropy at low temperatures. The near-horizon AdS(2) x R-3 geometry of the purely electrically charged brane thus appears to be unstable under the addition of a small magnetic field. Based on this observation, we propose a formulation of the third law of thermodynamics (or Nernst theorem) that can be applied to black holes in the AdS/CFT context. We also find interesting behavior for smaller, non-supersymmetric, values of the Chern-Simons coupling k. For k = 1 we exhibit exact solutions corresponding to warped AdS(3) black holes, and show that these can be connected to asymptotically AdS(5) spacetime. For k <= 1 the entropy appears to go to a finite value at extremality, but the solutions still exhibit a mild singularity at strictly zero temperature. In addition to our numerics, we carry out a complete perturbative analysis valid to order B-2, and find that this corroborates our numerical results insofar as they overlap.