Holographic stripes.

We construct inhomogeneous asymptotically anti-de Sitter black hole solutions corresponding to the spontaneous breaking of translational invariance and the formation of striped order in the boundary field theory. We find that the system undergoes a second-order phase transition in both the fixed density and fixed chemical potential ensembles, for sufficiently large values of the axion coupling. We investigate the phase structure as a function of the temperature, axion coupling, and the stripe width. The bulk solutions have striking geometrical features related to a magnetoelectric effect associated with the existence of a near-horizon topological insulator. At low temperatures, the horizon becomes highly inhomogeneous and tends to pinch off.