Quantum Hall ferromagnets, cooperative transport anisotropy, and the random field Ising model

We discuss the behavior of a quantum Hall system when two Landau levels with opposite spins and combined filling factor near unity are brought into energetic coincidence using an in-plane component of magnetic field. We focus on the interpretation of recent experiments under these conditions [Zeitler et al., Phys. Rev. Lett. 86, 866 (2001); Pan et al., Phys. Rev. B 64, 121305 (2001)], in which a large resistance anisotropy develops at low temperatures. Modeling the systems involved as Ising quantum Hall ferromagnets, we suggest that this transport anisotropy reflects domain formation induced by a random field arising from isotropic sample surface roughness.