Metastable Resistance Anisotropy Orientation of 2D Electrons in High Landau Levels

The strongly anisotropic resistivity tensor observed in half-filled high Landau levels is widely believed to arise from a collective electronic phase resembling a unidirectional, or striped, charge density wave. In this picture, a preferential orientation of the stripes over macroscopic sample sizes accounts for the observed twofold-symmetric transport anisotropy. Since the high resistance direction is usually found to lie along the [1-10] axis of the host GaAs/AlGaAs heterostructure, the stripes are assumed to run parallel to [110]. Remarkably, the origin of the native symmetry-breaking potential responsible for this orientation remains unknown. Here we report on new experiments revealing that this potential can in fact have two orthogonal local minima. It is possible to initialize the system in the higher of the two minima and then observe its relaxation toward equilibrium. Depending on the temperature, the equilibration time can range from seconds to days. The double-minimum symmetry-breaking potential suggests that the piezoelectric interaction in GaAs may play a role in determining the stripe orientation.