Possible Nematic to Smectic Phase Transition in a Two-Dimensional Electron System at Half-Filling

Liquid crystal phases can exist in two-dimensional electron systems (2DES) subjected to a perpendicular magnetic field in a half-filled Landau level in the quantum Hall regime. Theory predicts the existence of a low-symmetry liquid crystal smectic phase that breaks both rotational and translational symmetries. However, previous experiments in 2DES are most consistent with the presence of an anisotropic nematic phase that breaks rotational symmetry but preserves long-range translational symmetry. We report three novel transport phenomena at half filling that indicate a change in symmetry at lowest temperatures in ultra-low disorder 2DES: a non-monotonic temperature dependence of the sample resistance R_xx, dramatic onset of large time-dependent fluctuations in R_xx, and a sharp feature in the differential resistance reminiscent of depinning observed in other crystalline electronic states. Taken together, these data suggest that a sequence of symmetry-breaking phase transitions occurs as temperature is lowered at half filling: first a transition from an isotropic liquid to a nematic phase and finally to a liquid crystal smectic phase.