Two-dimensional electrons in excited Landau levels: evidence for new collective states

Recent magneto-transport experiments on high mobility two-dimensional electron systems have provided strong evidence that a new class of collective electronic states exists at high Landau level occupancy. Among the phenomena observed are strongly anisotropic resistances near half-filling and curious re-entrant integer quantum Hall states in the flanks of high Landau levels. These results have been widely interpreted as evidence for the existence of an unusual class of charge density wave states. This paper emphasizes the underlying arguments for the existence of new phases and reviews the charge density wave interpretation.