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

Recent magneto-transport experiments on high mobility two-dimensional elect ron systems have provided strong evidence that a new class of collective el ectronic 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 th e underlying arguments for the existence of new phases and reviews the char ge density wave interpretation. (C) 2001 Elsevier Science Ltd. All rights r eserved.