Polaron pinning effects in superlattices at high electric and magnetic fields

Current-voltage characteristics of GaAs/AlAs superlattices have been measur ed using long-pulsed magnetic fields up to 37 T applied parallel to the gro wth axis. In the region of Wannier-Stark localisation a combined Wannier-St ark Landau ladder of states is obtained and a number of resonant features c orresponding to inter-well transitions are observed. At low magnetic fields , their voltage positions increase with magnetic field. However at magnetic fields above about 18 T, their voltage positions become independent of mag netic field owing to a pinning effect associated with polaronic coupling to the longitudinal-optic phonon mode. (C) 1998 Elsevier Science B.V. All rig hts reserved.