ANOMALOUS CARRIER LIFETIME ENHANCEMENT AND EFFECTIVE-MASS DISCONTINUITY OBSERVED DURING MAGNETIC-FIELD-INDUCED SUBBAND DEPOPULATION IN A WIDE PARABOLIC QUANTUM-WELL

GaAs/AlGaAs wide parabolic quantum wells (WPQWs) contain uniform, quas i-two-dimensional slabs of electrons, with thickness similar to 1000 A ngstrom. Electronic subbands are depopulated by magnetic fields B in t he well plane. Small additional perpendicular fields induce Shubnikov- de Haas (SdH) oscillations which we use to determine the carrier densi ty, effective mass, and scattering lifetime, throughout the two subban d to one subband transition. Densities in each subband are consistent with a model in which electron-electron interactions simply renormaliz e the parabolic confining potential. Discontinuities in carrier mass a nd lifetime at depopulation are best explained by a model in which int eractions flatten the potential profile. The second subband SdH lifeti me is enhanced near depopulation, even though the onset of two subband transport increases sample resistance. Electrons in the lowest subban d prove unexpectedly effective in screening local alloy disorder; this does not agree with long-standing explanations for the limited mobili ty in WPQW structures. (C) 1998 Elsevier Science B.V. All rights reser ved.