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, quasi-two-dimensional slabs of electrons, with thickness ∼ 1000 Å. Electronic subbands are depopulated by magnetic fields B in the well plane. Small additional perpendicular fields induce Shubnikov-de Haas (SdH) oscillations which we use to determine the carrier density, effective mass, and scattering lifetime, throughout the two subband to one subband transition. Densities in each subband are consistent with a model in which electron-electron interactions simply renormalize the parabolic confining potential. Discontinuities in carrier mass and lifetime at depopulation are best explained by a model in which interactions flatten the potential profile. The second subband SdH lifetime is enhanced near depopulation, even though the onset of two subband transport increases sample resistance. Electrons in the lowest subband prove unexpectedly effective in screening local alloy disorder; this does not agree with long-standing explanations for the limited mobility in WPQW structures.