Carrier-lifetime enhancement and mass discontinuity inferred from transport in a parabolic quantum well during subband depopulation

In GaAs/AlₓGa_1-xAs parabolic quantum wells, subbands are depopulated by a magnetic field in the well plane. A small additional perpendicular field induces Shubnikov--de Haas (SdH) oscillations, which we have used to determine the carrier density, effective mass, and lifetime, throughout the two-subband-to-one-subband transition. The masses of carriers in the first and second subbands differ by 50% near the threshold of population of the second subband. The second subband SdH lifetime is enhanced near the threshold, even though the onset of two-subband transport increases the sample resistance.