Single-particle subband spectroscopy in a parabolic quantum well via transport experiments

The transport properties of a parabolic quantum well are investigated at low temperatures 0.5 K<T<4 K. A front-gate voltage is used to tune the carrier density and hence the subband structure over a wide range. Various orientations of a magnetic field B with respect to the normal of the sample 0°≤α≤90° are investigated. For α=0°, where B is oriented along the surface normal of the sample, quantum Hall states can be suppressed and recovered depending on the carrier density of the sample. This effect is explained quantitatively by a self-consistent calculation that considers the interplay of occupied Landau levels belonging to different subbands. A suppressed quantum Hall state can also be recovered via a tilted magnetic field. The resonant-subband-Landau-level coupling leads to a repulsion of the levels and, therefore, to the creation of an energy gap. For T≤0.5 K the spin splitting of the Landau levels appears in the magnetoresistance. In the regime of carrier densities where the quantum Hall plateau corresponding to filling factor ν=4 is suppressed, a double minimum structure for ν=3 is observed, reflecting the different exchange enhancement of the spin splitting of the two lowest subbands. For magnetic fields oriented in the plane of the sample, a band structure arises that leads to an anisotropic conductivity for current flow perpendicular and parallel to the magnetic-field orientation.