Landau-level interplay in InGaAs double quantum wells

Shubnikov–de Haas (SdH) and Hall measurements have been used to investigate a pair of adjacent two-dimensional electron gases (2DEGs) which were formed in two n_0.53Ga_0.47As quantum-wells, separated by a thin In_0.52Al_0.48As barrier, grown lattice-matched on InP. This double quantum-well system consists of two asymmetric InGaAs quantum wells, 9 nm and 7 nm respectively, separated by a 4.5 nm InAlAs barrier. The existence of two occupied electronic subbands with differing electron densities can clearly be identified by beating effects in the SdH oscillations. By applying a substrate bias the electron densities can be tuned and the beating is shifted. In the simultaneously performed Hall measurements additional features can be observed: Hall measurements with different total electron densities reveal plateaus for integer filling factors ν (with ν = ν₁ + ν₂, ν₁and ν₂both integers, corresponding to the two subbands). Some even filling factors become suppressed and recover with changing electron density. Also, for some densities an odd filling factor is observed. The systematic tuning of the electron densities via the application of a bias voltage to the front gate reveals two Landau fans, one for each electronic system, respectively, crossing each other. The electron densities for both electronic systems can be identified by analysing the SdH spectra. As a function of the front-gate voltage, these densities seem to show evidence for an anticrossing of the two electronic states and therefore for a strong coupling between the states.