Population process of the upper subband inAlxGa1−xAs-GaAs quantum wells

Specially tailored gated ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$-GaAs quantum wells have been prepared where the occupation of the upper subband occurs at low carrier densities. A detailed investigation by magneto-capacitance versus voltage measurements clearly reveals that in the vicinity of the population threshold of the upper subband there exist two minima corresponding to one and the same filling factor in the lower subband. We can nicely explain this unique behavior by modeling the total density of states which results from an interplay of Landau levels of both occupied subbands.