Magnetotransport in HgTe double quantum well*

We present a study of a double quantum well (DQW) made of two-dimensional layers with inverted energy band spectrum: HgTe. The magnetotransport reveals a considerably larger overlap of the conduction and valence subbands, than is known for HgTe single quantum wells (QW). Thus, the critical field $B_c$ for opening the gap in the energy spectrum shifts towards much higher fields with respect to $B_c$ in single QWs. The accompanying specific features in magnetotransport, such as multiple inversions in $\rho_{xy}$(B ), zero-filling-factor state with a concomitant manifestation of its insulator character in $\rho_{xx}$(B ), etc., also move towards higher fields, where the quantum Hall regime is well realized. The overlap can be regulated by a gate voltage $V_g$ and the coexisting electrons and holes were found in the whole investigated range of positive and negative $V_g$. The electron density n remains almost constant in the whole range of investigated $V_g$, while the hole density p drops down passing through the charge-neutrality point. This difference between n and p stems from an order of magnitude larger density of states for holes than for electrons. We analyze our observations on the basis of a calculated picture of magnetic levels in a DQW.