We present the first experimental study of the double-quantum-well (DQW) system made of 2D layers with inverted energy band spectrum: HgTe. The magnetotransport reveals a considerably larger overlap of the conduction and valence subbands than in known HgTe single quantum wells (QW), which may be regulated here by an applied gate voltage Vg. This large overlap manifests itself in a much higher critical field Bc separating the range above it with a plain behavior of the Hall magnetoresistance ρxy(B), where the quantum peculiarities shift linearly with Vg, and the range below with a complicated behavior. In the latter case specific structures in ρxy(B) are formed like a double-N -shaped ρxy(B), reentrant sign-alternating quantum Hall effect with transitions into a zero-filling-factor state etc., which are clearly manifested here due to better magnetic quantization at high fields, as compared to the features seen earlier in a single HgTe QW. The coexisting electrons and holes were found in the whole investigated range of positive and negative Vg as revealed (i) from fits to the low-field N -shaped ρxy(B), (ii) from the Fourier analysis of oscillations in ρxx(B) and (iii) from a specific behavior of ρxy(B) at high positive Vg. A peculiar feature here is that the found electron density n remains almost constant in the whole range of investigated Vg while the hole density p drops down from the value a factor of 6 larger than n at extreme negative Vg to almost zero at extreme positive Vg passing through the charge neutrality point. We show that this difference between n and p stems from an order of magnitude larger density of states for holes in the lateral valence subband maxima than for electrons in the conduction subband minimum. We analyze our observations on the basis of a calculated picture of magnetic levels in a DQW and suggest that their specificity is due to (i) a nonmonotonic course of the valence subband magnetic levels and an oscillating behavior of the valence subband top versus field due to lateral maxima in the energy spectrum, (ii) a reduced gap between the lowest electron and the highest hole magnetic levels where the electron-and hole-type localized states are superposed, and (iii) a possible formation of the interlayer electron-hole excitons.

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We have measured the temperature (2.9 K < T < 50 K) and magnetic field (0 T < B < 9 T) dependences of longitudinal and Hall resistivities for HgCdTe/HgTe/HgCdTe system with HgTe quantum well width of 20.3 nm. The activation analysis of the experimental magnetoresistivity traces has been used as a quantitative tool to probe inter-Landau level distances. The activation energies were determined from the temperature dependence of the longitudinal resistivity in the regions of quantized Hall plateaus (for the filling factors ν of 1, 2 and 3) and the indications of the large values of the g factor ≅ 30-75 were found.

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