Unconventional reentrant quantum Hall effect in a HgTe/CdHgTe double quantum well

We report on the observation of an unconventional structure of the quantum Hall effect (QHE) in a p−type HgTe / CdxHg1−x Te double quantum well (DQW) consisting of two HgTe layers of critical thickness. The observed QHE is a reentrant function of magnetic field between two i = 2 states (plateaus at ρyx = h / ie2 ) separated by an intermediate i = 1 state in the shape of a flat-top peak placed on the remarkably long i = 2 plateau. This anomalous i = 1 peak separates two different regimes: (i) a traditional QHE at relatively low fields corresponding to a small density of mobile holes ps and (ii) a high-field QHE with a 2-1 plateau-plateau transition corresponding to a much larger ps. The high-field part is strongly sensitive to external influences such as gate voltages, in contrast to the low-field part, which is much less responsive. We explain the observed behavior by analyzing the calculated trajectories of the Fermi level EF (B) between hole-like and electron-like Landau levels (LLs). At low fields, EF is captured by the lateral maximum (LM) of the valence subband, and only holes in the center of the Brillouin zone contribute to QHE, while holes in the LM are inactive. In contrast, at fields above the reentry, EF rises significantly higher than LM, approaching the zero-mode LLs, and all holes come into play in QHE. At intermediate fields, the reentrance is caused by a combination of two factors in the specific energy spectrum of this DQW: (i) the superposition of an electron-like LL on hole-like LLs and (ii) the stabilizing influence of the LM reservoir on EF (B).