Landau-level hybridization and the quantum Hall effect in
InAs/GaSb electron-hole systems

Kyoichi Suzuki, Kei Takashina, and Yoshiro Hirayama
Physical Science Laboratory

 In InAs/GaSb heterostructures, the conduction band in InAs and the valence band in GaSb overlap. Consequently, a two-dimensional electron gas (2DEG) in the InAs and a two-dimensional hole gas (2DHG) in the GaSb can coexist in close proximity. A unique quantum Hall effect (QHE) has been known to occur in these structures where the Hall resistance is quantized by the difference between2DEG and 2DHG Landau-level (LL) filling-factors (νe, νh). Its mechanism had yet to be clarified, and had been thought to be a complex combination of QHE arising in both 2DEG and 2DHG.
 We clarified the mechanism using back-gated structures (Fig. 1) and detailed magneto-transport measurements. The samples consisted of an InAs 2DEG layer on the surface side and a GaSb 2DHG layer on the substrate side. We succeeded in controlling the 2DHG density with the gate voltage, while keeping the 2DEG density constant by the surface potential. Changing the thickness of the inserted AlSb layer between the InAs and GaSb layers, which works as a potential barrier for both 2DEG and 2DHG, the hybridization strength between 2DEG and 2DHG wavefunctions can be controlled. Under magnetic field, in samples without wavefunction hybridization, the electron and hole energy levels split into LLs shown as the dashed lines in Fig. 2. In samples where wavefunction hybridization is allowed, however, LLs also hybridize (solid lines) and a new energy structure is formed, in which the region of νeh becomes effectively the band gap. Net carriers (difference of 2DEG and 2DHG densities) fill the hybridized LLs above the band gap. Therefore, conventional 2DEG-like QHE occurs, according to the net carrier density.
 We believe our results will become useful for realizing new physics phenomena reflecting electron-hole interactions and correlations, such as Bose-Einstein condensation of the dipole excitons, and contribute to the development of mid-infrared device applications using inter-subband transitions and inter-layer tunneling.

[1] K. Suzuki et al., Phys. Rev. Lett. 93 (2004) 016803.


Fig. 1. Sample potential profile

Fig. 2. Landau-level hybridization
  Dashed lines: Electron and hole Landau
        -levels without hybridization.
  Solid lines: Hybridized Landau-levels.

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