We have studied experimentally the low temperature conductivity of mesoscopic size InAs/GaSb quantum well Hall bar devices in the inverted regime. Using a pair of electrostatic gates we were able to move the Fermi level into the electron-hole hybridization state, and observe a mini gap. Temperature dependence of the conductivity in the gap shows residual conductivity, which can be consistently explained by the contributions from the free as well as the hybridized carriers in the presence of impurity scattering, as proposed by Naveh and Laikhtman [Euro. Phys. Lett., 55, 545-551 (2001)]. Experimental implications for the stability of proposed helical edge states will be discussed.
It was recently proposed by Kane and Mele
1, and independently, by Bernevig and Zhang 2 that a novel transport phenomenon, termed the quantum spin Hall effect (QSHE) should arise in a class of two-dimensional topological insulators (TI). The QSHE phase is characterized by an odd number of Kramer pair states at the edges, and an insulating gap in the bulk. Edge states are robust against disorder due to the topology of the bands and result in helical edge transport with counterpropagating spin-up and spin-down channels. Four-probe transport measurements on such structures at zero magnetic field give a near quantized conductance of 2e 2 /h for mesoscopic samples. Just as theoretically predicted by Bernevig et al , giving rise to a phase transition from a normal insulator to TI via a continuously varying parameter.In CQW, carriers (electrons and holes) are separated in different wells and the bulk energy gap arises due to the hybridization of the e-and the h-bands at a finite k-vector in momentum space. This finite k character is in contrast to that in the HgTe QWs, where the carriers are in the same well and the energy gap opens in the zone center. It is theoretically realized that disorder (which always exists in real materials) could play different roles in the two cases 7 for the experimental observation of QSHE. In this paper we present an experimental study of lowtemperature conductivity in mesoscopic size CQW Hallbar devices. Analyses of our results indicate intricate contributions from the free as well as the hybridized carriers to the bulk CQW conductivity in the presence of a finite amount of disorder. Interestingly, such contributions appear to be independent of the amount of scattering but vary strongly with CQW band parameters corroborated the existence of a tunneling-induced mini-gap. These results convincingly show that a mini-gap exists due to e-h hybridization in the inverted regime of InAs/GaSb CQWs. In contrast, a true bulk insulator, which shows temperature activated conductivity has yet to be experimentally confirmed. Moreover, in light of the current interest in observing the QSHE, transport in the mesoscopic-size devices should be measured 4 . Work reported here presents a first experimental study in this direction.InAs/GaSb CQW was grown by molecular beam epitaxy on silicon-doped N + (100) GaAs substrate. The structu...