Finite conductivity in mesoscopic Hall bars of inverted InAs/GaSb quantum wells

Abstract

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...

Topological Insulator Materials

“…The band gap in this quantum well arises from anti-crossing of the two subbands at finite momentum and hence is very small ($4 meV), which makes clean observation of the helical edge state very difficult. 73,76) Reasonably convincing evidence for the TI phase was obtained via observation of the 2e 2 =h quantization of the zero-bias Andreev reflection conductance through Nb point contacts, 74) but more recently, direct observation of the conductance quantization to 2e 2 =h has been achieved by introducing disorder to the InAs/GaSb interface by Si doping to localize the unwanted bulk carriers. 75) Other possible candidates of 2D TIs include Bi bilayer, 77) Na 2 IrO 3 , 78) and graphene with artificially enhanced SOC.…”

Observation of Edge Transport in the Disordered Regime of Topologically InsulatingInAs/GaSbQuantum Wells

“…In contrast, no Hall signal is detected for E F in the electron or hole bands, i.e., right and left white portions of the same figure, respectively. Entry into the topological bulk gap is verified via Hall measurements at finite magnetic fields [11,20]. The wave vector at which electron and hole bands from InAs and GaSb, respectively, cross is k cross ∼ 1.67 × 10 6 cm −1 , and shows little dependence on the back gate bias.…”

“…The Fermi velocity is estimated from the crossing vector and the gap size to be v F ∼ 1.8 × 10 4 m=s, giving an unusually small orbital g factor g ∼ 0.47 [22]. In this case, magnetic fields of even up to 10 T would open up a gap in the spectrum of only ≲0.3 meV, which is comparable to the level broadening [20] and, hence, would not be apparent in transport measurements. Besides the low effective g factor, the weak magnetic field dependence may be explained via the fact that the Dirac point of the edge states may actually be "hidden" between the two hole band maxima [unlike presented in the inset of Fig.…”

“…In fact, helical edge states are expected to be robust to normal dephasing [18] as brought on by electron-electron and electron-phonon interactions [24], while particularly sensitive to spin-flip dephasing processes [18]. In addition, the energy scale of the inelastic scattering such as from nearby charge puddles [15] is ℏv F =l inelastic ≲ 70 mK, where the inelastic scattering length l inelastic is several μm [20], and, hence, a weak temperature dependence observed here may not be all that surprising. Finally, a low Fermi velocity would indicate that correlations may be important in the edge physics of the TI InAs=GaSb [16], albeit the presence of the electrostatic gates leads to significant screening.…”

“…[9], with 125 A InAs and 100 A GaSb double quantum wells in the inverted regime, hosting electron and hole 2D layers, respectively. The wells are Si doped in order to suppress the bulk conductivity, arising from reduced hybridization of electron hole states due to disorder [19,20]. Samples are fabricated via standard lithography techniques, ion milling, and ion beam deposition of electrical contacts and SiO 2 dielectric layer, which defines the front electrostatic gate.…”