We present an experimental study of low temperature electronic transport in the hybridization gap of inverted InAs/GaSb composite quantum wells. Electrostatic gate is used to push the Fermi level into the gap regime, where the conductance as a function of sample length and width is measured. Our analysis shows strong evidence for the existence of helical edge modes proposed by Liu et al [Phys. Rev. Lett., 100, 236601 (2008)]. Edge modes persist inspite of sizable bulk conduction and show only a weak magnetic field dependence -a direct consequence of gap opening away from zone center.Topological insulators (TI) are a novel phase of matter, where the role of the spin-dependent magnetic field is played by the spin-orbital interactions. In extension of the paradigm to 3D, TI surfaces emerge as "half-graphene" with an odd number of Dirac cones. 5In 2D, the TI phase is also known as quantum spin Hall insulator (QSHI) and is characterized by an energy gap in the bulk and topologically protected helical edge states. Quantized conductance, taken as the evidence for the QSHI phase, has been experimentally observed in the inverted HgTe/CdTe quantum wells (QWs). 6,7Liu et al 8 have proposed that QSHI should arise in another semiconductor system, the hybridized InAs/GaSb QWs, where a rich phase diagram including band insulator and QSHI can be continuously tuned via gate voltages. Here we present a systematic transport study of high quality InAs/GaSb devices tuned into the QSHI state, where we observe slowly-propagating helical edge modes that are largely immune to a conductive bulk. Exploring this system should have a far-reaching impact, since InAs makes a good interface with superconductors, 9 a prerequisite for fabricating TI/superconductor hybrid structures; 10 the latter are predicted to host exotic Majorana fermion modes and are viable for fault-tolerant quantum computing.A common characteristic to all TIs is band inversion, which in InAs/GaSb is achieved by tuning energy levels in two neighboring electron and hole QWs. Hybridization of electron-hole bands leads to a gap opening, which has been experimentally well established, albeit always with a non-zero residual conductivity. 11,12In an early theoretical study, 13 the origin of the residual conductivity has been ascribed to the level-broadening due to scattering. Interestingly, in the "clean limit", the gap conductivity is finite, yet independent of scattering parameters, such as sample mobility. Motivated by the QSHI proposal, and a few times larger than the expected contribution from the edge. Nevertheless, bulk conductivity diminishes as the band inversion is reduced, 14 promoting the QSHI. In this Letter we study the length and width dependence of conductance in the hybridization regime and find direct evidence for the existence of helical edge modes proposed by Liu et al.8 Surprisingly, edge modes persist alongside the conductive bulk and show only weak magnetic field dependence. This apparent decoupling between the edge and bulk is a direct consequence of gap ...

We have engineered electron-hole bilayers of inverted InAs=GaSb quantum wells, using dilute silicon impurity doping to suppress residual bulk conductance. We have observed robust helical edge states with wide conductance plateaus precisely quantized to 2e 2 =h in mesoscopic Hall samples. On the other hand, in larger samples the edge conductance is found to be inversely proportional to the edge length. These characteristics persist in a wide temperature range and show essentially no temperature dependence. The quantized plateaus persist to a 12 T applied in-plane field; the conductance increases from 2e 2 =h in strong perpendicular fields manifesting chiral edge transport. Our study presents a compelling case for exotic properties of a one-dimensional helical liquid on the edge of InAs=GaSb bilayers. Introduction.-Symmetry protected topological order is a new paradigm in classification of condensed matter systems, describing certain system observables, such as charge or spin conductance, via topological invariants, i.e., distinct system characteristics which remain unchanged under smooth deformations of its band structure [1,2]. In addition to topological considerations, time reversal symmetry (TRS) has been widely believed to be a necessary ingredient for the emergence of the quantum spin Hall (QSH) insulating phase, commonly characterized via the Z 2 topological invariant [3][4][5][6]. Applying a magnetic field breaks the TRS and removes the topological protection of the helical liquid (HL) from backscattering. In fact, in the first realization of the QSH phase in HgTe=CdTe quantum wells, strong magnetic field dependence has been reported [6,7] albeit only in larger devices; nevertheless, it has been theoretically shown [8] that strong backscattering of the helical edge in magnetic field appears only in the case of sufficient disorder in the system, suggesting that the presence of magnetic fields is not a sufficient condition to gap out the edge states, and the ultimate fate of HL under TRS breaking may depend on the exact microscopic details of the system. Here we present data of robust HL edge states in engineered semiconductor systems that are immune to disordered bulk, as well as perturbations from external magnetic fields.The quantum spin Hall insulating state is here realized in InAs=GaSb quantum wells where electron-hole bilayer naturally occurs due to the unique broken-gap band alignment of InAs and GaSb [9]. In particular, the conduction band of InAs is some 150 meV lower than the valence band of GaSb, which results in charge transfer between the two layers, and emergence of coexisting 2D sheets of electrons and holes, trapped by wide gap AlSb barriers, as shown in Fig. 1(a). The positions of the electron and hole subbands

We observe edge transport in the topologically insulating InAs=GaSb system in the disordered regime. Using asymmetric current paths we show that conduction occurs exclusively along the device edge, exhibiting a large Hall signal at zero magnetic fields, while for symmetric current paths, the conductance between the two mesoscopicly separated probes is quantized to 2e 2 =h. Both quantized and self-averaged transport show resilience to magnetic fields, and are temperature independent for temperatures between 20 mK and 1 K. DOI: 10.1103/PhysRevLett.112.026602 PACS numbers: 72.25.Dc, 73.23.-b, 73.63.Hs Two-dimensional (2D) topological insulators (TI) are a novel class of materials that are insulating in the bulk but which display uniquely conductive edge channels [1][2][3][4]. These one-dimensional (1D) edge modes are helical, with the spin direction tied to the electron direction of motion, and are protected from backscattering by the time reversal symmetry (TRS) [5,6]. Applying magnetic fields breaks the TRS, removing the topological protection of the 1D helical liquid (HL) from single particle backscattering, resulting in a gap opening in the edge spectrum. Such HL channels were first observed in transport measurements in HgTe=CdTe quantum wells, and much of the HL phenomenology has been confirmed and elucidated in those first experiments [7,8]. Recently, Du et al. reported quantized transport in the inverted regime of Si-doped InAs=GaSb quantum wells in mesoscopic samples [9], where the existence of helical edge states was proposed in Ref.[10] and the first experimental evidence provided in Ref.[11] through scaling arguments due to the presence of residual bulk carriers. Unlike that observed in HgTe=CdTe [7], quantized transport in InAs=GaSb persists to magnetic fields of several Tesla [9,11], challenging the common understanding of 2D TIs in terms of TRS protected edge states and associated Z 2 topological invariant.Much remains to be learned about the nature and robustness of HL, in particular, to TRS breaking and disorder. In addition, edge transport in InAs=GaSb has so far only been indirectly assessed in ballistic samples [9,11]. In this Letter we study TI InAs=GaSb quantum wells in the disordered regime [12], where the total device size is much larger than the ballistic length of the HL, and show that transport in the topological regime manifestly occurs along the sample perimeter and is quantized to values consistent with the existence of a HL. Similar to ballistic regime studies [9,11], the conduction is also seen to be only weakly dependent on externally applied magnetic fields of up to 1 T. We argue that this behavior is due to the reduced effective g factor of the edge states originating from their small Fermi velocity v F . In addition, the edge states do not exhibit significant variation in transport properties for temperatures between 20 mK and 1 K measured. This is in contrast to theoretical studies, which have predicted power law corrections to the edge conductance as a function of temperatur...

We present an experimental study of S-N-S junctions, with N being a quantum spin Hall insulator made of InAs/GaSb. A front gate is used to vary the Fermi level into the minigap, where helical edge modes exist [Phys. Rev. Lett. 107, 136603 (2011)]. In this regime we observe a ~2e(2)/h Andreev conductance peak, consistent with a perfect Andreev reflection on the helical edge modes predicted by theories. The peak diminishes under a small applied magnetic field due to the breaking of time-reversal symmetry. This work thus demonstrates the helical property of the edge modes in a quantum spin Hall insulator.