Edge Transport in Quantum Spin Hall Insulator-Superconductor Heterostructures

For heterostructures formed by a quantum spin Hall insulator (QSHI) placed in proximity of a superconductor (SC), no external magnetic field is necessary to drive the system into a phase supporting Majorana bound states. This fact makes QSHI-SC very attractive for the realiation of non-Abelian electronic states and fault-tolerant qubits. A QSHI state can be realized in double quantum wells formed by InAs and GaSb by properly choosing the relative thickness of InAs and GaSb. In this talk, I will present theoretical and experimental results for the transport properties of QSHI-SC heterostructures formed by a InAs/GaSb double quantum well, designed to be in the QSHI phase, and superconducting Ta. In particular we studied the dependence of the differential conductance on the magnitude of an external magnetic field, B. Our detailed theoretical results show that the dependence of the differential conductance on B is affected by the position, in energy, of the Dirac point for the helical modes. In particular, I will show the difference between the transport properties of QSHI-SC systems in which the Dirac point is within the bulk gap, and those in which the Dirac point is "buried", i.e., below the top of the bulk valence band. By comparing the theoretical results to the experimental measurements we conclude that in our current devices the Dirac point is buried. I will then discuss the implications of having a buried Dirac point for the realization of Majorana bound states in QSHI-SC devices.