Symmetry protected Josephson supercurrents in three-dimensional topological insulators

Abstract

Coupling the surface state of a topological insulator to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, that is, in the true topological regime relevant for investigating Majorana modes. Here we report measurements of d.c. Josephson effects in topological insulator-superconductor junctions as the chemical potential is moved through the true topological regime characterized by the presence of only surface currents. We compare our results with three-dimensional quantum transport simulations, and determine the effects of bulk/surface mixing, disorder and magnetic field; in particular, we show that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. Our results thus clarify key open issues regarding the nature of supercurrents in topological insulators.

Aharonov–Bohm oscillations in a quasi-ballistic three-dimensional topological insulator nanowire

“…Angle-resolved photoemission spectroscopy of the crystal shows a bulk conduction band located B0.20 eV above the Dirac point, as well as gapless linear surface states inside the bulk bandgap 16 . Previous results on similar samples 16 demonstrate that the resistance maximum coincides well with the charge neutrality point, and that the discrepancy, in gate voltage, between these and the Dirac point should be less than 1 V. Nanowires are obtained via mechanical exfoliation of a bulk TI crystal (Bi 1.33 Sb 0.67 )Se 3 onto Si/SiO 2 substrates, where the doped substrate acts as a global backgate (see Methods). While chemical or mechanical etching allows for effective production of nanowires from layered thin films, these processes typically introduce additional damage and defects along the edges 17,18 .…”

“…Three-dimensional TI crystals (Bi 1.33 Sb 0.67 Se 3 ) were grown using a modified floating zone method 16 . TI nanowires were obtained by mechanical exfoliation ('scotch tape method') of bulk (Bi 1.33 Sb 0.67 )Se 3 crystals on 300 nm SiO 2 /highly n-doped Si substrates 29 .…”

Majorana Fermion Surface Code for Universal Quantum Computation

“…Proximity-induced superconductivity and supercurrents have been observed in a number of TI materials [46][47][48][49][50][51][52]. Low-temperature scanning tunneling microscopy (STM) experiments have found a proximity-induced superconducting gap on TI surface states, and the tunneling spectrum of Abrikosov vortices shows a zero-bias conductance peak, which is robust in a range of a magnetic field and splits at higher fields [12].…”

Topological Insulator Materials

“…6) It is useful to note that such a proximity-induced superconducting state on the surface of a TI has singlet Cooper pairs, but nonetheless it is topologically nontrivial due to the Berry phase born by the surface Dirac electrons; therefore, such a surface can be considered a 2D topological superconductor. There have been a number of experimental reports to confirm the superconducting proximity effect in the topological surface states, 171,231,[260][261][262][263][264][265][266][267][268] but the existence of Majorana fermions has not been elucidated.…”