Phys. Rev. Lett. 121, 097701 (2018)

Topological crystalline insulators are a subclass of topological insulators. Rather than spin–orbit coupling, a particular crystalline symmetry gives rise to non-trivial metallic surface states in these otherwise insulating materials. If made superconducting, these insulators will turn into topological superconductors. These superconductors host Majorana quasiparticles, which are electron and hole in one, and may be useful to build qubits with built-in protection against errors.

Credit: APS

Rodney Snyder and co-workers have now fabricated a Josephson junction from superconducting aluminium with a weak link made from the topological crystalline insulator Pb0.5Sn0.5Te. In addition to the common signatures observed in trivial Josephson junctions, transport measurements unveil features that indicate deviations from the ordinary sinusoidal current–phase relation. Because additional transport channels with high transmission give rise to such skewing, this is good evidence for the supercurrent not being entirely carried by the low-conductance bulk states. Instead, part of the supercurrent could be channelled through 1D spin-polarized states induced by symmetry-breaking step edges at the surface of a topological crystalline insulator. Alternatively, the 2D surface states of the topological insulator could account for the high-conductance channels. Either way, these experiments point to the realization of a topological superconductor, this time made from a topological crystalline insulator.