Search for Superconducting Edge Mode in Topological Materials

Abstract

The introduction of topology brings us new understanding to the electronic structure in condensed matter. Predicted effects and materials with non-trivial topology, such as the quantum hall effect, the quantum anomalous hall effect, 3D topological insulators, topological semimetals, etc. have been realized in various systems. In particular, the combination of the edge state in topological materials and superconducting pairing is of theoretical interest and may have great potential in topological quantum computations.

The Cr-doped (Bi,Sb)2Te3 thin film, a magnetic-doped 3D topological insulator, exhibits quantized hall conductance and chiral edge states at zero external field, with the chirality and the Chern states determined by the direction of magnetization. This is known as the quantum anomalous hall effect. We studied the transition of Chern states by driving the system into different magnetization directions with an external magnetic field. We observed a novel transition of Chern states that depends on dissipation in the system, which can be associated with a macroscopic dissipative quantum tunneling. In addition, the superconducting pairing and chiral edge states are predicted to host a zero energy mode. We realized a QAH-SC hybrid system based on Al and Cr-doped (Bi,Sb)2Te3 and observed signatures of a zero energy mode in the chiral edge states with by superconducting gap.

In topological superconductors, attention has focused on topological edge states that carry unpaired excitations. An interesting question is whether edge states can carry a supercurrent rigorously protected against hybridization with the bulk supercurrent. MoTe2 is a Weyl semimetal and superconductor at ultra-low temperature or under high pressure. In MoTe2 micro-flakes with Au contacts, we observed a novel oscillation mode that shows up as a scalloped pattern in Ic vs. B. The observations of frequency scaling with sample area, a frequency chirp effect and phase coherence in the oscillation mode provide strong evidence for the co-existence of edge supercurrent and bulk supercurrent. We proposed a free energy model based on fluxoid quantization and observed excitation branches in agree with the model.