A Study on Interacting Condensates in Superconducting Topological Materials

Abstract

Topological superconductors have been a focus of intense research efforts, because of their potential application in building fault-tolerant topological quantum computers. One approach to seeking topological superconductivity is investigating topological materials that transition to superconductors. In this Thesis, I present a series of discoveries that concern interacting condensates in the bulk and the edge states of superconducting topological materials. A type-II Weyl semimetal MoTe$_2$ hosts an intrinsic edge supercurrent. In a MoTe$_2$ device with gold electrodes, the critical current modulates periodically with respect to the applied magnetic field. The area associated with this period scales with increasing field and converges to the physical area of device, which suggest the fluoxid quantization by the one-dimensional superconducting edge mode as the origin. When a superconducting MoTe$_2$ crystal is coupled to a s-wave superconductor, such as niobium, the incompatibility between the two condensates is manifested as various experimental anomalies. The phenomena include asymmetric noise content in fluoxid-induced oscillations, anti-hysteretic central peaks, and bimodal stochastic switching of critical current. Analyses of these observations reveal that the unique gap function of device, described by the correlation method of proximity effect, switches between different symmetries as the field changes the relative strengths of the pair condensates. The symmetry that dictates the edge pairing can be inferred from the noise content in edge oscillations, which also uncovers the novel blockade mechanism of s-wave condensate by the intrinsic one. Covering the entire physical edge of a MoTe$_2$ crystal with niobium gives rise to nonlocal effects. In a completely enveloped device, s-wave symmetry is enforced in the distant, uncapped segments of edge. Enclosing a crystal by the edge with niobium suppresses all bulk pairings, which suggests that the bulk states are sensitive to the entirety of edge states. Anomalous modulations of critical current are observed in a kagome superconductor CsV$_3$Sb$_5$ with niobium electrodes. The period of oscillations is unstable and changes during field scans. The area associated with this period is always larger than the physical area of device. The incompatibility between the intrinsic CsV$_3$Sb$_5$ and s-wave condensates appears as anti-hysteretic zero-bias differential resistance and jagged central peaks.