Josephson Effects in the Iron-Based Superconductor FeTe$_{1-x}$Se$_{x}$

Abstract

The iron-based superconductor FeTe$_{1-x}$Se$_x$ has emerged as a promising platform for combining superconductivity and topology in a single system, for the realization of topological quantum computing. Besides this, FeTe$_{1-x}$Se$_x$ hosts rich physical phenomena such as S$\pm$ superconductivity, Majorana bound states, and higher-order topological superconductivity, among others. Despite the interest in superconducting devices based on FeTe$_{1-x}$Se$_x$, there have been relatively few demonstrations of Josephson junctions in FeTe$_{1-x}$Se$_x$-based systems.

In this dissertation we measured Josephson effects in a FeTe$_{1-x}$Se$_x$-based device and found three signatures of unconventional Josephson junction behavior. This first signature was the existence of two distinct Josephson diffraction patterns under applied RF irradiation, which likely arises from flux flow and a phase slip line in FeTe$_{1-x}$Se$_x$. The second signature was the emergence of sudden jumps in the DC current at which Shapiro steps arise, as a function of applied RF power. This was measured by mapping $\frac{dV}{dI}$ vs DC current and RF power. We provide two potential explanations for this phenomenon based on non-equilibrium superconductivity. The third signature is a minimum critical current at zero magnetic field when RF irradiation is present which resembles the $\pi$-Josephson junctions formed as a consequence of multiband superconductivity.