Large Second-Order Josephson Effect in Planar Superconductor-Semiconductor Junctions

TL;DR

This paper reports the discovery of a large second-order Josephson harmonic in planar superconductor-semiconductor junctions, demonstrated through multiple experimental signatures and supported by numerical modeling, with implications for understanding superconducting quantum devices.

Contribution

It provides the first evidence of a significant second harmonic in planar Josephson junctions, using experimental data and numerical modeling to analyze its properties and origins.

Findings

Half-periodic SQUID oscillations tunable by gate voltages.

Presence of half-integer Shapiro steps.

Observation of kinks near half-flux quantum in diffraction patterns.

Abstract

We investigate the current-phase relations of Al/InAs-quantum well planar Josephson junctions fabricated using nanowire shadowing technique. Based on several experiments, we conclude that the junctions exhibit an unusually large second-order Josephson harmonic, the $\sin(2\varphi)$ term. First, superconducting quantum interference devices (dc-SQUIDs) show half-periodic oscillations, tunable by gate voltages as well as magnetic flux. Second, Josephson junction devices exhibit kinks near half-flux quantum in supercurrent diffraction patterns. Third, half-integer Shapiro steps are present in the junctions. Similar phenomena are observed in Sn/InAs quantum well devices. We perform data fitting to a numerical model with a two-component current phase relation. Analysis including a loop inductance suggests that the sign of the second harmonic term is negative. The microscopic origins of the observed effect remain to be understood. We consider alternative explanations which can account for some but not all of the evidence.