Josephson spectroscopy study of kagome superconductors toward the deep point-contact regime

TL;DR

This study explores the behavior of Josephson currents in kagome superconductors using deep point-contact scanning tunneling microscopy, revealing deviations from expected conductance dependence and highlighting implications for exotic physics research.

Contribution

It demonstrates the effects of deep point-contact regime on Josephson current measurements and identifies optimal conditions for atomic-scale probing of pair-density wave states.

Findings

Zero-bias conductance deviates from quadratic dependence in deep contact regime.

Saturation of zero-bias conductance caused by series resistance.

Optimal regime identified for studying pair-density wave states in kagome superconductors.

Abstract

Josephson scanning tunneling microscopy (JSTM) has emerged as an important technique for probing the superconducting order parameter at the atomic scale. However, the Josephson current in JSTM may behave quite differently when the coupling strength varies. Here, we push the junction to the deep point-contact regime, reaching a normal-state junction resistance of only 0.15 $h/2e^2 \simeq 2~{\rm k}\Omega$. We demonstrate, using kagome superconductors, that the zero-bias conductance, a key characteristic of the Josephson current, deviates strongly from the quadratic dependence on the normal-state conductance upon entering the deep point-contact regime. Furthermore, we observe a striking saturation of the zero-bias conductance, which we show arises from the series resistance in the circuit. This also serves as a cautious reminder when interpreting zero-bias conductance saturation or quantization in studies of exotic physics such as that of Majorana zero modes if the tip-sample junction resistance is extremely small. Finally, we identify an optimum regime where JSTM can be used as an atomic-scale probe for studying pair-density wave states in materials with low superconducting transition temperature, such as AV3Sb5 kagome superconductors.