Fractional Order Shapiro Steps in Superconducting Nanowires

TL;DR

This paper investigates fractional Shapiro steps in superconducting nanowires under microwave radiation, revealing fractional steps at higher frequencies and modeling this with a multi-valued current-phase relationship.

Contribution

It demonstrates the emergence of fractional Shapiro steps at high frequencies and introduces a numerical model based on a multi-valued CPR for nanowires.

Findings

Integer Shapiro steps at 0.9-4 GHz

Fractional steps of 1/2, 1/3, 1/4, 1/6 at higher frequencies

Modeling with multi-valued current-phase relationship

Abstract

We expose superconducting nanowires to microwave radiation in order to study phase lock-in effects in quasi one dimensional superconductors. For sufficiently high microwave powers a resistive branch with Shapiro steps appears in the voltage-current characteristics. At frequencies in the range of 0.9-4 GHz these steps are of integer order only. At higher frequencies steps of 1/2, 1/3, 1/4 and even 1/6 order appear. We numerically model this behavior using a multi-valued current-phase relationship (CPR) for nanowires.