Half-integer Shapiro steps at the 0-pi crossover of a ferromagnetic Josephson junction

TL;DR

This paper studies the current-phase relation in ferromagnetic Josephson junctions near the 0-pi crossover, revealing fractional Shapiro steps indicative of a sin(2*phi) relation caused by energy level splitting in the ferromagnet.

Contribution

It demonstrates the presence of half-integer Shapiro steps in Nb/CuNi/Nb junctions at the 0-pi crossover, indicating a doubled Josephson frequency and a sin(2*phi) current-phase relation.

Findings

Observation of residual supercurrent at the crossover temperature.

Detection of half-integer Shapiro steps under high frequency excitation.

Evidence of a sin(2*phi) current-phase relation due to energy level splitting.

Abstract

We investigate the current-phase relation of S/F/S junctions near the crossover between the 0 and the pi ground states. We use Nb/CuNi/Nb junctions where this crossover is driven both by thickness and temperature. For a certain thickness a non-zero minimum of critical current is observed at the crossover temperature. We analyze this residual supercurrent by applying a high frequency excitation and observe the formation of half-integer Shapiro steps. We attribute these fractional steps to a doubling of the Josephson frequency due to a sin(2*phi) current-phase relation. This phase dependence is explained by the splitting of the energy levels in the ferromagnetic exchange field.