Direct Observation of a Temperature Dependent 0-pi Junction Transition in a Mesoscopic High Critical Temperature Grain Boundary Junction

TL;DR

This study reports the first direct observation of a temperature-dependent 0-pi transition in a high critical temperature grain boundary Josephson junction, revealing nonmonotonic behavior and a sign change in the critical current.

Contribution

It provides experimental evidence of a temperature-induced 0-pi transition in a high-Tc grain boundary junction, supported by phase-sensitive measurements and theoretical models.

Findings

Critical current sign changes at temperature T*

Transition from 0-SQUID to pi-SQUID at T*

Agreement with models involving mid-gap states

Abstract

We have observed, for the first time, a temperature dependent 0-pi junction crossover in a 45 symmetric [001] tilt grain boundary junction. Experimental data, obtained by a double phase-sensitive test based on a low inductance dc SQUID, show an anomalous nonmonotonic temperature dependence of the Josephson current, clear evidence of a critical current sign change at a temperature T*. At the same temperature, the SQUID undergoes a transition from a 0-SQUID to a pi-SQUID. Results are in good agreement with theoretical models taking into account mid-gap states in low transparency junctions.