Degenerate ground state and anomalous flux hysteresis in an YBa2Cu3O7 grain boundary r.f. SQUID

TL;DR

This study investigates flux hysteresis and trapped flux in a YBa2Cu3O7 rf SQUID with grain boundary junctions, revealing temperature-dependent phenomena including a degenerate ground state and anomalous flux behavior.

Contribution

It introduces a model incorporating a temperature-dependent second harmonic in the current-phase relationship to explain observed flux hysteresis anomalies.

Findings

Broadening of flux distribution from 15 K to 30 K

Bifurcation at 35 K indicating degenerate ground state

Disappearance of bifurcation and emergence of secondary loops above 40 K

Abstract

We report measurements of the flux hysteresis curves and trapped flux distribution in an YBa2Cu3O7 r.f. SQUID containing two closely spaced grain boundary Josephson junctions in parallel. Broadening of the flux distribution from T = 15 K to 30 K is followed by a bifurcation at T = 35 K which corresponds to a degenerate ground state. Above T ~ 40 K the bifurcation disappears, the flux distribution narrows significantly and small secondary loops appear in the hysteresis curves. This behaviour can be modelled qualitatively if we assume a temperature dependent second harmonic term in the current-phase relationship of the junctions.