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

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.
10 pages, 7 figures