Observations and model of a new ac-loss valley in a YBCO superconductor

We report on measurements of the ac susceptibility of sintered YBCO pellets, with ZnO added, as a function of temperature T, for several different ac magnetic field amplitudes, Hac, in the presence of static bias magnetic field Hb directed along Hac. For each choice of Hac, the imaginary part of the ac susceptibility versus T traces a peak, denoted χ''max, centred at a temperature denoted Tmax. We find that χ''max versus both Hac and Tmax traces a valley. Calculations of hysteresis losses, exploiting the critical state model where the critical current density jc = α(T)/Bn, reproduce the experimental data quite well.