Hysteretic method of lower critical field determination in high-Tc superconductors

The per unit volume per cycle AC hysteretic loss of samples with different radii, R0 (filament radii, in the case of multifilamentary materials), can be meaningfully compared as function of field-sweep amplitude, Hm, by normalizing the loss to some reference radius, Rref.. The R0-normalized Qh can then be used to compare the Jcs of different samples in both the low- and high-field regimes. A new function, Qh=(10/3 pi 2JcR0)((3/2)Hc1(Hm-Hc1)2+(Hm-Hc1)3), has been developed for describing the low-field hysteretic loss in transverse-field cylindrical samples. Two methods are outlined for using this equation to extract Hc1 from thick high-Jc samples-samples which defy the conventional deviation-from-linearity approach. Applications of these procedures to samples of melt-processed YBCO and multifilamentary NbTi are presented.