Scaling of magnetization and some basic parameters of Ba1-xKxBiO3+y superconductors near Tc

Reversible magnetization and hysteresis loops have been studied over wide ranges of magnetic field and temperature for single crystals of Ba1-xKxBiO3+y (x=0.34, 0.37, 0.41, and 0.46). The main goals were to derive critical parameters of superconductors with a cubic structure as well as to explain the origin of flux pinning and peak (fishtail) effect observed in all samples. Values of the electron-phonon interaction constant of lambda(ph)similar to 1, similar to 0.76, and 0.9 for Ba1-xKxBiO3+y with x=0.34, x=0.37, and x=0.41, respectively, have been obtained. The data show that this family belongs to oxide superconductors with at least an intermediate strength of coupling. The pinning force density has been scaled into a single curve for wide field and temperature ranges. The functional form of the scaling curve reflects various mechanisms of flux pinning. For Ba0.54K0.46BiO3+y the peak effect is mainly due to a dynamical contribution. However, the fishtail for Ba1-xKxBiO3+y with x = 0.34, 0.37, and 0.41 has its origin in the crossover between two pinning mechanisms induced by external magnetic field and/or temperature. For the Ba0.59K0.41BiO3+y crystal a seeded growth-induced anisotropy was found to be responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic-field orientations. [S0163-1829(98)00442-1].