Vibrating reed study of the flux line dissipation of ceramic and single-crystal (Ba, K)BiO₃

Vibrating reed (VR) and dc and ac magnetic susceptibility measurements were performed on a nominal single crystal of composition (Ba_0.64K_0.36)BiO₃ in an applied magnetic field H. Field-cooling and field-sweep data revealed multiple peaks in the reed dissipation 1/Q located below the superconducting transition temperature T_c(≈29.6 K for H=0). A shoulder or onset (with increasing T) of dissipation appears for T≤18 K, which may be a signature of a flux lattice melting transition. VR data for dense ceramic samples of composition (Ba_0.625K_0.375)BiO₃ (T_c=28.6K) exhibit a relatively narrow and smooth peak in 1/Q that corresponds well to a broad, intermediate-temperature peak in the crystal data. Resistivity experiments demonstrate that the single ceramic peak occurs well below the temperature at which the electrical resistance R≈0, suggesting that the higher-temperature crystalline peaks are positioned close to the upper critical field line and may be strongly dependent upon grain size or surface properties. Both ac and dc susceptibility data show no clear evidence for multiple phases or gross compositional inhomogeneities in the crystalline sample. Our results demonstrate that the VR technique is an extremely sensitive method to probe sample inhomogeneities and their role in flux pinning phenomena.