Heat conduction in Ba1?xKxBiO3

Abstract

We have investigated heat conduction of single crystal Ba 1-x K x BiO 3 in the temperature range of 2-300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivity κ(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higher T c ) exhibit a strong suppression of K and a glass-like temperature dependence. In contrast, those with a higher potassium content (lower T c ) show an increase as temperature decreases with a peak near 23 K. Field dependence of κ(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop in κ(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior of κ(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.