HEAT-CONDUCTION IN BA1-XKXBIO3

We have investigated heat conduction of single crystal Ba1-xKxBiO3 in the temperature range of 2-300 K and in a magnetic field of up to 6 Te sla. Temperature dependence of thermal conductivity kappa(T) reveals t he participation of both electrons and phonons with their relative con tributions that depend critically on the potassium doping concentratio n. Crystals underdoped with potassium (samples with higher T-c) exhibi t a strong suppression of kappa and a glass-like temperature dependenc e. in contrast, those with a higher potassium content (lower T-c) show an increase as temperature decreases with a peak near 23 K. Field dep endence of kappa(H) is also very sensitive to the level of potassium d oping. Crystals exhibiting a large phonon contribution show an initial drop in kappa(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 en ters a mixed state. The high field behavior of kappa(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in th is material.