Low-temperature specific heat of several high-temperature superconductors in the superconducting and mixed states

Abstract

The low-temperature specific heat of high quality, well characterized sintered polycrystalline samples of superconducting Ba$\sb{0.6}$K$\sb{0.4}$BiO$\sb3,$ YBa$\sb2$Cu$\sb3$O$\sb{6.7},$ and Bi$\sb2$Sr$\sb2$CaCu$\sb2$O$\sb{\rm 8+x}$ were measured in the superconducting state. Careful analysis of the data below 10K indicate that the contribution to the specific heat which has a linear dependence on temperature, which has been observed in samples of YBa$\sb2$Cu$\sb3$O$\sb{7-\delta},$ is not a universal feature of the high temperature superconductors. We observed that the linear term in YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ is weakly dependent on the oxygen content, $\delta.$ We argue that a linear term of 4 mJ/mole K$\sp2$ is intrinsic to this compound, and while the origin of this linear term is still unknown it does not appear to be associated with the superconducting state.

The low-temperature specific heat of the YBa$\sb2$Cu$\sb3$O$\sb{6.7}$ and Bi$\sb2$Sr$\sb2$CaCu$\sb2$O$\sb{\rm 8+x}$ samples were also measured in the mixed state in applied magnetic fields up to 3T. A strong field dependence in the specific heat was observed in the YBa$\sb2$Cu$\sb3$O$\sb{6.7}$ sample, while the specific heat of Bi$\sb2$Sr$\sb2$CaCu$\sb2$O$\sb{\rm 8+x}$ was independent of field. The presence of a large field dependence at low-temperatures in the mixed state is not in agreement with the predictions of London theory. We mention several possible explanations for the large field dependence of the specific heat, including novel vortex structures, vortex kink nucleation, vortex core interactions, proximity effect coupling, and nodes in the superconducting gap.