Thermal diffusivity in poly- and single-crystalline YBCO superconductors

The thermal diffusivity, a, and the specific heat, C, of YBa2Cu3O7−δ of poly-crystals, synthesized by the usual method, and single-ones, by the QMG method were measured by using the transient method. The thermal conductivity, κ, is evaluated by κ=ϱaC, where ϱ is the density. Both the thermal diffusivity parallel to the ab-plane for the QMG crystal and that for the poly one show the gradual increase around Tc with decreasing temperature. The value of a parallel to the c-axis for the single crystal specimen do not show a definite increase at Tc with decreasing temperature. Temperature dependence of κ parallel to the ab-plane is interpreted in terms of the phonon transport theory for the ordinary condition, but that parallel to the ifc-axis is not explained without considering the condition of the interlayer tunneling along c-axis.