ANISOTROPIC LOW-TEMPERATURE HEAT-TRANSPORT IN YBA2CU3O6.9 SINGLE-CRYSTAL

We report measurements of the low-temperature thermal conductivity of YBa2Cu3O7-delta (delta congruent-to 0.1) single crystals (T(c) = 84 K) both parallel (kappa(a,b)) and perpendicular (kappa(c)) to the CuO2 planes. Whereas kappa(c) (T) is found to be identical, within experimental resolution, with the phonon contribution kappa(ph) (T), kappa(a,b) (T) contains an additional term linear in temperature, AT. We ascribe AT to the contribution of unpaired electronic carriers residing in the "chain layers". Measurements performed in external magnetic fields B less-than-or-equal-to 8 T support this interpretation. Our observations can be explained by an "internal multilayer" (IML) model in which it is assumed that strong super-conductivity is generated within the CuO2 layers and weak superconductivity is induced in the "chain layers" by the proximity effect. The fit of the experimental results to the IML model reveals that approximately 15% of the electronic carriers remain unpaired in YBa2Cu3O7 below T = 1 K.