THERMAL-CONDUCTIVITY OF TWINNED YBA2CU3O7-X AND TWEEDED YBA2(CU0.95FE0.05)3O7-X IN A MAGNETIC-FIELD - EVIDENCE FOR INTRINSIC PROXIMITY EFFECT

The behavior of the thermal conductivity kappa of twinned YBa2Cu3O7-x and tweeded YBa2(Cu0.95Fe0.05)3O7-x polycrystals in a magnetic field up to 5 T is presented. Data as a function of field are shown to depart from the usual linear behavior of thermal magnetoresistivity (TMR) kappa-1 (B) for conventional superconductors. A set of stretched exponential laws, of the form kappa-1 (B) - kappa-1 (0) = CB exp(-pB(q)) has been found to describe our data with q decreasing with temperature from 1 to 1/2 and from 1/2 to 1/4 for pure and iron-doped samples, respectively, in contrast to the unique interpolation formula (q = 1/4) for TMR proposed recently by Richardson et al. [1] to fit their data on twinned and untwinned single crystals. Such a complex behavior of TMR in the high field region is supposed to originate from extended defect mediated intrinsic proximity effect in both samples. The proposed interpretation is based on incorporating two types of phonon scatterers, free (unpinned) and pinned (by the defect cores) vortices, into a conventional picture of phonon-vortex scattering mechanism