SUPERCONDUCTING FLUCTUATIONS IN THE THERMAL-CONDUCTIVITY OF BI2SR2CACU2O8 AND DYBA2CU3O7-X MATERIALS

The thermal conductivity kappa of Bi2Sr1.8Ca1.2Cu2O8 and DyBa2Cu2O7-x polycrystals is analyzed near the critical temperature (T-c=79.5 and 8 3.8 K, respectively) in order to extract the superconducting fluctuati on contribution kappa(fl). The fluctuationless background is calculate d in a formal way, taking into account both an electronic and a phonon contribution above and below the critical temperature. The fluctuatio n contribution to the electronic thermal conductivity is then extracte d. A crossover from a critical to a Gaussian fluctuation regime (\T-G- T-c\approximate to 2.2 K) is observed on the Bi2Sr1.8Ca1.2Cu2O8 sample both above and below T-c, followed by a crossover from a three-dimens ional (3D) to a 2D Gaussian behavior (\T-VL-T-c\approximate to 3.7 K) as the sample temperature is further moved away from T-c. On the other hand, a 3D behavior only is observed on the DyBa2Cu2O7-x material in the normal and superconducting states. In both systems, the critical e xponents are found to be those theoretically predicted. Moreover, the crossover temperatures are consistent with those expected from the dif ferently anisotropic structures of these compounds. From the crossover temperatures and the thermal conductivity fluctuation contribution am plitudes, quite realistic values for the Ginzburg-Landau parameter k(G L), the interlayer coupling energy J(c), and the electronic mean free path l(e) near T-c are found in those materials.