Superconducting fluctuations in the thermal conductivity of Bi2Sr2CaCu2O8 and DyBa2Cu3O7-x materials

The thermal conductivity κ of Bi2Sr1.8Ca1.2Cu2O8 and DyBa2Cu2O7-x polycrystals is analyzed near the critical temperature (Tc=79.5 and 83.8 K, respectively) in order to extract the superconducting fluctuation contribution κfl. The fluctuationless background is calculated in a formal way, taking into account both an electronic and a phonon contribution above and below the critical temperature. The fluctuation contribution to the electronic thermal conductivity is then extracted. A crossover from a critical to a Gaussian fluctuation regime (|TG-Tc|∼2.2K) is observed on the Bi2Sr1.8Ca1.2Cu2O8 sample both above and below Tc, followed by a crossover from a three-dimensional (3D) to a 2D Gaussian behavior (|TVL-Tc|∼3.7K) as the sample temperature is further moved away from Tc. 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 exponents are found to be those theoretically predicted. Moreover, the crossover temperatures are consistent with those expected from the differently anisotropic structures of these compounds. From the crossover temperatures and the thermal conductivity fluctuation contribution amplitudes, quite realistic values for the Ginzburg-Landau parameter kGL, the interlayer coupling energy Jc, and the electronic mean free path le near Tc are found in those materials. copyright 1997 The American Physical Society