3-DIMENSIONAL FLUCTUATION CONDUCTIVITY IN SUPERCONDUCTING SINGLE-CRYSTAL K3C60 AND RB3C60

THE superconducting transition temperature, T(c), defines the point at which the free energies of the superconducting and normal states of a material become equal. Just above T(c), thermodynamic fluctuations produce small, transient regions of the superconducting state, giving rise to an anomalous increase in the normal-state conductivity known as paraconductivity. This situation is analogous to the fluctuating regions of correlated spins found near the Curie-Weiss transition in ferromagnets. Such, fluctuations are of theoretical significance in that they provide a direct probe of critical phenomena in general, and a stringent test of scaling theories, which describe the approach to the critical point. Paraconductivity effects are strongly dependent on the dimensionality of the system, although for conventional superconductors, three-dimensional fluctuation conductivity has to our knowledge never been observed. Here we report the observation of pure, three-dimensional paraconductivity in single crystals of the recently discovered1 superconductors K3C60 and Rb3C60. In addition to probing the critical state near T(c), these measurements allow the indirect determination of the residual, normal-state resistivity.