The superconducting transition temperature, C, defines the point at which the free energies of the superconducting and normal states of a material become equal. Just above 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 discoveredsuperconductors Kand Rb In addition to probing the critical state near c, these measurements allow the indirect determination of the residual, normal-state resistivity.
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