The value of the dynamic critical exponent z is studied for two-dimensional superconducting, superfluid, and Josephson Junction array systems in zero magnetic field via the Fisher-Fisher-Huse dynamic scaling. We find z ≃ 5.6 ± 0.3, a relatively large value indicative of non-diffusive dynamics. Universality of the scaling function is tested and confirmed for the thinnest samples. We discuss the validity of the dynamic scaling analysis as well as the previous studies of the Kosterlitz-Thouless-Berezinskii transition in these systems, the results of which seem to be consistent with simple diffusion (z = 2). Further studies are discussed and encouraged.

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We present evidence for nonequilibrium two stage melting in Bi 2 Sr 2 CaCu 2 O 81d . Samples cooled in low applied magnetic fields display a sharp hysteretic drop of the resistance which is associated with a first-order vortex lattice freezing transition. Using an eight-terminal geometry, we have measured a second sharp feature in the liquid phase which suggests thermal interlayer decoupling of vortex lines. The melting transition temperature decreases with increasing current, however, indicating that these measurements were made under nonequilibrium conditions. [S0031-9007(97)02320-X]