Wan, Hebboul, and Garland Reply: The theoretical work of Horovitz [l] concludes that our recent measurements in BizSrzCaCuzOs-~(BSCCO) [2] can be interpreted in terms of a fluxon transition. His predictions seem to be consistent with our main findings, namely, the existence of two transition temperatures and a pronounced peak in the secondary voltage. A key feature of the Horovitz theory, ho~ever, is the existence of "fluctuating" fluxons which mediate the coupling between thermally excited vortices on adjacent CuO bilayers. Since fluxons are a consequence of the Josephson interaction, his theory also appears to be consistent with our general interpretation of the data. Clearly, more work is needed to establish the exact role of vortex and fluxon excitations in these layered superconductors.

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Measurements of the zero-field resistive transition of Bi2Sr2CaCu20s single crystals suggest that a bulk zero-resistance state develops through a sequential process characterized by two transition temperatures. The upper transition corresponds to Josephson coupling of parallel CuO bilayers, and the lower transition to the conventional Kosterlitz-Thouless dissociation of bound vortex excitations. Measurements using a "flux transformer" electrode geometry indicate a pronounced peak in the secondary voltage, which is suppressed by an external magnetic field. PACS numbers: 74.40.+k, 74.50.+r, 74.60.Ge, 74.72.HsThe unusual transport properties of the high temperature superconductor I^S^CaiC^Og-y (BSCCO) highlight the importance of understanding the superconducting phase transition in strongly anisotropic materials. It is known [1] that the superconducting transition of this material shows quasi-two-dimensional behavior, as evidenced by an exponential square-root singularity in the temperature dependence of p a b(T)> the electrical resistivity measured in the crystalline a-b plane, and by a power law in the current-voltage (IV) characteristics which increases from a linear (Ohmic) to a cubic dependence at T c , and which becomes increasingly nonlinear at lower temperatures. Such behavior is commonly seen in two-dimensional superconducting films [2].By contrast, along the c axis, the normal-state resistivity p c (T) of BSCCO is very high, of the order 10 5 -10 6 of pab, with the thermally activated temperature dependence of a semiconductor and a broadened superconducting transition in which a critical current develops below T c [3]. These properties are consistent with the view that superconductivity in BSCCO originates in stacked bilayers of CuO, oriented parallel to the a-b plane, with adjacent bilayers coupled by the Josephson interaction [4].Although much recent work [5] has focused on the properties of quantized flux lines threading these bilayers, little is known about the details of the resistive transition. More generally, it is not yet established how the transition to bulk superconductivity proceeds in planar, anisotropic materials in which out-of-plane Josephson coupling modifies the conventional 2D transition [6]. Here, we address this issue through detailed measurements of the //«0 transition of BSCCO single crystals. We find that the development of a bulk zero-resistance state takes place through a sequential process characterized by two transition temperatures. At the upper transition temperature, designated 7£, the Josephson interaction couples two-dimensional CuO bilayers. At the lower transition temperature, designated T? (for BSCCO, T c c~T ? b^2 K), the bilayers undergo a Kosterlitz-Thouless-type transition to a zero dissipation state. In the temperature interval Tc b

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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]