We have observed power-law behavior in the current-voltage characteristics of single crystals of Bi2Sr&Ca&Cu208+~and Bii 6Pbo 4Sr2Ca2Cu30 both in zero and in applied magnetic fields. The observed power-law behavior, V~I ' ' near the transition with a characteristic Nelson-Kosterlitz jump in the exponent a( T) at T = T, gives evidence for a Kosterlitz-Thouless (KT) transition in wihch vortexantivortex pairs are dissociated within the superconducting planes. In a magnetic field, the KT transition is suppressed because field-induced vortices induce dissipation and reduce the stability of vortexantivortex pairs. The interaction of vortex pairs with interlayer Aux lines has been discussed.The transport properties of high-T, superconductors have several interesting and important features. Among them, the transport dimensionality and the non-ohmic behavior observed near the superconducting transition temperature need to be explained clearly. It is well known that the high-T, superconductors have twodimensional (2D) properties due to the layered structure of the CuOz superconducting planes which interact weakly with each other. Kosterlitz-Thouless (KT) (Ref. 1) behavior has therefore been expected, and the onset of a resistive state in several high-T, materials is described in terms of a KT transition. The power law of the current-voltage (I V) characte-ristics (V~I ' ') is described in the framework of the KT model near to the mean-field critical temperature T,o.In the KT model of a 2D system the phase transition at a critical temperature TKT is determined by the dissociation of vortex-antivortex pairs (pancake vortices) which interact mutually with a logarithmic potential U(r) =2m. Ektt T ln(r lg), where g is the Ginzburg-Landau (GL) coherence length, and nK=P d/16' k+TA. the stiffness constant of the KT theory. d is the thickness of the fluctuating superconducting sheet and k is the penetration depth in the ab plane. When T"T ( T", the pairs can be broken by an applied current density causing dissipation and a non-ohmic behavior as the temperature is increased. This gives a sudden jump in the exponent a( T) = 1+srK from 3 to 1 at a temperature TzT which gives the 2D nature of the superconductors.The observed power law in the I-V characteristics, suggesting a scaling law, cannot be derived from the standard fluxflow or flux-creep phenomena. The most important characteristic feature of the direct evidence of KT transition is the universal Nelson-Kosterlitz jump in the power-law exponent as a function of temperature. In this paper, we report the observation of the Nelson-Kosterlitz jump in a in zero magnetic field and also the behavior of the exponent a in external applied magnetic fields in single crystals of Bi&Sr&Ca, Cuzos+r (BSCCO) and Bii 6Pbo 4SrzCazCu30"(BPSCCO). We show how an external field plays a crucial role in the dynamics of the KT transition.The BSCCO and BPSCCO crystals used in our studies were grown by a self-flux method. The details of the growth process have been discussed elsewhere.Chemical analysi...
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