Observation of Kosterlitz-Thouless Transition and Conductivity Fluctuation in Superconducting Ybco Thin Films
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OBSERVATION CONDUCTIVITY FILMS
OF KOSTERLITZ-THOULESS TRANSITION SUPERCONDUCTING YBCO FLUCTUATION IN
AND THIN
Q. Y. YING*' and H. S. KWOK State University of New York at Buffalo, Institute on Superconductivity, Amherst, NY. 14260 * Also Department of Physics and Astronomy, SUNY at Buffalo, Amherst, NY 14260 ABSTRACT Kosterlitz-Thouless properties of laser deposited high Tc superconducting thin films of YBa2Cu30 7 _x were examined. The exponent of the power law dependence of V on I showed a "universal jump" at Tc. 1D like conductivity fluctuation was also found at temperatures slightly above the mean-field transition temperature Tco, which may be the result of imperfections of the sample.
INTRODUCTION In the last decade, there has been considerable efforts to investigate both theoretically and experimentally the Kosterlitz-Thouless (KT) properties in 2D superconductors, namely superconducting thin films [1]1. In the high Tc oxide superconductors, there exists an intrinsic 2D Cu-O structure. Kosterlitz-Thouless properties has been examined in YBa 2 Cu 3 0 7 thin films for renormalization of mean-field penetration depth study [2], in bulk, single crystal YBa 2 Cu 3 0 7 and Bi 2 Sr 2 CaCu2 0 8 samples [3-6]. Also phase transition with vortex pair unbinding was observed in YBa2Cu30 7 films with optical response study [7]. Most recently, KT transition was found in T12 Ba 2 CaCu2 08 films [81. It is generally believed that 2D Cu-O plane in the structure plays an important role in high Tc superconductivity. It is interesting to study the dimensionality behavior in those materials. Excess conductivity above the transition temperature provides the dimensionality information since it depends on reduced temperature differently [9]. Results previously reported in the literature showed properties of 3D, 2D and crossover of the dimensionality [ 10]. In this report, Kosterlitz-Thouless transition in c-axis oriented YBa2Cu307_x superconducting thin films was observed from I-V measurement. It was also found that the excess conductivity above the mean-field transition temperature Tco showed a 1D behavior and additional complications at higher temperatures. EXPERIMENT The sample under study was made by laser deposition using a 193 nm ArF excimer laser [11]. X-ray diffraction peaks showed c-axis preferential orientation perpendicular to the (100) MgO substrate. The film thickness was 0.12 jtm. It was patterned into a strip using the laser etching technique [12]. The strip was 80 jtm wide and 6.5 mm long. Standard four-probe method was used for the I-V measurement near zero magnetic field (0.5 G). The temperature stability during each measurement was
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