On the Resistive State of High Temperature Superconductors in Magnetic Field
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ON THE RESISTIVE STATE OF HIGH TEMPERATURE SUPERCONDUCTORS IN MAGNETIC FIELD Y. lYE*, S. NAKAMURA-, T. TAMEGAI*, T. TERASHIMA** AND Y. BANDO** * The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106 Japan ** Institute for Chemical Research, Kyoto University, Uji, Kyoto 611 Japan ABSTRACT Experimental studies of the resistive state of high temperature superconductors in the mixed state were carried out on thin film samples of YBa 2Cu 3 O7 _- and Bi 2Sr 2CaCu 2Os+y. Precise angular dependence measurements on YBa 2 Cu 3 07-, epitaxial film have revealed not only the critical field anisotropy with respect to the a, b, and c-axes but also a feature associated with twin boundaries. Dependence on the angle between the transport current and the magnetic field is investigated and discussed in terms of flux dynamics in highly anisotropic layered superconductors. Peculiar behavior of the Hall effect in the resistive state is reported. INTRODUCTION The mechanism of energy dissipation in the resistive state of high temperature superconductors (HTSCs) in magnetic field is the subject of much current interest. While its technological implication, i.e. loss of the current-carrying ability without energy dissipation, is certainly an issue of great concern, the occurrence of dissipative process offers a unique opportunity to access a previously unexplored field of superconductivity, because it allows us to carry out various types of transport measurements in a reasonably wide region of the superconducting phase. In this paper, we report on our recent experimental studies of transport in the mixed state of the HTSCs. In particular, we discuss the dependence of resistivity on the magnetic field orientaion relative to the crystal axes and to the transport current direction, The behavior of the Hall effect in the superconducting transition region is also treated. Some parts of the work we present here have been published.[1-31 EXPERIMENTAL Samples used in the present work are thin film samples of Bi 2 Sr 2CaCu 2Os+, and YBa 2 Cu3 0 7 y. Highly c-axis oriented polycrystalline films of Bi 2 Sr 2 CaCu2O8 +y were prepared in ISSP, Univ. of Tokyo by single-target rf-sputter deposition onto MgO (100) substrates.[1I Epitaxial films of YBa 2 Cu307_r were fabricated in ICR, Kyoto Univ. by activated reactive coevaporation technique on SrTiO3 (100) substrates.[4] The YBa 2 Cu 3 0 7 y films were heavily twinned, consisting of densely interwoven stripe regions with interchanged a- and b-axes. The average spacing between the twin boundaries is not precisely known. The length scale is presumably much less than a micron, since the twin patterns are not resolved by usual polarized optical microscopy. All the samples were prepared in a standard Hall bar pattern. Electrical leads were attached with silver paste either directly or on evaporated silver pads. The resistivity and Hall measurements were done by usual d.c. methods. Two sets of superconducting magnet systems were used for the magnetotransport experiments.
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