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CRITICAL CURRENT DENSITIES IN Bi 2Sr 2CaCu 208+d Shunji Nomura*,

Yutaka Yamada+, Tomohisa Yamashita, Eriko Yoneda, Hisashi Yoshino and Ken Ando

Research & Development Center TOSHIBA CORPORATION Komukai-Toshiba cho, Kawasaki 210, JAPAN present address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA02139 a p present address: Kernforschungzentrum Karlsruhe GmbH, D7500 Karlsruhe 1, Weberstrasse 5, 3640 West Germany ABSTRACT Critical current densities and upper critical fields were measured for a single crystal of the high Tc oxide superconductor, Bi 2 Sr2CaCu 2 08+d, within the ab basal plane and along the c axis. The anisotropy in critical current densities was observed to be Jc,/Jc,=lO in agreement with the anisotropy in resistivity for the normal state. The magnetic field dependence of the critical current densities can be interpreted by the anisotropy in the upper critical fields. The scaling of the critical current density with the magnetic field was found. The critical current density scaled to zero at fields Bc 2 in the ab plane and along the c axis which were in good agreement with the upper critical fields measured by transport. The anisotropy in flux pinning force density along the b axis, Fp//b, and along the a axis, Fp,,a, was found to be Fp,,b/Fp,,a=3. The data provided strong evidence for flux pinning by the modulated structure in this system.

INTRODUCTION Oxide superconductors have been recently discovered in the Bi-Sr-Ca-Cu-O system'. Bi2Sr2CaCu 2 O8+d with 80 K critical temperature was identified2"4. This compound is similar to its predecessor YBa 2 Cu3 0 7 in regard to structural features, such as Cu-0 pyramids and in physical properties, such as a very high upper critical field Bc 2 , short coherence lengths •, and large anisotropies in transport phenomena, reflecting its two-dimensionality. Critical current densities in single crystals of Bi2Sr2CaCu2O8+d have been reported by transport5,6 as well as by 7 8 magnetization , . However, there exist important differences between the two materials such as the absence of twinning, superconductive properties of being invariant to oxygen atmosphere and the presence of a modulated structure in the Bi-Sr-Ca-Cu-0 system. An important question is the origin of the pinning centers in those oxide superconductors. It is interesting to investigate roles of a modulated structure on superconductivity, especially on flux pinning in Bi2Sr2CaCu2O8+d single crystals without twinning. In this letter, the authors report an anisotropy in critical current densities under a magnetic field and the upper critical field for transport in the ab plane and along the c axis in single crystals Bi2Sr 2 CaCu2O8+d at the boiling point of liquid nitrogen (77.3K). The angular dependence of the critical current densities between the crystal axes and an applied magnetic field was measured. The anisotropy in the flux pinning forces can be explained by the presence of the modulated structure in this system.

EXPERIMENTAL PROCEDURE Singl