High Critical Current Density in Ultrathin Ybco Films
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HIGH CRITICAL CURRENT DENSITY IN ULTRATHIN YBCO FILMS X.X. XI**, W. SCHAUER* ,V. WINDTE*, 0. MEYER,G. LINKER, Q. LI** AND J. GEERK Kernforschungszentrum Karlsruhe,INFP, ITP,P.O.Box 3640,D-7500 Karlsruhe,FRG. ** Physics Department, Rutgers University, Piscataway,NJ 08854 ABSTRACT Measurements of the critical current density Jc for ultrathin YBa 2 Cu 3 07_. (YBCO) films are presented.A high Jc value at 77K in zero field of 2xl0 A/cm2 was obtained in a 30nm thick film and even a 5nm thick film could carry a supercurrent of 3.5xl05A/cm2 at 4.2K in zero field. A very weak field-dependence of J was observed when the field was in the (a,b) plane showing greatly depressed dissipation. INTRODUCTION There has been much effort in preparing ultrathin films of high-Tc superconductors(HTSC) due to its importance in both application and fundamental research. One of the fundamental issues in HTSC is the flux motion and dissipation[l]. While the dissipation mechanism in HTSC is still not completely understood,the study of dissipation when one of the sample dimensions of HTSC is approaching the coherence length is of significance. In a previous paper[2] we have shown that by inverted cylindrical magnetron sputtering YBa 2 Cu 3 07_x(YBCO) films with full superconducting transition above liquid helium temperature can be made down to about 3nm on SrTiO3 substrate. In this paper we present experimental data of high critical current densities and their field-dependence in these films.The results provide unique information for the understanding of flux motion in HTSC. RESULTS The films were deposited by an inverted cylindrical magnetron sputtering method described elsewhere[3].Details of the ultrathin film preparation have been published previously[2J and we demonstrated that metallic transport behaviour and zero resistance temperature above 4.2K was obtained in 3nm thick films on SrTiO3 and 6nm thick films on MgO. In an exceptional case, full superconductivity was even found in a film on SrTiO3 with a thickness of only 2nm. The growth of the ultrathin films was characterized by Xray diffraction measurements. In Fig.l X-ray diagrams of two films on SrTiO3 and MgO substrates with thicknesses of 6nm and 8 nm respectively are displayed. It can be seen that only distinct (001) lines were detected. This shows that the films were grown with the c-axis perpendicular to the substrate surface and the (a,b) planes are parallel to the film.In other words,thin films with several unit cells of YBCO in the normal direction were prepared. The critical current density Jc was determined by fourpoint resistive measurements using a lItV voltage criterion. The Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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