Effect of thickness reduction on vortex-glass transition temperature in YBa 2 Cu 3 O 7-δ thin films

Abstract

We investigated the current-voltage characteristics and critical scaling behavior in c-axis oriented YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) (YBCO) films whose thicknesses are from 18 nm to 230 nm, in magnetic fields applied perpendicular to the film surface. The vortex-glass transition temperature (T<SUB>g</SUB>) decreased as the thickness decreased, and wide temperature critical scaling region was observed in thinner films. We note that the reduction of T<SUB>g</SUB> is similar to the reported result of the vortex-glass transition of narrow YBCO strip lines, and that wide temperature critical scaling region was also observed in the vortex-glass transition of Bi<SUB>2</SUB>Sr<SUB>2</SUB>Ca<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>x</SUB> (Bi-2223) films. From the analogy to the study of Bi-2223 films, we suggest that, in the thinner YBCO films, the vortex-glass correlation length parallel to the c-axis (xi) <SUB>g</SUB>// is limited by the film thickness, and that wide critical scaling region originates from the enhanced thermal fluctuation effect brought by the reduced correlation volume through the interruption of vortex correlation along c-axis.