Thickness dependence of flux-line-lattice melting in high-Tc superconducting films.

Abstract

Civale et al. have recently measured the irreversibility line of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ (YBCO) films in the magnetic field-temperature plane, ${\mathit{B}}_{\mathrm{irr}}$(T), as a function of film thickness. We are able to reproduce the observed shift of ${\mathit{B}}_{\mathrm{irr}}$(T) to lower temperatures as the film thickness is reduced within a model of flux-line-lattice melting without pinning. While pinning must be important, this suggests ${\mathit{B}}_{\mathrm{irr}}$(T) is at least partly an intrinsic property. The onset of the shift of ${\mathit{B}}_{\mathrm{irr}}$(T) begins for films thinner than 1000--1500 \AA{} in YBCO. The present model predicts that the onset will begin for films thinner than 100--150 \AA{} in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$.