Renormalization of the Mean-Field Superconducting Penetration Depth in Epitaxial YBa2Cu3

Abstract

The a-b-plane penetration depth X(T) in YBa2Cu3C>7 films, determined from kinetic inductance measurements, follows the temperature dependence of weak-coupling mean-field BCS theory near T c to within ~0.5 K of T c , resolution limited either by homogeneity or for the thinnest films by phase fluctuations. The Kosterlitz-Thouless transition in a thin film is the first observation in a clean type-II superconductor, noted by the universal superfluid jump and the algebraic-decay law with a square-root cusp exponent.A kinetic inductance method in which supercurrent flow is induced in the a-b basal plane of oaxis-oriented epitaxial YBa2Cu3C>7 films grown on SrTiChOOO) was used to measure the weak-field in-plane component of the penetration depth X. [ Properties of the high-r c cuprates such as the short coherence length £o, comparatively long mean free path /, large a-b vs c anisotropy, superconducting energy gap A(T), together with thermodynamic fluctuations near T c will influence the detailed temperature dependence of X(T), which is a measure of the superelectron density and of the observed dissipation, which is associated with vortex motion. For temperatures not too close to T c , the weak-coupling mean-field BCS theory with A/&r c = 1.76 and X(0)=0.15 /im provides a good fit to X(T) in films with the sharpest transitions. Broadening by film inhomogeneity correlates with a systematic increase in the magnitude of X(T). Phase fluctuations driven by the presence of quantized vortices are clearly observed in 500-A films, where the entire film fluctuates as a two-dimensional (2D) superconductor. The data presented here are the first to show the Kosterlitz-Thouless (KT) transition 2 in an intrinsic (/ > £o) type-II superconductor, as distinct from the earlier restriction to high-resistivity dirty-limit oxides. 3 The intriguing possibility of renormalization by 2D fluctuations within the CuC>2 planes is consistent with the data.Various films from 500 to 2000 A thick were prepared by coevaporation from Cu, Y, and BaF2 sources onto polished SrTiO 3 (100) and by reaction at 800 °C in 0 2 with water vapor used to catalyze oxidation. 4 The method yields high-quality c-axis perpendicular films with 10-//m grains, critical current density ^:2xl0 6 A cm ~2 at 77 K, and a-b -plane resistivity of 400 /iftcmat 295 K. We found the penetration-depth measurement itself to be a sensitive and critical test of film quality.The penetration depth X was calculated from a measurement of the complex ac sheet impedance of the film, Z~R + icoL. 1 Under conditions of low dissipation, the sheet kinetic inductance of the superconducting state dominates, and is given by L =4KC ~2d ~lX 2 , where d is the film thickness. The London equation may be inadequate very close to 7 C , because of fluctuations, the finite transition width, and the appearance of the dissipative component R. Applied perpendicular magnetic fields enhance L (and X) and contribute to R.The apparatus contained astatically wound 1-mmdiam drive and receive coils positioned with a 1.5-m...

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