Microwave measurements of the absolute London penetration depth in double-sided YBa2Cu3O7−x thin films on sapphire

Abstract

The absolute values of the London penetration depth λL, were measured in epitaxial (001) YBa2Cu3O7−x (YBCO) thin films prepared by simultaneous sputter deposition on both sides of 3 in. r-cut sapphire wafers buffered with thin (001) CeO2 layers. The measurements were performed by using a technique, which is based on the effect produced on the quality factor Q of a microwave disk resonator by a gold layer deposited on the YBCO electrodes. The observed change of Q can be transformed into λL value with an accuracy determined mainly by the uncertainty of the YBCO film thickness. The λL(T) data obtained by this technique revealed a good agreement with the variation of the resonant frequency with temperature, which is conventionally used for measurements of the variation of the effective London penetration depth ΔλL eff(T) with temperature. At temperatures above ∼60 K this λL(T) dependence was very close to the predictions of the Gorter–Casimir model. At lower temperatures a linear λL(T) behavior was observed with the slope higher for thinner YBCO films. The absolute λL values were higher than those reported in literature for λab (screening currents flow in the crystallographic a–b plane) of YBCO single crystals. For example, our measured λL (4.2 K) values were in the range of 195–220 nm compared to literature data for λab quoted around 140 nm.