Low-field aspects of magnetic-flux dynamics of inhomogeneous superconductors

Abstract

Material is presented showing that certain inhomogeneous superconducting samples, including both low-T c and high-T c compounds, respond to weak changes in applied magnetic field in a way which in suitable designs makes them usable as simple quantum interferometers. Usually, S uperconducting QU antum I nterference D evices (SQUIDs) are considered to consist of a superconducting loop with one or two well-defined Josephson junctions. Here we demonstrate that an anodized niobium tube, which consists of an inhomogeneous niobium thin film covering a quartz cylinder has a typical SQUID-like response, when operated as a radio-frequency (RF) SQUID. The physical background appears to be that very small changes in applied magnetic field can induce macroscopic changes in the flux configuration in an inhomogeneous superconductor, where the critical current density is a rapidly varying function of both position and field. The effect is directly demonstrated by comparing the shielding properties of high-quality epitaxial films and low critical current density inhomogeneous films of the high temperature superconductor YBa 2Cu 3O 7−x. The shielding properties have been measured by using a simple set-up including an RF-SQUID made out of ceramic YBa 2Cu 3O 7−x, the sample, and a small field coil, operated in a well-shielded environment.