Field modulated microwave absorption in high T/sub c/ superconductors

Abstract

Modified electron spin resonance (ESR) techniques were used to measure low-field (<200 Oe) microwave absorption of high-T/sub c/ cuprates YBa/sub 2/Cu/sub 3/O/sub 7-x/. A series of experiments on polycrystalline and single-crystal samples in which the angle between the DC and the modulating magnetic fields was varied showed that the microwave absorption obeys results predicted from thermodynamic considerations of fluxoids. The absorption takes place in the normal regions created by flux penetration at low fields in these samples. The change in magnetically modulated microwave absorption on passing through T/sub c/ is explained by the decrease in absorption that occurs when part of the sample becomes superconducting and by the rapid variation of the penetration depth near T/sub c/. The technique is an extremely sensitive method of detecting superconductivity in very small samples and for studying the superconducting state. Possible applications such as microwave attenuators, magnetic memories, and gaussmeters are discussed.