Quasiparticle magnetospectroscopy in superconductors: A detailed look at the shape of the penetration layer

Microwave magnetoabsorption peaks in single-crystal superconducting Al have been identified by comparison with a detailed calculation of quasiparticle energy levels. The peaks correspond to resonant transitions between states bound in the magnetic-field penetration layer as predicted by Pincus. By fitting the observed spectra to the calculations, we are able to extract the penetration length lambda and a second parameter describing the shape of the nonlocal-field penetration.