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 calculation, we are able to extract the penetration length $\lambda$ and a second parameter describing the shape of the nonlocalfield penetration.