Surface States in Superconducting Indium

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Abstract
Measurements of the microwave (10-40-GHz) surface impedance of superconducting indium in a magnetic field have revealed the distinct peak structure that is associated with the excitation of quantum-mechanical surface-bound quasiparticle states. The impedance peaks are found to depend on frequency and temperature as predicted by the model. We have discovered a sensitive dependence of the peaks on the surface roughness that can be understood qualitatively in terms of Pincus's theory of the surface states. Following Garfunkel, we have carried out calculations of the surface impedance that correctly give peaks in the impedance derivative at values of magnetic field where the surface electron binding energy equals the microwave photon energy. The calculated peaks have the experimentally observed frequency-temperature scaling.