Magnetic Field Dependence of the Surface Resistance of Pure and Impure Superconducting Aluminum at Photon Energies near the Energy Gap

The magnetic field dependence of the microwave absorptivity of superconducting aluminum single crystals has been determined in the frequency range 15-100 GHz, covering the spectrum near the superconducting energy gap. Measurements were made on samples of varying purity, from the purest available aluminum to aluminum doped with 3-at.% silver. In the pure case, the energy of the absorption edge was found to shift with static magnetic field by an amount of the order pv, where p is the Fermi momentum and v is the drift velocity associated with the Meissner current. The addition of impurities was found to reduce the shift in the absorption edge as expected. Furthermore, the impurities coupled with the static field to induce an unexpected effect, namely, an absorption peak near the energy of the zero-field absorption edge. The amplitude of the absorption peak was found to increase monotonically with both magnetic field and with concentration of the silver impurity. A suggestion is made that impurities and magnetic field induce a change in the BCS coherence factors (from case II to case I) which, in turn, is responsible for the absorption peak.