Millimeter-Microwave Studies of Energy-Gap Anisotropy in Superconductors

Measurements of the absorption of polarized microwaves have been made on superconducting single crystals of aluminum and zinc as a function of frequency in the region where the photon energy spans the superconducting energy gap. When plotted as isotherms of surface resistance ratio versus photon energy the results for aluminum show, in three different crystal faces (100, 110, and 111), two rapid changes in slope occurring at values of energy about 10% apart. The photon energies at these slope changes are interpreted as indicating the positions of two different superconducting energy gaps. We also find small differences in the energy-gap values (∼3%) among the three different crystal faces. All the measured values of energy gaps at $T=0\mathrm{}$ in aluminum lie between $3.04k{T}_c$ and $3.50k{T}_c$, where $T_c=1.175\mathrm{}°$K. Preliminary results on a single crystal of zinc yield an energy gap at $T=0\mathrm{}$ of $(3.00\mathrm{}\pm 0.15)k{T}_c$, where $T_c=0.838\mathrm{}°$K.