Magnetic Field Dependence of the Microwave Surface Impedance of Superconducting Tantalum

The changes in the superconducting surface resistance ΔRS=RS(H)-RS(0) and reactance ΔXS=XS(H)-XS(0) of Ta with applied magnetic field H have been measured at a frequency of 8.8 GHz, using a dielectric-resonator technique. The quantity ΔRS is found to be positive for a reduced temperature t=TTC just less than 1, negative for $0.70<t<0.98$, and positive again for $t<0.70$, while ΔXS is always positive with a peak at about $t=0.97$, a broad minimum centered at $t=0.85$, and a continuous rise for lower temperatures. The dependence of ΔRS and ΔXS on the angle γ between Hrf, the microwave magnetic field, and H with both fields in the sample plane is investigated. The magnitude of this dependence is found to be a function of t.