Magnetic Field Dependence of the Microwave Surface Impedance of Superconducting Tantalum

The changes in the superconducting surface resistance $\Delta R_S=R_S\mathrm{}\left(H\right)-R_S\mathrm{}\left(0\right)\mathrm{}$ and reactance $\Delta X_S=X_S\mathrm{}\left(H\right)-X_S\mathrm{}\left(0\right)\mathrm{}$ of Ta with applied magnetic field $H$ have been measured at a frequency of 8.8 GHz, using a dielectric-resonator technique. The quantity $\Delta R_S$ is found to be positive for a reduced temperature $t=\frac{T}{T_C}$ just less than 1, negative for $0.70<t<\mathrm{}0.98\mathrm{}$, and positive again for $t<\mathrm{}0.70\mathrm{}$, while $\Delta X_S$ is always positive with a peak at about $t=0.97\mathrm{}$, a broad minimum centered at $t=0.85\mathrm{}$, and a continuous rise for lower temperatures. The dependence of $\Delta R_S$ and $\Delta X_S$ on the angle $\gamma$ between ${\mathbf{H}}_{\mathrm{rf}}$, 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 $t$.