Anomalies in the Magnetic Field Dependence of the Surface Impedance of Superconducting Sn

Abstract

We have made an extensive study of the microwave surface resistance of superconducting Sn as a function of magnetic field. At reduced temperatures $t\gtrsim 0.6$ and microwave frequencies in the range of 28 to 56 Gc/sec, we find some unusual peaks in the absorption derivative. The anomalous peaks appear at the onset of a range of magnetic field where the resistance decreases with increasing field. The position in field for such a peak is found to vary with temperature approximately as ${(1-t^4)}^{\frac{1}{3}}$. The amplitude diminishes rapidly with decreasing temperature. The peak position depends linearly on the microwave frequency. The effect is anisotropic with respect to the orientation of the magnetic field in a given sample plane, as well as with the choice of sample plane for a given orientation of the field. The amplitude of the peaks depends on the polarization of the rf current relative to the magnetic field. A very light etching of the sample surface destroys the signals.