Magnetic Field Dependence of the Surface Impedance of Superconducting Tin

The surface impedance at 1 kMc/sec of superconducting Sn has been investigated as a function of temperature, $T$, and of static magnetic field, $H$, both longitudinal and transverse to the rf current. The following unexpected phenomena have been observed. The rf resistance, $R$, and the reactance, $X$, may each decrease with increasing $H$. A decreasing $X$ and an increasing $R$ may coexist. A variation of either the crystalline or static field orientation with respect to the rf magnetic field can change the sign of [$R\left(H\right)-R\left(0\right)\mathrm{}$] and of [$X\left(H\right)-X\left(0\right)\mathrm{}$]. For $T<\mathrm{}2.0\mathrm{}°$K, both $R$ and $X$ are monotonically increasing functions of $H$. In this range of $T$, the approximation of a quadratic dependence for $R$ on longitudinal and transverse $H$ becomes increasingly unsatisfactory with decreasing $T$. The quadratic coefficient of the variation of $X$ with longitudinal $H$ also decreases with decreasing $T$, but the corresponding coefficient in transverse $H$ increases, $1.2°\mathrm{K}<T<\mathrm{}2.0\mathrm{}°\mathrm{K}$. The relation between these unexpected behaviors and previous experimental and theoretical studies is discussed.