Superconductivity of Sn-Zn Eutectic Alloys

An experimental study has been made of the superconducting transitions in Sn-Zn eutectic alloys in the temperature range 3.0-3.7°K, employing magnetization measurements on spherical samples. The samples were about 4 mm in diameter, and the laminar spacing varied from 1 to 8μ depending on the speed with which the parent ingot had been cooled from the melt. The measurements showed magnetic isotherms approaching those of a type-I superconductor, but with frozen flux of varying degree. The critical-field-versus-temperature relation was similar to that of pure Sn, providing evidence that the laminations behave like a continuous superconducting material. The transition temperature $T_c$ was found to decrease with decreasing laminar spacing. This effect was compared quantitatively with the predictions of proximity-effect theory. The two significant theoretical parameters derived from the data were ${\rho }_s$, the effective resistivity ratio of the Sn-rich phase, and $T_{\mathrm{cn}}$, the effective transition temperature of the Zn-rich phase. The large-spacing $T_c$ data indicated an effective ${\rho }_s$ of about 0.12, in reasonable agreement with the expected magnitude. At smaller spacing the experimental $T_c$ was higher than expected. Alternatively stated, $T_{\mathrm{cn}}$ was 2.7°K, which is larger than the accepted Zn transition temperature 0.9°K.