Superconducting Transitions of Isotopes of Zinc

The effect of isotopic mass on the superconducting transition temperature $T_c$ of Zn has been investigated in three highly purified samples with average isotopic masses of 63.96, 65.91, and 67.94. By extrapolating critical magnetic field curves to zero field, values of 0.855, 0.846, and 0.836°K were obtained for $T_c$ of ${\mathrm{Zn}}^{64}$, ${\mathrm{Zn}}^{66}$, and ${\mathrm{Zn}}^{68}$, respectively. These values were found to satisfy the relation $T_c\propto M^{-z}=M^{-0.5(1-\zeta )}$ with $z$ taken as 0.37 and $\zeta$ as 0.25. Critical field curves measured between 0.39°K and $T_c$ were found to have a maximum deviation from a parabola of about 5.7%. Specific heats calculated from these critical field curves are compared with direct measurements made by others. The superconducting-to-normal transition of a sample was observed to broaden as the applied magnetic field was reduced below some small value. Ratios of transition widths in "zero" field and in the earth's field ranged from about 2 to over 30, depending on the sample.