Critical Magnetic Fields of Superconducting Molybdenum

Critical magnetic fields of two specimens of molybdenum have been measured down to 0.12°K. Sample $A$ with a listed impurity content of less than 27 parts per million was measured as received. Its transition temperature in zero applied magnetic field is 0.958°K and its critical magnetic field at the absolute zero of temperature ($H_0$) is 99.7 G. Sample $B$ was formed by the levitation melting of a sample similar to Sample $A$. Its $T_c$ and $H_0$ are 0.930°K and 98.2 G, respectively. The shapes of the transitions, observed in the presence of an externally applied magnetic field, indicate that these two samples are "soft" or "Meissner-type" superconductors. A calculation which takes into account the fact that the critical field curve is not strictly parabolic leads to values for $\gamma$, the electronic specific heat coefficient, of 1.59 and 1.61 mJ/mole-${\mathrm{deg}}^2$ for Samples $A$ and $B$, respectively.