Superconductivity of Compacted Platinum Powder at Very Low Temperatures

We report the observation of superconductivity in compacted, high purity platinum powder of average grain size $\approx 2\mu \mathrm{m}$. The transition into the superconducting state was found by measuring resistivity and ac susceptibility as well as dc magnetization. The transition temperature $T_C$ strongly depends on the compression of the powder; we found $0.62\le T_C\le 1.38\mathrm{mK}$ for packing fractions of $0.8\ge f\ge 0.5$, respectively. The corresponding critical magnetic fields are $6.6\le B_C\left(0\right)\le 67\mu \mathrm{T}$.