Critical-Field Study of Superconducting Aluminum

Abstract

The critical field of pure aluminum has been measured down to 0.28\ifmmode^\circ\else\textdegree\fi{}K. Because the measurement technique requires a magnetic field gradient, both normal and superconductive phases were present in the sample, thus avoiding superheating and supercooling effects. The movement of the phase boundary in response to a change in magnetic field was continuously observed, its position at equilibrium leading to a determination of the critical field. Hysteresis in the transition was 0.3% of the critical field, with reduction to arbitrarily small values apparently possible. The deviation curve shows the usual weak-coupling characteristics and, apart from a low-temperature anomaly attributed to uncertainties in the vapor-pressure thermometry, good agreement with BCS theory. The measurements yield ${T}_{c}=1.175\ifmmode\pm\else\textpm\fi{}0.001$\ifmmode^\circ\else\textdegree\fi{}K and ${(\frac{d{H}_{c}}{\mathrm{dT}})}_{{T}_{c}}=158\ifmmode\pm\else\textpm\fi{}4$ Oe/\ifmmode^\circ\else\textdegree\fi{}K, while analysis of the low-temperature data indicates ${H}_{0}=104.8\ifmmode\pm\else\textpm\fi{}0.6$ Oe with $\ensuremath{\gamma}$ in the range 1.23 to 1.30 mJ/mole${(\mathrm{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{K})}^{2}$.