Critical-Field Study of Superconducting Aluminum

Abstract

The critical field of pure aluminum has been measured down to 0.28°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 $T_c=1.175\mathrm{}\pm 0.001$°K and ${\left(\frac{d{H}_c}{\mathrm{dT}}\right)}_{T_c}=158\mathrm{}\pm 4$ Oe/°K, while analysis of the low-temperature data indicates $H_0=104.8\mathrm{}\pm 0.6$ Oe with $\gamma$ in the range 1.23 to 1.30 mJ/mole${\left(\mathrm{°}\mathrm{K}\right)}^2$.