Critical-Field Study of Superconducting Aluminum

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 Tc=1.175±0.001^∘{}K and (dHcdT)_Tc=158±4 Oe/^∘{}K, while analysis of the low-temperature data indicates H₀=104.8±0.6 Oe with γ in the range 1.23 to 1.30 mJ/mole(^∘K)².