The penetration depth in several hard superconductors

Abstract

The magnetic penetration depth in the superconducting intermetallic compounds V 3Si, V 3Ga, V 3Ge and V 3Pt and in tantalum was studied by means of a coil whose inductance was made to depend on this parameter. Difficulty was experienced in finding samples of the intermetallic compounds with sharp enough superconducting transitions to allow quantitative measurements to be made. However, quantitative results were obtained for V 3Si and tantalum which showed that below the transition region, in the temperature range studied, the magnetic field penetration depth varied linearly with y = (1 − (T/T c) 4) − 1 2 . For V 3Si the slope of this line, λ 0, is found to be 2,300 Å to an accuracy +10 and −30 per cent. For tantalum, λ 0 is 1,500 Å ±50 per cent. From these figures, the BCS theory is used to find a zero temperature penetration depth λ(0), and a coherence distance, ξ 0. The relative values of λ(0) and ξ 0 confirm that these materials are hard superconductors in the negative surface energy sense.