Abstract
Accurate magnetization experiments on niobium (Γ > 2 000) from the superconducting transition temperature of 9·20(±0·03)°K to 1·5°K produced highly reversible curves. D.c. resistance measurements—yielding Hc3/Hc2 ratios of 3·4 to 3·65—suggest this to be partly due to poor surface finish. Temperature change altered the shape of the magnetization curves, which displayed a discontinuity in dM/dH at 1·005Hc1 and continuous non-linear approach to M = 0 at Hc2. Data were consistent with a λ-transition at Hc1. Thermodynamic critical fields Hc(t) obeyed the parabolic law, indicating strong coupling: 2e(0) was 3·67(±0·05) kTc. Nearer Tc, the energy gap followed the BCS expression e(t)/e(0) = B1/2(1 − t)1/2 for B = 3·9(±0·2). Other parameters were ξ0 = 390(±15), λL(0) = 333(±15), S/St = 0·79, and κ0 = 0·815(±0·015). The Maki functions κ1(t), κ2(t) were in fair accord with theory, but changed rapidly with temperature, κ1(0)/κ(1) being 1·68, 30% too high. This discrepancy is not explained in non-local corrections to GLAG theory. Temperature variation of Hc1/Hc differed from theoretical prediction.
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