Field variation of the superconducting penetration depth

Abstract

The high-frequency technique developed previously has been used to investigate the dependence of the penetration depth in superconducting tin on the strength of the applied field. A steady magnetic field was applied transverse to a thin cylindrical specimen forming part of a 3 cm. resonator, any resulting change in penetration depth being revealed as a shift in the resonant frequency. The change was greatest close to the transition temperature (3.72° K), but even here it amounted to no more than 3% at the critical field strength. As the temperature was lowered the effect became smaller at first, reaching a very low value at 3° K, and then increased once more to about 2 % at 1.7° K. The effect of a steady magnetic field on the high-frequency resistance was also studied, though not in great detail. Above 3° K the resistance decreases slightly until at a field strength rather greater than ½ H c a sudden increase announces the formation of the intermediate state. At lower temperatures the effect of the steady field is to increase the resistance slightly. It is suggested that the very small change in penetration depth even at the critical field strength is evidence of the existence of long-range order in the superconducting state over a distance of 10 -4 cm. or more, and the very sharp resistance transition in pure superconductors is adduced as further evidence of this hypothesis.