Penetration Properties of Superconductors. New Results on an Old Problem.

The standard interpretation of superconductivity (S) is based on the London theory (LT) with minor modifications adopted in the GL and BCS theories. In all theories the bulk of the S phase is totally inert and microscopic parameters of the S state can be obtained solely from the surface properties, i.e., the penetration depth λ and the induction profile B vs depth z. Herewith, B(z) is close to exponential and does not depend on the applied field H₀ in the LT and BCS, while in the GL it does. According to the standard interpretation of the flux lines, λ² ~1/H₀. An attempt to verify the field dependence of λ was made by Pippard in 1950 using a microwave resonator (9 GHz). Pippard concluded that λ could be considered as a field independent quantity, but in 25 years he noted that the problem remained unsolved. To sort out with this and other problems in the standard interpretation was the goal of the reported project. Low-energy and bulk muSR experiments were performed utilizing the continuous and pulsed muon sources on high-purity In and Nb films and single crystal In, Sn and Nb samples. The entire B(z) [from H₀ down to 0] was measured for the first time. In all cases B(z) is close to linear and does not depend on the field. Measured using a continuous muon source, the muon depolarization rate in the bulk (where B=0) is proportional to the field strength H within the samples. However, the data obtained using a pulsed muon source do not show this effect.