It was recalled that the behavior of a type II superconductor in the presence of a magnetic field was governed by 2 characteristic length scales, the London penetration depth and the coherence length. Magnetization measurements of MgB2 powder were presented which revealed an anisotropy of the upper critical field and hence a coherence length of 6. By using the technique of small-angle neutron scattering, it was shown that this anisotropy was not mirrored by the London penetration depth, which was almost isotropic. This result could be explained by the superconductivity residing in two distinct electronic bands of the material, only one of which was highly anisotropic.

Experimental Evidence for Anisotropic Double-Gap Behavior in MgB2. R.Cubitt, S.Levett, S.L.Budko, N.E.Anderson, P.C.Canfield: Physical Review Letters, 2003, 90[15], 157002 (3pp)