Measurements were made of the transverse field muon spin precession in the flux-lattice state of the 2-gap superconductor MgB2 and of the electron-doped compounds Mg1-xAlxB2 in magnetic fields up to 2.8T. It was shown the effect of the 2 gaps on the internal field distribution in the flux-lattice, from which 2 coherence length parameters and the doping dependence of the London penetration depth were determined. This was an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB2 single crystal. The data agreed quantitatively with STM and thus validated a new phenomenological model for the internal fields.

Effect of Two Gaps on the Flux-Lattice Internal Field Distribution - Evidence of Two Length Scales in Mg1-xAlxB2 from muSR. S.Serventi, G.Allodi, R.De Renzi, G.Guidi, L.Romano, P.Manfrinetti, A.Palenzona, C.Niedermayer, A.Amato, C.Baines: Physical Review Letters, 2004, 93[21], 217003