Effect of two gaps on the flux lattice internal field distribution: evidence of two length scales from muSR in Mg1-xAlxB2

TL;DR

This study uses muSR measurements to investigate how two superconducting gaps influence the internal magnetic field distribution in MgB2 and Mg1-xAlxB2, revealing two distinct length scales and validating a new phenomenological model.

Contribution

It provides the first muSR evidence of two length scales in the flux lattice of two-gap superconductors, linking microscopic measurements with vortex structure models.

Findings

Identification of two coherence lengths in MgB2 and Mg1-xAlxB2

Quantitative agreement with STM observations of vortex size

Validation of a new phenomenological model for internal fields

Abstract

We have measured the transverse field muon spin precession in the flux lattice (FL) state of the two gap superconductor MgB2 and of the electron doped compounds Mg1-xAlxB2 in magnetic fields up to 2.8T. We show the effect of the two gaps on the internal field distribution in the FL, from which we determine two coherence length parameters and the doping dependence of the London penetration depth. This is an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB2 single crystal. Our data agrees quantitatively with STM and we thus validate a new phenomenological model for the internal fields.