Effect of quasiparticle delocalization on μSR measurements of a multiband superconductor in the vortex state

We present μSR measurements of the low temperature magnetic field dependence of the vortex core size in the multiband superconductor NbSe2. The spatially extended bound core states associated with the smaller energy gap rapidly delocalize with increasing magnetic field, resulting in a shrinking of the vortex core size. The more tightly bound core states associated with the larger energy gap, on the other hand, lead to a field-independent core size for fields greater than 4 kOe. The field dependence of the extracted magnetic penetration depth is explained in terms of the effect of delocalized quasiparticles on the spatial field variation around the vortex cores.