The magnetic field dependence of the vortex core size in the multiband superconductor NbSe2 has been determined from muon spin rotation measurements. The spatially extended nature of the quasiparticle core states associated with the smaller gap leads to a rapid field-induced shrinkage of the core size at low fields, while the more tightly bound nature of the states associated with the larger gap leads to a field-independent core size for fields greater than 4 kOe. A simple model is proposed for the density of delocalized core states that establishes a direct relationship between the field-induced reduction of the vortex core size and the corresponding enhancement of the electronic thermal conductivity. We show that this model accurately describes both NbSe2 and the single-band superconductor V3Si.

We report muon spin rotation measurements on a single crystal of the marginal type-II superconductor V. The measured internal magnetic field distributions are modeled assuming solutions of the Ginzburg-Landau (GL) equations for an ideal vortex lattice obtained using (i) an iterative procedure developed by E.H. Brandt, Phys. Rev. Lett. 78, 2208Lett. 78, (1997 and (ii) a variational method. Both models yield qualitatively similar results. The magnetic penetration depth λ and the GL coherence length ξ determined from the data analysis exhibit strong field dependences, which are attributed to changes in the electronic structure of the vortex lattice. The zero-field extrapolated values of λ and the GL parameter κ agree well with values obtained by other experimental techniques that probe the Meissner state.

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