The magnetic field distribution in the vortex state of YNi2B2C has been probed by muon spin rotation (µSR). The analysis based on the London model with nonlocal corrections shows that the vortex lattice has changed from hexagonal to square with increasing magnetic field H. At low fields the vortex core radius, ρv(H), decreases with increasing H much steeper than what is expected from the √ H behavior of the Sommerfeld constant γ(H), strongly suggesting that the anomaly in γ(H) primarily arises from the quasiparticle excitations outside the vortex cores.74.60. Ec, 76.75.+i The recent studies of the flux-line lattice (FLL) state in ordinary s-wave superconductors have revealed that the electronic structure of vortices is much more complicated than that of a simple array of rigid cylinders containing normal electrons. One of the unexpected phenomena within this conventional model is the non-linearity in the magnetic field dependence of the Sommerfeld constant γ(H) (electronic specific heat coefficient) observed in CeRu 2 1 , NbSe 2 2 , and YNi 2 B 2 C 2 . According to the above simple model where the quasiparticle excitations are confined within the cores of vortices (with a radius ξ) in s-wave superconductors, one would expect that γ(H) is proportional to the number of vortices per unit cell and thus to the applied magnetic field H. However, experiments have revealed that this is not the case for any of the above compounds. 1,2 Instead, they find a field dependence like γ(H) ∝ √ H which is expected for d-wave superconductors having more extended quasiparticle excitations along nodes in the energy gap. The recent study on the effect of doping in YNi 2 B 2 C and NbSe 2 indicates that the anomalous field dependence is observed only in the clean limit 2 , suggesting the importance of nonlocal effects in understanding the field dependence of γ(H). Moreover, it has been reported that the vortex core radius depends on applied magnetic field and shrinks at higher fields in NbSe 2 3 and in CeRu 2 4 .Another complication especially for borocarbides (RNi 2 B 2 C, R = rare earth) is that a square FLL is formed in some of these compounds at high magnetic fields, whereas a hexagonal FLL is realized at low fields. 5-8 This is not expected for the local model with isotropic intervortex interactions and thereby suggests the importance of considering electronic structure (or the Fermi surface) and the associated nonlocal corrections in the specific compound.We report on µSR measurements of the magnetic field dependence of theâ-b magnetic penetration depth λ, the effective vortex core radius ρ v , and the apex angle of the FLL θ in single crystalline YNi 2 B 2 C. We demonstrate that the proper reconstruction of the field profile with a square FLL is obtained from the µSR spectra only when the nonlocal corrections are considerred. 9 The field dependence of λ turned out to be linear over the entire magnetic field range of observation. More importantly, it was found that ρ v shrinks sharply with increasing magnetic field and levels ...
