Nonlocal effects and shrinkage of the vortex core radius in YNi₂B₂C probed by muon spin rotation

The magnetic field distribution in the vortex state of YNi₂B₂C has been probed by muon spin rotation. 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, ρν(H), decreases with increasing H much steeper than what is expected from the √H behavior of the Sommerfeld constant y(H), strongly suggesting that the anomaly in γ(H) primarily arises from the quasiparticle excitations outside the vortex cores.