The magnetic field distribution in the vortex state of YNi2B2C has been probed by muon spin rotation (muSR). 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, rho_v(H), decreases with increasing H much steeper than what is expected from the sqrt(H) behavior of the Sommerfeld constant gamma(H), strongly suggesting that the anomaly in gamma(H) primarily arises from the quasiparticle excitations outside the vortex cores.
@article{arxiv.cond-mat/0201038,
title = {Nonlocal Effects and Shrinkage of the Vortex Core Radius in YNi2B2C Probed by muSR},
author = {K. Ohishi and K. Kakuta and J. Akimitsu and W. Higemoto and R. Kadono and J. E. Sonier and A. N. Price and R. I. Miller and R. F. Kiefl and M. Nohara and H. Suzuki and H. Takagi},
journal= {arXiv preprint arXiv:cond-mat/0201038},
year = {2009}
}