  We have measured the low temperature heat capacity Cp and microwave surface
impedance Zs in the vortex state of YNi_2B_2C. In contrast to conventional
s-wave superconductors, Cp shows a nearly \sqrt H-dependence. This \sqrt
H-dependence persists even after the introduction of the columnar defects which
change the electronic structure of the vortex core regime dramatically and
strongly disturb the regular vortex lattice. On the other hand, flux flow
resistivity obtained from Zs is nearly proportional to H. Taken together, these
results indicate that the vortex state of YNi_2B_2C is fundamentally different
from the conventional s-wave counterparts, in that the delocalized
quasiparticle states around the vortex core play a much more important role,
similar to d-wave superconductors.
