Low energy quasiparticle excitation in the vortex state of borocarbide superconductor YNi2B2C

We measured the heat capacity C-p and microwave surface impedance Z, in the vortex state of ;YNi2B2C. In contrast to conventional s-wave superconducto rs, C-p shows a rootH dependence. This rootH dependence persists even after the introduction of the columnar defects which change the electronic struc ture of the vortex core regime and destroy the regular vortex lattice. On t he other hand, flux how resistivity is nearly proportional to H. These resu lts indicate that the vortex state of YNi2B2C is fundamentally different fr om the conventional s-wave counterparts, in that the delocalized quasiparti cle states around the vortex core an important, similar to d-wave supercond uctors.