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 superconductors, C-p shows a rootH dependence. This rootH dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime and destroy the regular vortex lattice. On the other hand, flux how resistivity is nearly proportional to H. These results indicate that the vortex state of YNi2B2C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core an important, similar to d-wave superconductors.