Heat capacity of YNi₂B₂C in the vortex state: Evidence of the extended s-wave state

The low temperature heat capacity C_p and microwave surface impedance Z_s in the vortex state of YNi₂B₂C have been studied. In contrast to conventional s-wave superconductors, C_p shows a nearly H^1/2-dependence. This H^1/2-dependence is little affected by the introduction of the columnar defects. On the other hand, flux flow resistivity obtained from Z_s increases linearly with H. These results indicate that in the vortex state of YNi₂B₂C the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.