Quasiparticle structure in the vortex state of anisotropic s-wave superconductor YNi₂B₂C

We have measured the low temperature heat capacity C_p in the vortex state of YNi₂B₂C. In contrast to conventional s-wave superconductors, C_p shows a nearly √H-dependence. This √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. 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 play a much more important role, similar to d-wave superconductors.