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

The low temperature heat capacity Cp and microwave surface impedance Z(s) i n the vortex state of YNi2B2C have been studied. In contrast to conventiona l s-wave superconductors, C-p shows a nearly H-1/2-dependence. This H-1/2-d ependence is little affected by the introduction of the columnar defects. O n the other hand, flux flow resistivity obtained from Z(s) increases linear ly with H. These results indicate that in the vortex state of YNi2B2C the d elocalized quasiparticle states around the vortex core play a much more imp ortant role, similar to d-wave superconductors. (C) 2001 Elsevier Science B .V. All rights reserved.