Microwave Flux Flow Resistivity of Clean Limit Superconductor YNi2B2C

We measured the microwave flux flow resistivity ρf of the clean limit s-wave superconductor YNi2B2C. We show that ρƒ increases linearly with H. This result shows that the number of the quasiparticle inside the vortex core increases in proportion to H and provides a strong evidence that the size of the vortex core does not change with H. This is inconsistent with the recent µ SR and STM results which claim that the core shrinks with H. Moreover, core shrinkage with decreasing T in constant H (Kramers-Pesch effect) was not observed down to 1.5K (~0.1Tc). The present results imply that the YNi2B2C is a s-wave superconductor with the anisotropic energy gap.