Microwave flux flow resistivity of clean limit superconductor YNi2B2C

We measured the microwave flux flow resistivity pf of the clean limit s-wave superconductor YNi2B2C. We show that rho(f) 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 mu 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 The present results imply that the YNi2B2C is a s-wave superconductor with the anisotropic energy gap.