Quasiparticle density of states of clean and dirty s-wave superconductors in the vortex state

The quasiparticle density of states (DOS) in the vortex state has been probed Ly specific heat measurements under magnetic fields (H) fur clean and dirty s-wave superconductors, Y(Ni1-xPtx)(2)B2C and Nb1-xTaxSe2. We find that the quasiparticle DOS per vortex is appreciably H-dependent in the clean-limit superconductors, while it is H-independent in the dirty superconductors as expected from a conventional rigid normal electron core picture. We discuss possible origins for our observations in terms of the shrinking of the vortex core radius with increasing H.