Anomaly of quasi-particle density of states in the vortex state of NbSe₂

Magnetic field H dependence of the electronic specific heat coefficient y was measured in various NbSe₂ crystals in the vortex states. With increasing residual resistivity ratio, γ(H) changed from conventional γ ∝ H behavior to the strongly sublinear function of H. The sublinear H dependence of γ is similar to that of nodal superconductors, while NbSe₂ is a nodeless superconductor. From measurements on the sample containing columnar defects, we found that the γ anomaly is brought by quasi-particle states outside of the vortex cores. This result suggests that excess quasi-particle states are generated by Doppler energy shift caused by supercurrent around vortices.