Theory of an inherent spin-density-wave instability due to vortices in superconductors with strong Pauli effects

A spin-density-wave (SDW) instability mechanism enhanced by vortices under fields is proposed to explain the high field and low-temperature phase in CeCoIn ₅. In the vortex state strong Pauli effect and nodal gap conspire to enhance the momentum-resolved density of states over the normal state value exclusively along the nodal direction, providing a favorable nesting condition for SDW with Q=(2k _F,2k _F,0.5) only at high fields (H). We can consistently understand observed mysteries of the field-induced SDW confined below H _c2, such as facts that Q is directed to the nodal direction independent of H, SDW diminishes under tilting field from the ab plane, and the SDW transition line in (H,T) has a positive slope.