We compare the theoretical anisotropic upper critical field $H_{ C}(\Theta,T)$ of a quasi-two-dimensional d-wave superconductor with recent $H_{ c2}$ data for the layered organic superconductor $\kappa-(BEDT-TTF)_2Cu(NCS)_2$. We find agreement both with regard to the angular and the temperature dependence of $H_{ C}$. This supports the suggestion that the Fulde-Ferrell-Larkin-Ovchinnikov state (FFLO state) exists in this material for exactly plane-parallel orientation of the magnetic field. Indications of precursor states, occurring for small deviations from the plane-parallel field direction, are also pointed out and further measurements for confirming the existence of the FFLO state are proposed.
Categories: cond-mat.supr-con