Has the FFLO state been observed in the organic superconductor $\kappa -$(BEDT-TTF${)}_2$Cu(NCS${)}_2$ ?

Abstract

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 $\kappa -(BEDT-TTF{)}_{2}Cu(NCS{)}_2$. 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.

7 pages, 4 figures, accepted for publication in Journal of Physics: Condensed Matter, issue 28 (cover date 17 July), pp L471--476