Proximity of Antiferromagnetism and d-wave superconductivity in $\kappa$-BEDT-TTF: A V-CPT study

The organic conductors $\kappa$-(BEDT-TTF)$_2$X (X=Cu(NCS)$_2$, Cu[N(CN)$_2$]Br and Cu[N(CN)$_2$]Cl) have attracted attention because the superconducting phase exists in the vicinity of an antiferromagntic insulating phase [1]. The salts with the first two substitutions are superconductors with critical temperature near 10K. The third one is a commensurate insulator, which transits to a superconducting phase at ambient pressure. We use the 2D, half-filled Hubbard model on a triangular lattice to describe these compounds. The ground state antiferromagnetism and superconductivity is investigated using variational cluster perturbation theory (V-CPT) [2-3]. The on-site repulsion $U$ and the diagonal hopping $t'$ are varied and a phase diagram of the $\kappa$-family of BEDT is obtained. [1] H. Kino and H. Fukuyama, J. Phys. Soc. Jpn. 65 (1996) 2158. [2] M. Potthoff, M. Aichorn and C. Dahnken, Phys. Rev. Lett. 91 (2003) 206402. [3] D. Senechal and A.-M. S. Tremblay, cond-mat/0410162