Antiferromagnetism and Superconductivity in layered organic conductors: Variational cluster approach

The $\kappa$-(ET)${}_2$X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping $t^{\mathrm{\prime }}$ and Hubbard repulsion $U$. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (V-CPT). For large $U$, N'eel antiferromagnetism exists for $t^{\mathrm{\prime }}<t_{c2}^{\mathrm{\prime }}$, with $t_{c2}^{\mathrm{\prime }}\sim 0.9$. For fixed $t^{\mathrm{\prime }}$, as $U$ is decreased (or pressure increased), a $d_{x^2-y^2}$ superconducting phase appears. When $U$ is decreased further, the a $d_{xy}$ order takes over. There is a critical value of $t_{c1}^{\mathrm{\prime }}\sim 0.8$ of $t^{\mathrm{\prime }}$ beyond which the AF and dSC phases are separated by Mott disordered phase.