Superconducting dome in doped quasi-2d organic Mott insulators: a paradigm for strongly-correlated superconductivity

Layered organic superconductors of the BEDT family are model systems for the interplay of the Mott transition with superconductivity, magnetic order and frustration. Recent experimental studies on a hole-doped version of BEDT compounds reveal an enhancement of superconductivity and a rapid crossover between two different conducting phases above the superconducting dome. One of these phases is a Fermi liquid, the other not. Using plaquette cellular dynamical mean field theory with state of the art continuous-time quantum Monte Carlo calculations, we study this problem with the two-dimensional Hubbard model on the anisotropic triangular lattice. Phase diagrams as a function of temperature $T$ and interaction strength $U/t$ are obtained for anisotropy parameters $t'=0.4t$, $t'=0.8t$ and various fillings. As for cuprates, we find, at finite doping, a first-order transition between two normal-state phases. For $T$ above the critical point of the first-order transition, there is a Widom line where crossovers occur. The results are in broad agreement with experiment. This suggests that for compounds with intermediate to high frustration, very light-doping should reveal the first-order transition and associated crossovers. These crossovers could leave traces in the superconducting phase. We also predict that destroying the superconducting phase by a magnetic field should reveal the first-order transition. Finally, we predict that electron-doping should also lead to an increased range of $U/t$ for superconductivity but with a reduced maximum $T_c$. This work also clearly shows that the superconducting dome here is tied to the Mott transition and its continuation as a transition separating pseudogap phase from correlated metal in doped compounds, as in the cuprates. Contrary to heavy fermions for example, the maximum $T_c$ is definitely not attached to an antiferromagnetic quantum critical point.