d-wave superconductivity, antiferromagnetism and spin liquid in quasi-twodimensional organic superconductors

The self-energy-functional approach is a powerful many-body tool to investigate different broken symmetry phases of strongly correlated electron systems. We use the variational cluster perturbation theory (also called the variational cluster approximation) to investigate the interplay between the antiferromagnetism and d-wave superconductivity of -(ET)2 X conductors. These compounds are described by the so-called dimer Hubbard model, with various values of the on-site repulsion U and diagonal hopping amplitude t'. At strong coupling, our zero-temperature calculations show a transition from Néel antiferromagnetism to a spin-liquid phase with no long range order, at around t' ~ 0.9. At lower values of U, we find d-wave superconductivity. Taking into account the point group symmetries of the lattice, we find a transition between x2 y 2 d and dxy pairing symmetries, the latter happening for smaller values of U.