(kappa)–(BEDT–TTF)2X organic crystals: Superconducting versus anti-ferromagnetic instabilities in the Hubbard model on an anisotropic triangular lattice

A Hubbard model at half-filling on an anisotropic triangular lattice has been proposed as the minimal model to describe conducting layers of (kappa)–(BEDT–TTF)2X organic materials. The model interpolates between the square lattice and decoupled chains. The (kappa)–(BEDT–TTF)2X materials have many similarities with cuprates, such as the presence of unconventional metallic properties and the close proximity of superconducting and anti-ferromagnetic phases. As in cuprates, spin fluctuations are expected to play a crucial role in the onset of superconductivity. We perform a weak-coupling renormalization-group analysis to show that a superconducting instability occurs. Frustration in the anti-ferromagnetic couplings, which arises from the underlying geometrical arrangement of the lattice, breaks the perfect nesting of the square lattice at half-filling. The spin-wave instability is suppressed and a superconducting instability predominates. For the isotropic triangular lattice, there are again signs of long-range magnetic order, in agreement with studies at strong-coupling.