Symmetrized mean-field description of magnetic instabilities in k-(BEDT-TTF)_2Cu[N(CN)]_2Y salts

Abstract: We present a convenient mean-field method, and apply it to study the metallic/antiferromagnetic interface of kappa-(BEDT-TTF)(2)Cu[N(CN)](2)Y organic superconductors (BEDT-TTF is bis-ethylendithio-tetrathiafulvalene, Y = Cl,Br). The method, which fully exploits the crystal symmetry, allows one to obtain the mean-field solution of the two-dimensional Hubbard model for very large lattices (typically 6 x 10(5) sites), yielding a reliable description of the phase boundary in a wide region of the parameter space. The metal/antiferromagnet transition appears to be second order, except for a narrow region of the parameter space, where the transition is very sharp and possibly first order. The coexistence of metallic and antiferromagnetic properties is only observed for the transient state in the case of smooth second order transitions. The relevance of the present results to the complex experimental behavior of centrosymmetric kappa-(BEDT-TTF)(2)Cu[N(CN)](2)Y salts is discussed.