ИСТИНА

In recent years, organic superconductors based on BEDT-TTF molecules are actively studied. This is caused by the fact that they are good candidates for the role of quantum spin liquid — a special magnetic state, which has a short-range, but not long-range order. Dimers of BEDT-TTF molecules each have one localized electron and effectively form layers of a triangular lattice. Due to the inherent property of geometric frustration, the triangular lattice does not have a staggered antiferromagnetic order, but allows the formation of many other magnetic structures. At normal conditions (BEDT-TTF)2X compounds are insulators but with pressure they go to superconducting state and then to normal one. Experimental phase diagram of the organic superconductors is similar to the cuprate superconductors. It contains antiferromagnetic and paramagnetic insulator states, superconducting and normal metal phases. Theoretical calculations within the Hubbard model confirm rich magnetic phase diagram for the triangular lattice. We construct the ground state magnetic phase diagrams of the Hubbard model in the Hartree–Fock and slave boson approaches taking into consideration collinear and noncollinear (spiral) magnetic phases. We investigate the effect of electron correlations, doping and next-nearest neighbor hopping to the stability of magnetic structures. Half-filling phase diagrams allow us to study the metal-insulator transition and find a region of possible spin-liquid state formation. Slave boson method was previously used for the construction of magnetic phase diagrams in the Hubbard model. It allows one to calculate the ground state energy of spiral magnetic order with account of possible phase separation and to construct ground state phase diagrams. The approach was approved for frustrated lattices as well. In [J. Phys.: Condens. Matter 28, 505601 (2016)] the magnetic phase diagram of the face-centered cubic lattice is constructed. The slave boson method can be generalized to consider superconducting order as well.