Coulomb interaction effects in the spin susceptibility of an organic Dirac fermion system

In an organic conductor α-(BEDT-TTF)₂I₃, a two-dimensional correlated Dirac electron system is realized under hydrostatic pressure. Thanks to the sublattice-selective NMR measurement and theoretical analysis, it has been revealed that the magnetization is affected not only by on-site Coulomb interaction but also by inter-site Coulomb interaction. The random phase approximation (RPA) analysis based on the Hubbard model clarified that the on-site Coulomb repulsion causes the ferri magnetic polarization. On the other hand, the renormalization group approach based on 2x2 Wyel Hamiltonian showed that the inter-site long-range Coulomb interaction induces the fermi velocity renormalization. However, we noticed that these two correlation effects are competitive since the ferri magnetism is stabilized by inter-band thermal excitations, which are supsressed by the velocity renormalization.
In this study, we examine the spin susceptibility in a extended Hubbard model describing α-(BEDT-TTF)₂I₃. Employing the Fock type selfenergy and RPA, the ferri magnetism and velicity renormalization are successfully obtained at the same time. We also evaluate the self-energy with screening effect by thermally excited carriers.