Two-dimensional extended Hubbard model: doping, next-nearest neighbor hopping and phase diagrams

Abstract

Using the strong coupling diagram technique, we investigate the extended Hubbard model on a two-dimensional square lattice. This approach allows for charge and spin fluctuations and a short-range antiferromagnetic order at nonzero temperatures. The model features the first-order phase transition to states with alternating site occupations (SAO) at the intersite repulsion $v=v_c$. In this work, we show that doping decreases $v_c$. For a nonzero next-nearest neighbor hopping $t'$, less mobile carriers produce a stronger fall in $v_c$. For half-filling and $t'=0$, we consider phase diagrams for a fixed temperature $T$, on-site $U$, and intersite repulsions. The diagrams contain regions of SAO, Mott insulator, and several metallic states distinguished by their densities of states. Two of them are characterized by a dip and a peak at the Fermi level. The dip originates from the Slater mechanism for itinerant electrons, while the peak to bound states of electrons with localized magnetic moments. The existence of these two metallic regions in the phase diagram is a manifestation of the Pomeranchuk effect. The boundary between these regions reveals itself as a kink in the curve $v_c(T)$ and as a maximum in the $T$ dependence of the double occupancy $D$. Our calculated $D$ for different values of $v$, $U$, and $T$ are in semiquantitative agreement with Monte Carlo results.