Hubbard Model on a Triangular Lattice at Finite Temperatures

Using the strong coupling diagram technique, we find three phases of the half-filled isotropic Hubbard model on a triangular lattice at finite temperatures. The weak-interaction (\(U\lesssim 5t\)) and strong-interaction (\(U\gtrsim 9t\)) phases are similar to those obtained by zero-temperature methods—the former is a metal without perceptible spin excitations; the latter is a Mott insulator with the 120\(^\circ\) short-range spin ordering. Zero-temperature approaches predict a nonmagnetic insulating spin-liquid phase sandwiched between these two regions. In our finite-temperature calculations, the Mott gap in the intermediate phase is filled by the Fermi-level peak, which is a manifestation of the bound states of electrons with pronounced spin excitations. We relate the appearance of these excitations at finite temperatures to the Pomeranchuk effect.