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 ≲ 5 t ) and strong-interaction ( U ≳ 9 t ) 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 ∘ 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. [ABSTRACT FROM AUTHOR]