  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 ($U\lesssim5t$) and strong-interaction
($U\gtrsim9t$) 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.
