  The one-electron density of states for the half-filled Hubbard model on a
triangular lattice is studied as a function of both temperature and Hubbard U
using Quantum Monte Carlo. We find three regimes: (1) a strong-coupling
Mott-Hubbard regime, characterized by a gap which persists even at high
temperatures; (2) a weak-coupling paramagnetic regime, characterized by the
absence of a pseudogap at any finite temperature; and (3) an
intermediate-coupling (spiral) spin-density-wave regime, characterized by a
pseudogap which appears when U is increased beyond a critical (temperature
dependent) value. The behavior of the sqrt(3) times sqrt{3} adlayer structures
on fourth-group semiconductor surfaces is briefly commented upon in the light
of the above discussion.
