  We investigate the metal-insulator transition in the half-filled Hubbard
model on a two-dimensional triangular lattice using both the
Kotliar-Ruckenstein slave-boson technique, and exact numerical diagonalization
of finite clusters. Contrary to the case of the square lattice, where the
perfect nesting of the Fermi surface leads to a metal-insulator transition at
arbitrarily small values of U, always accompanied by antiferromagnetic
ordering, on the triangular lattice, due to the lack of perfect nesting, the
transition takes place at a finite value of U, and frustration induces a
non-trivial competition among different magnetic phases. Indeed, within the
mean-field approximation in the slave-boson approach, as the interaction grows
the paramagnetic metal turns into a metallic phase with incommensurate spiral
ordering. Increasing further the interaction, a linear spin-density-wave is
stabilized, and finally for strong coupling the latter phase undergoes a
first-order transition towards an antiferromagnetic insulator. No trace of the
intermediate phases is instead seen in the exact diagonalization results,
indicating a transition between a paramagnetic metal and an antiferromagnetic
insulator.
