In the large-U limit, the ground state of the half-filled, nearest-neighbor Hubbard model on the triangular lattice is the three-sublattice antiferromagnet. In sharp contrast with the square-lattice case, where Goldstone modes never have a charge component, it is shown that beyond leading order in t/U the three Goldstone modes on the triangular lattice, at finite q, are a linear combination of spin and charge. This leads to non-vanishing conductivity at any finite frequency, even though the magnet remains insulating at zero frequency. More generally, non-collinear spin order should lead to such gapless insulating behavior.

ER -