Long-range Neel order in the triangular Heisenberg model

We have studied the Heisenberg model on the triangular lattice using quantu m Monte Carlo techniques (up to 144 sites) and exact diagonalization (up to 36 sites). By studying the spin gap as a function of the system size we ha ve obtained robust evidence for a gapless spectrum, confirming the existenc e of long-range Neel order. Our best estimate is that in the thermodynamic limit the order parameter m dagger = 0.41 +/- 0.02 is reduced by about 59% from its classical value and the ground state energy per site is e(o) = -0. 5458 +/- 0.0001 in units of the exchange coupling. We have identified the g round state correlations that are important at shea distances.