We have analyzed the two-dimensional antiferromagnetic Heisenberg model on
the triangular lattice using a Schwinger boson mean-field theory. By expanding
around a state with local 120∘ order, we obtain, in the limit of
infinite spin, results for the excitation spectrum in complete agreement with
linear spin wave theory (LSWT). In contrast to LSWT, however, the modes at the
ordering wave vectors acquire a mass for finite spin. We discuss the origin of
this effect.Comment: 15 pages REVTEX 3.0 preprint, 6 postscript figures ( uuencoded and
compressed using the script uufiles ) are submitted separately