MARSHALL-PEIERLS PHASE RULE AND LIEB-MATTIS LEVEL ORDERING IN NON-BIPARTITE LOW-DIMENSIONAL ANTIFERROMAGNETS

We present a number of arguments in favor of the suggestion that the M arshall-Peierls sign rule and the Lieb-Mattis level ordering survive t he frustration in the square-lattice Heisenberg antiferromagnet with f rustrating next-nearest-neighbor (diagonal) bonds (J(1)-J(2) model) fo r relatively large values of the parameter J(2)/J(1).