Niemeier self-dual lattices and topological phase transitions

A topological phase transition in two-dimensional nonlinear sigma-models on tori, connected with self-dual (unimodular) 24-dimensional Niemeier lattices, is considered. It is shown that critical properties of these transitions are determined by corresponding Coxeter numbers of lattices. A case of general integer-valued lattices with minimal norm equal 1 or 2 and a possible application to string theory are discussed.