Abstract
Recently a new type of singularity has been discovered in the spectrum of an fcc lattice problem with second-neighbor interactions. Arising as a consequence of a surface of analytic critical points, it does not fit into van Hove's classification of the isolated variety. A simple modification of his scheme accommodates this case and predicts further singularities, characteristic of one- and two-dimensional lattices, in three-dimensional model problems. Implications for real materials are discussed.

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