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A tight binding model is introduced to describe the strong interaction limitof excitonic ordering. At stoichiometry, the model reduces in the strongcoupling limit to a pseudo-spin model with approximate U(4) symmetry. Excitonicorder appears in the pseudo-spin model as in-plane pseudo-magnetism. The U(4)symmetry unifies all possible singlet and triplet order parameters describingsuch states. Super-exchange, Hunds-rule coupling, and other perturbations actas anisotropies splitting the U(4) manifold, ultimately stabilizing aparamagnetic triplet state. The tendency to ferromagnetism with doping(observed experimentally in the hexaborides) is explained as a spin-floptransition to a different orientation of the U(4) order parameter. The physicalmechanism favoring such a reorientation is the enhanced coherence (and hencelower kinetic energy) of the doped electrons in a ferromagnetic backgroundrelative to the paramagnet. A discussion of the physical meaning of variousexcitonic states and their experimental consequences is also provided.Comment: 16 pages, 5 figure

Excitonic order at strong coupling: Pseudospin, doping, and ferromagnetism