Abstract
A pseudospin approach is developed for the study of quantum spin systems. The method is based on generalized coherent states in the symmetric representations of the unitary algebra U(2 s+1), where s is the total spin at each site, and on the associated Holstein-Primakoff theory. A systematic 1/ n éxpansion is derived which enables one to study magnets with weak as well as strong single-site anisotropies. While the method applies to a variety of quantum spin systems in arbitrary lattice dimensions, particular emphasis is placed here on the spin-1 planar ferromagnetic chain observed in CsNiF 3. The 1/ n expansion for that system yields a twofold spectrum of elementary excitations, the usual magnon and a collective mode. It is also found that a sharp crossover transition occurs for intermediate couplings in analogy with the Gross-Witten third-order transition in the large- n limit of simple gauge models.
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