Elementary excitations in quadrupolar systems |
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Authors: | Jean Sivardiere |
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Institution: | Département de Recherche Fondamentale, Centre d''Etudes Nucléaires de Grenoble B.P. 85 Centre de tri, 38041 Grenoble Cedex, France |
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Abstract: | We consider isotropic quadrupolar ordered systems and derive elementary excitations at low temperature. A Holstein-Primakoff type transformation and a linear approximation are used. For , the spectrum is made of four degenerate acoustic branches. For S ? 2, only two degenerate branches satisfy the Goldstone theorem: they describe Δm = ± 1 excitations similar to librons in molecular crystals. The two degenerate branches describing Δm = ± 2 excitations have a gap at k = 0 although the hamiltonian is isotropic. For a special cubic hamiltonian, a Goldstone mode is found in the spectrum and related to a continuous degeneracy of the ground state. A comparison between dipolar and quadrupolar systems is presented. |
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