Abstract
We consider S ⩾ 3 2 isotropic quadrupolar ordered systems and derive elementary excitations at low temperature. A Holstein-Primakoff type transformation and a linear approximation are used. For S = 3 2 , 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 S = 3 2 cubic hamiltonian, a Goldstone mode is found in the spectrum and related to a continuous degeneracy of the ground state. A comparison between S = 1 2 dipolar and S = 3 2 quadrupolar systems is presented.
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