Optical absorption and static spin correlation functions in the one-dimensional ferromagnet CsNiF3

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Abstract
The authors report the temperature dependence (1.63A2g to 3T2g( Gamma 3) excitonic transition in the one-dimensional easy-plane ferromagnet CsNiF3. They have found a linear dependence of its position (or of the first moment) on the temperature, a quite unusual behaviour which is claimed to be due to its 1D character. The authors have calculated these moments in terms of one- and two-ion static spin correlation functions. In particular, the linear shift of the first moment with temperature is qualitatively well explained. They discuss quantitatively their results in the light of 1D-Heisenberg, X Y or easy-plane models.