N. Cavadini, Ch. Rüegg, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka
Temperature renormalization of the magnetic excitations in S=1/2 KCuCl₃
Eur. Phys. J. B 18, 565-571 (2000)                 

Abstract: A complete temperature characterization of the spin dynamics in the unconventional S=1/2 antiferromagnet KCuCl₃ is presented from single crystal inelastic neutron scattering studies. KCuCl₃ features a quantum disordered singlet ground state and a finite spin gap to triplet excitations of dimer origin. Three dimensional magnetic correlations support the dispersive propagation of the excitations in the whole reciprocal space. Upon increasing the temperature, a renormalization in the energy, in the intensity and in the damping rate of the triplet modes is reported. The experimental observations can be described within the framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the thermally activated decrease of the occupation difference n₀-n₁ between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Néel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.