Temperature renormalization of the magnetic excitations in S=1/2 KCuCl3

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 with 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 have described within 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 n(0)-n(1) between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Neel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.