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

A complete temperature characterization of the spin dynamics in the unconve ntional S = 1/2 antiferromagnet KCuCl3 is presented from single crystal ine lastic neutron scattering studies. KCuCl3 features a quantum disordered sin glet ground state with a finite spin gap to triplet excitations of dimer or igin. Three dimensional magnetic correlations support the dispersive propag ation 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 observation s can have described within framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the the rmally activated decrease of the occupation difference n(0)-n(1) between si nglet and triplet dimer states. This is the expression of kinematic constra ints which are of minor importance for classical magnons in Neel ordered an tiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.