N. Cavadini, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka
Magnetic excitations and spin gap in KCuCl₃
Physica B 540, 276-278 (2000)                 

Abstract: Detailed knowledge of the dynamical spin-spin correlation function S(k, w) turns out to be a necessity in characterizing unconventional quantum spin systems - in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S = 1/2 Cu²⁺ compound KCuCl₃. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet-triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl₃ as a quantum spin liquid near to the dimer limit.