N. Cavadini, W. Henggeler, A. Furrer, K. Krämer, H. U. Güdel, H. Mutka, A. Wildes and P. Vorderwisch
Triplet Excitations in a Family of S=1/2 Unconventional Antiferromagnets
in "Neutron Scattering in Novel Materials", A. Furrer (ed.)
World Scientific, Singapore, 237-251 (2000)           

Abstract: Cu²⁺ magnetic insulators KCuCl₃ and TlCuCl₃ are unconventional antiferromagnets of dimer nature. Both assume a quantum disordered singlet ground state, and show a finite spin excitation gap to triplet states. Despite the lack of classical Néel order, true three-dimensional magnetic interactions are realized in the above compounds. Recent inelastic neutron scattering experiments are presented, which elucidate the nature of triplet modes under this circumstance. Model predictions around the dimer limit are derived and successfully compared to the observed dynamic properties. The latter are representative of a large class of gapped quantum systems, for which a general phase diagram is discussed.