N. Cavadini, G. Heigold, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka
Magnetic excitations in the quantum spin system TlCuCl₃
Phys. Rev. B 63, 172414/1-4 (2001)                 

Abstract: The S = 1/2 quantum system TlCuCl₃ has a nonmagnetic singlet ground state and a finite-energy gap to triplet excited states. At H_c~6 T it undergoes field-induced three-dimensional magnetic ordering. Its dynamical spin properties in zero field were studied by inelastic neutron scattering on single crystals. The elementary spectrum consists of well-defined triplet waves of dimer origin. From the observed energy dispersion, the underlying spin-exchange coupling scheme is rationalized. Appreciable three-dimensional correlations are reported, in accordance with the high-field phase. An antiferromagnetic Heisenberg model in the strong-coupling approximation satisfactorily accounts for the energy and intensity dependence of the triplet excitations in the whole reciprocal space. TlCuCl₃ is characterized as a strongly coupled spin system in the vicinity of a quantum critical point. A comparison with the parent compound KCuCl₃ is proposed.