Magnetic excitations in the quantum spin system TlCuCl3 -: art. no. 172414

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 similar to6 T it undergoes held-induced three-dimensional magnetic ordering. Its dynamical spin properties in zero held 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.