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

The S = 1/2 quantum system TlCuCl3 has a nonmagnetic singlet ground state a nd a finite-energy gap to triplet excited states. At H-c similar to6 T it u ndergoes held-induced three-dimensional magnetic ordering. Its dynamical sp in properties in zero held were studied by inelastic neutron scattering on single crystals. The elementary spectrum consists of well-defined triplet w aves of dimer origin. From the observed energy dispersion, the underlying s pin-exchange coupling scheme is rationalized. Appreciable three-dimensional correlations are reported, in accordance with the high-field phase. An ant iferromagnetic Heisenberg model in the strong-coupling approximation satisf actorily accounts for the energy and intensity dependence of the triplet ex citations in the whole reciprocal space. TlCuCl3 is characterized as a stro ngly coupled spin system in the vicinity of a quantum critical point. A com parison with the parent compound KCuCl3 is proposed.