Magnetic excitations in the quantum spin system<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">TlCuCl</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The $S=\frac{1}{2}$ quantum system ${\mathrm{TlCuCl}}_{3}$ has a nonmagnetic singlet ground state and a finite-energy gap to triplet excited states. At ${H}_{c}\ensuremath{\sim}6\mathrm{T}$ 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. ${\mathrm{TlCuCl}}_{3}$ is characterized as a strongly coupled spin system in the vicinity of a quantum critical point. A comparison with the parent compound ${\mathrm{KCuCl}}_{3}$ is proposed.

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