The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We have investigated the triplet dispersion as a function of temperature by inelastic neutron scattering experiments in zero magnetic field. The description of Troyer, Tsunetsugu, and Würtz provides an appropriate quantum statistical model for dimer spin systems at finite temperature, where many-body correlations become particularly important [Ch. Rüegg et al., Phys. Rev. Lett. 95, 267201 (2005)]. The temperature-dependence of the excitation spectrum is subsequently investigated by the same experimental technique at finite field around the quantum critical point at Hc, where the triplet gap is closed at T=0 K and field-induced BEC of magnons occurs. The observed renormalization effects in the quantum critical region as well as the phase with long-range magnetic order will be discussed and compare to the zero-field results.
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