Quantum Statistics of Interacting Dimer Spin Systems
  The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important. 
Ch. R\"uegg, B. Normand, M. Matsumoto, Ch. Niedermayer, A. Furrer, K.   W. Kr\"amer, H. U. G\"udel, Ph. Bourges, Y. Sidis an...