Ch. Rüegg, N. Cavadini, A. Furrer, K. Krämer, H.-U. Güdel, P. Vorderwisch, K. Habicht, H. Mutka and A. Wildes
Spin dynamics in the BEC phase of the S=1/2 quantum spin system TlCuCl₃
J. Magn. Magn. Mater. 272-276, 195-196 (2004)                 

Abstract: The copper salt S=1/2 TlCuCl₃ is a three-dimensional quantum spin system with a singlet ground-state of dimer origin. A spin energy gap D0.7 meV separates the ground-state from the triplet waves, which propagate along all directions in reciprocal space. Across the quantum critical point at H_c=D/gm_B Bose–Einstein condensation of the triplet quasi-particles has been observed. The inelastic neutron scattering spectra at H>H_c proof the coexistence of a gapless Goldstone mode, which has a linear dispersion, with renormalized quadratic Zeeman modes.