Spin dynamics in the BEC phase of the S=1/2 quantum spin system TlCuCl3

Abstract The copper salt S =1/2 TlCuCl 3 is a three-dimensional quantum spin system with a singlet ground-state of dimer origin. A spin energy gap Δ ≈0.7 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 = Δ / gμ 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.