Spin dynamics of the spin dimer system ${\mathrm{TlCuCl}}_3$ probed by Raman spectroscopy

Phonon and magnetic excitations in the strongly coupled spin dimer system ${\mathrm{TlCuCl}}_3$ have been studied by inelastic light scattering. We report the observation of a broadened two-magnon continuum with an unusual temperature dependence: upon cooling the spin gap is reduced and the peak energy of the continuum shifts to lower energy. This is attributed to dynamic spin-phonon interaction which leads to a phonon-modulated spin gap and exchange coupling constant. In addition, due to the large bandwidth and small spin gap of ${\mathrm{TlCuCl}}_3,$ the interaction of thermally induced triplets is taken into account. Our results suggest the significance of spin-phonon coupling and thermally induced triplets to understand the collective spin dynamics of ${\mathrm{TlCuCl}}_3.$