Interplay of triplets and lattice degrees of Freedom in the coupled spin dimer system KCu Cl3

We report inelastic light scattering measurements of the three-dimensional coupled spin dimer system KCu Cl3 with a spin gap of Δ=31 K. We observe several distinct features, (i) equally intense two- and three-magnon continua, showing substantial temperature dependence of spectral weight and onset energy, and (ii) nonmonotonic temperature dependence of optical phonon modes and the presence of pronounced multiphonon scattering. The former points to the significance of delocalized triplets while the latter uncovers substantial lattice anomalies. This suggests that the studied system, showing the Bose-Einstein condensation of triplets, is characterized by an intriguing interplay between delocalized triplets and lattice degrees of freedom.