Temperature-dependent lifetime of spin excitations in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ (R=Eu, Y)

We have measured the light scattering by spin-pair excitations in single crystals of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ (R=Y,Eu) for several polarization directions in the temperature range of 10 to 700 K. A strong broadening of the ${\mathit{B}}_{1\mathit{g}}$ and an enhancement of the ${\mathit{A}}_{1\mathit{g}}$ component is observed with increasing temperature. For all temperatures agreement between calculations and experimental light-scattering data has been obtained with J=810 ${\mathrm{cm}}^{\mathrm{-}1}$ and a zone-edge spin-excitation damping Γ which changes from 184±50 ${\mathrm{cm}}^{\mathrm{-}1}$ (T=10 K) to 913±100 ${\mathrm{cm}}^{\mathrm{-}1}$ (T=600 K). We conjecture that spin-lattice interaction significantly decreases the spin-excitation lifetime.