A nuclear-spin-lattice relaxation process in one-dimensional spin-Peierls systems below the transition to the dimerized state is proposed. In this indirect process, the nuclear spin flip is accompanied, in lowest order, by the scattering of three excitons. To calculate the relaxation rate, we extend the mean-field model for an alternating antiferromagnetic Heisenberg chain to include the second-order correction to the spectral weight function. Good agreement is obtained with the experimental proton relaxation data for tetrathiafulvalene-biscis (1,2 perfluoromethylene 1,2 dithiolato)-copper, TTFCu${\mathrm{S}}_4$${\mathrm{C}}_4$ ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$. $\mathrm{for}\mathrm{temperatures}\mathrm{below}=\left(\frac{3}{4}\right)T_c$.

• Received 25 April 1977

DOI:https://doi.org/10.1103/PhysRevB.16.1870