Simple model for the linewidth of the two-magnon Raman feature observed in high-Tcsuperconductors

The conventional theory for two-magnon Raman scattering applied to ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ and other nonsuperconducting precursor phases of the cuprate-based high-$T_c$ materials yields a line shape much narrower than is observed. We modify this theory to allow for a finite magnon lifetime and to allow for a relaxation of momentum conservation in the scattering process. The former modification gives results in good agreement with the experiments; the latter does not. We argue that a probable source of the magnon damping is electronic excitations associated with the free carriers.