Two-magnon Raman scattering in insulating cuprates: Modifications of the effective Raman operator

Calculations of Raman scattering: intensities in spin-1/2 square-lattice He isenberg model, using the Fleury-London-Elliott theory, have so far been un able to describe the broad line shape and asymmetry of the two magnon peak found experimentally in the cuprate materials. Even more notably, the polar ization selection rules no violated with respect to the Fleury-Loudon-Ellio tt theory. There is comparable scattering in B-1g and A(1g) geometries wher eas the theory would predict scattering in only B-1g geometry. We review va rious suggestions for this discrepency and suggest that at least part of th e problem can be addressed by modifying the effective Raman Hamiltonian, al lowing for two magnon states with arbitrary total momentum. Such an approac h based on the Sawatzsky-Lorenzana theory of optical absorption assumes an important role of phonons as momentum sinks. It leaves the low-energy physi cs of the Heisenberg model unchanged but substantially alters the Raman lin e-shape and selection rules, bringing the results closer to experiments.