Abstract
We report the experimental results and propose a theory of resonant two-magnon Raman scattering from antiferromagnetic insulators. On the experimental side, we present the first measurements on YBa
2Cu
3O
6.1 at
T = 4 K, and find the resonant frequency at higher photon energies than previously believed. On the theoretical side, we argue that the conventional Loudon-Fleury theory does not work in the resonant regime, in which the energy of the incident photon is close to the gap between the conduction and valence bands, and identify the most relevant contribution to Raman intensity. We find good agreement between the theoretical prediction for the power-law behavior of the strength of the two-magnon scattering and the data. We argue that this agreement is evidence for the existence of
coherent quasiparticle states in these materials.