RESONANT 2-MAGNON RAMAN-SCATTERING IN ANTIFERROMAGNETIC INSULATORS

We report the experimental results and propose a theory of resonant tw o-magnon Raman scattering from antiferromagnetic insulators. On the ex perimental side, we present the first measurements on YBa2Cu3O61 at T = 4 K, and find the resonant frequency at higher photon energies than previously believed On the theoretical side, we argue that the convent ional Loudon-Fleury theory does not work in the resonant regime, in wh ich the energy of the incident photon is close to the gap between the conduction and valence bands, and identify the most relevant contribut ion to Raman intensity. We find good agreement between the theoretical prediction for the power-law behavior of the strength of the two-magn on scattering and the data. We argue that this agreement is evidence f or the existence of coherent quasiparticle states in these materials.