Exchange interaction effects on the optical properties of LuMnO3
  We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm-1 at temperatures between 4 and 300 K. A symmetry allowed on-site Mn $d$-$d$ transition near 1.7 eV is observed to blue shift (~0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in temperature dependence of the quasi-static dielectric constant epsilon_0 below the T_N ~90 K is overwhelmingly dominated by the phonon contributions. 
A.B. Souchkov, J.R. Simpson, H.D. Drew (University of Maryland, MD),   M. Quijada (NASA, Goddard Space Flight Ctr, MD), H. I...