We have measured the optical conductivity of single crystal ${\mathrm{L}\mathrm{u}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ from 10 to $45000{\mathrm{c}\mathrm{m}}^{-1}$ at temperatures between 4 and 300 K. A symmetry allowed on-site Mn $d\mathrm{\text{−}}d$ transition near 1.7 eV is observed to blueshift ($\sim 0.1\mathrm{e}\mathrm{V}$) 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 the temperature dependence of the quasistatic dielectric constant ${ϵ}_0$ below $T_N\sim 90\mathrm{K}$ is overwhelmingly dominated by the phonon contributions.

• Received 10 January 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.027203