The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to 28,000\wn and for temperatures 8 <T< 325 K\@. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at TN = 289 K has been observed. At low temperatures the splitting amounts to 15.0\wn. In addition, we studied the splitting of the cubic crystal field ground state of the Co2+ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below TN, magnetic dipole transitions between the exchange-split levels are identified and the energy-level scheme can be well described with a spin-orbit coupling λ=151.1\wn, an exchange constant J=17.5\wn, and a tetragonal crystal-field parameter D=−47.8\wn. Already in the paramagnetic state electric quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split d-manifold were identified at 8,000 and 18,500\wn.
@article{arxiv.0809.0997,
title = {Optical Spectroscopy in CoO: Phonons, Electric, and Magnetic Excitations},
author = {Ch. Kant and T. Rudolf and F. Schrettle and F. Mayr and J. Deisenhofer and P. Lunkenheimer and M. V. Eremin and A. Loidl},
journal= {arXiv preprint arXiv:0809.0997},
year = {2009}
}