Neutron scattering investigation of the d - d excitations below the Mott gap of CoO

Neutron scattering is used to investigate the single-ion spin and orbital excitations below the Mott-Hubbard gap in CoO. Three excitations are reported at 0.870 +/- 0.009 eV, 1.84 +/- 0.03, and 2.30 +/- 0.15 eV. These were parametrized within a weak crystal field scheme with an intraorbital exchange of J(dd) = 1.3 +/- 0.2 eV and a crystal field splitting 10Dq = 0.94 +/- 0.10 eV. A reduced spin-orbit coupling of lambda = -0.016 +/- 0.003 eV is derived from dilute samples of Mg0.97Co0.03O, measured to remove effects due to spin exchange and structural distortion parameters which break the cubic phase degeneracy of the orbital excitations complicating the inelastic spectrum. The peak at 1.84 eV, while reported using resonant x-ray and optical techniques, was absent or weak for nonresonant x-ray experiments and overlaps with the expected position of a (4)A(2) level. This transition is absent in the dipolar approximation but expected to have a finite quadrupolar matrix element that can be observed with neutron scattering techniques at larger momentum transfers. Our results agree with a crystal field analysis (in terms of Racah parameters and Tanabe-Sugano diagrams) and with previous calculations performed using local-density band theory for Mott insulating transition metal oxides. The results also demonstrate the use of neutron scattering for measuring dipole forbidden transitions in transition metal oxide systems.