Probing high-energy electronic excitations in NiO using inelastic neutron scattering

High-energy local multiplet excitations of the $d$ electrons are revealed in our inelastic neutron scattering measurements on the prototype magnetic insulator NiO. These become allowed by the presence of both nonzero crystal field and spin-orbit coupling. The observed excitations are consistent with optical, x-ray, and EELS measurements of $d$-$d$ excitations. This experiment serves as a proof of principle that high-energy neutron spectroscopy is a reliable and useful technique for probing electronic excitations in systems with significant crystal-field and spin-orbit interactions.

Figure 1

Panel (a) is the magnetic intensity map of energy-momentum space obtained on MAPS using incident neutron energy of 3 eV at room temperature. The background intensity due to phonons has been subtracted as discussed in the text. Panel (b) illustrates the theoretical calculation discussed in the text. Panels (c) and (d) illustrate integrated cuts in momentum and energy as noted. The open circles are experimental data and the solid (red) lines are theoretical calculations. As described in the text, the overall amplitude is the only adjustable parameter in the calculation.

Figure 2

Intensity map of expanded energy and momentum space. The solid white line would correspond to hydrogen recoil.

Figure 3

Panel (a) illustrates a false contour plot taken on MAPS with the $E_i=500$ meV. (b) The intensity of the peak at 96 meV as a function of momentum transfer. The solid and open points are from MARI ($E_i=750$ meV) and MAPS ($E_i=500$ meV), respectively. The solid red line is the free ion Ni${}^{2+}$ form factor. (c) The momentum-integrated scattering and solid line is the calculated density of states from Ref. 25.