Neutral $3d$ excitations in insulating $\mathrm{V}{\mathrm{O}}_2$ as seen with resonant inelastic x-ray scattering at the $V{L}_{3,2}$ edges

We present resonant inelastic x-ray scattering (RIXS) results on insulating monoclinic $\mathrm{V}{\mathrm{O}}_2$ with excitation across the $L_{2,3}$ edges of vanadium. The spectra measured near threshold in the $L_3$ region and in the $L_2$ region show a Raman behavior with a clear peak around $0.9\mathrm{eV}$ transferred energy. The analysis of the data allows the neutral excitation spectrum of the system up to about $2–2.5\mathrm{eV}$ to be determined. These excitations are due to $dd$ transitions, whose main contributions are discussed. The possibility of $\{d^1;d^1\to d^2;d^0\}$ charge fluctuations within a dimer in RIXS is also discussed. The results are compared with available information on optical constants and on electron energy loss spectroscopy. Due to the low symmetry of the system, there is a similarity between the Raman spectrum obtained with RIXS and the optical absorption spectrum.

Figure 1

(Color online) Upper panel: $L_{2,3}$ absorption curves of $\mathrm{V}{\mathrm{O}}_2$, with the vertical bars giving the energies used in RIXS. The absorption with linear polarization parallel to the $c$ axis is given by the thin line and that perpendicular to the $c$ axis by the thick line. The inset shows the geometry in scattering. Lower panel: a selection of RIXS spectra measured with horizontal linear polarization (one spectrum is in red for readability).

Figure 2

(Color online) The RIXS spectra with vertical polarization [black in (a)] and with horizontal polarization [red in (b)]. The sum of spectra A, B, C, E, and F is given in (c) with both polarizations. (d) The elastic contribution to the sum spectrum with horizontal polarization (red) is fitted with the combined line shape (blue), while the green spectrum with excitation C is given for comparison. The extracted experimental Raman spectrum is given by the upper black curve in reference to the horizontal dashed line.

Figure 3

(Color online) (a) Schematic origin of the $\mathrm{V}{\mathrm{O}}_2$ $d$ states and of the excitations ${\mathrm{E}}_1$, ${\mathrm{E}}_2$, and ${\mathrm{E}}_3$. See text for more explanations. (b) The subcomponents of the measured Raman spectrum. (c) Summary of the measured excitations in the final state of RIXS.

Figure 4

Comparison between the RIXS Raman spectrum of $\mathrm{V}{\mathrm{O}}_2$ (present work) and the optical absorption obtained from the optical conductivity of Ref. 12. The energy corresponds to the transferred energy in the RIXS spectrum and to incident photon energy in optical absorption.