Electronic structures ofCu2O,Cu4O3, and CuO: A joint experimental and theoretical study

A joint experimental and theoretical study is presented for the electronic structures of copper oxides including $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$, CuO, and the metastable mixed-valence oxide $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3$. The optical band gap is determined by experimental optical absorption coefficient, and the electronic structure in valence and conduction bands is probed by photoemission and electron energy loss spectroscopies, respectively. The experimental results are compared with many-body $GW$ calculations utilizing an additional on-site potential for $d$-orbital energies that facilitates tractable and predictive computations. The side-by-side comparison between the three oxides, including a band insulator $\left(\mathrm{C}{\mathrm{u}}_2\mathrm{O}\right)$ and two Mott/charge-transfer insulators (CuO, $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3)$ leads to a consistent picture for the optical and band-structure properties of the Cu oxides, strongly supporting indirect band gaps of about 1.2 and 0.8 eV in CuO and $\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_3$, respectively. This comparison also points towards surface oxidation and reduction effects that can complicate the interpretation of the photoemission spectra.