Electronic structure of the local-singlet insulatorNaCuO2 (original) (raw)

The insulating oxide NaCu02 has been studied by x-ray photoemission spectroscopy and subsequent cluster-model analysis. It is found that the d~d L charge-transfer energy (L:ligand hole) is negative and the ground state is dominated by the d L configuration. Using the Anderson impurity model, it is shown that strong 3d-ligand hybridization opens a band gap for a negative charge-transfer energy. This band gap corresponds to charge fluctuations mainly of the p-p type, d L +d L~d +d L, with a considerable mixture of d character into the p states, and not of the conventional Mott-Hubbard (d-d) type nor cf the charge-transfer (p-d) type. The magnitude of the gap is strongly affected by the geometrical arrangement of metal-oxygen local units, giving a natural explanation for the difference between the insulating NaCu02 and metallic LaCu03. The electronic structures of Fe + and Ni + oxides and their insulating versus metallic behaviors, which are expected to resemble those of the Cu3+ oxides, are also discussed. To generalize the above conclusions, a modification of the metal-insulator boundaries in the Zaanen-Sawatzky-Allen diagram is proposed to include compounds with small or negative chargetransfer energies. I. IIVI'RODUCTION Since the discovery of high-T, copper oxides, ' there has been increasing interest in the electronic properties of 3d transition-metal compounds, where electron-electron interaction plays an important role. Recently, much work has been done on these 3d transition-metal compounds following interpretations of photoemission spectroscopy data ' showing that photoemission spectroscopy is a powerful tool to investigate the electronic structure of these compounds. According to the classification scheme proposed by Zaanen, Sawatzky, and Allen (ZSA) based on photoemission-spectroscopy results, transition-metal compounds can be classi6ed into two regimes according to the relative magnitudes of the ligand-to-metal chargetransfer energy 6 and the d-d Coulomb repulsion energy U. In the Mott-Hubbard regime, where 5) U, the band gap is determined by charge fluctuations of the d-d type, d "+d"-+d"+'+d" ', and its magnitude is given by-U. In the charge-transfer regime, where 6 & U, charge fluctuations of the type d "+d"~d"+'+d "L (L =ligand hole), constitute a p-d-type band gap, whose magnitude is given byh. Systematics found in various 3d transition-metal compounds suggest that 5 is decreased with increasing valence of the transition-metal ion and becomes very small or even negative for compounds with unusually high valence states such as Cu +, Ni +, Fe +, etc. According to the above picture, it is expected that the charge-transfer-type band gaps collapse in these compounds, leading to metallic behaviors, but actually many high-valence compounds exist as insulators. In this paper, we have studied the electronic structure of one of the formally Cu + compounds, NaCu02. It is an insulator consisting of Cu02 chains as shown in Fig. l. Previously Steiner et al. have measured the Cu 2p xray photoemission spectroscopy (XPS) spectra of Na-CuOz, an x-ray-absorption spectroscopy (XAS) study of the Cu 2p core level has been reported by Sarma et al. They have studied this compound as a reference Cu + compound in discussing the existence of Cu + species in the high-T, copper oxide superconductors. Okada CU04 N~o Oo FIG. 1. Crystal structure of NaCu02. Cu04 squares share their edges with each other and constitute Cu02 chains.