In a recent Letter [Phys. Rev. Lett. 107, 187203 (2011)], Fujii et al. reported Mn 2p photoelectron emission spectra for (Ga,Mn)As recorded using hard x-rays. Due to the enhanced bulk sensitivity, hard-x-ray spectra reveal an extra low-binding-energy
peak, which is absent in surface-sensitive spectra recorded using soft x-rays. Based on Anderson-impurity-model calculations, Fujii et al. assigned the low-binding-energy peak to a cd6L2 final state, and related the variations in its intensity to variations in the As 4p-Mn 3d hybridization strength V. We show here that the definition of the charge-transfer energy considered by Fujii et al. is different from that considered in the Zaanen-Sawatzky-Allen diagram. We note that the Anderson impurity model is insufficient to describe low-binding-energy peaks in hard-x-ray core-level photoemission for transition-metal compounds on the verge of a metal-insulator transition. We propose a more plausible origin for the (Ga,Mn)As low-binding-energy peak, related to the nature of its metal-insulator transition.
The approximate location in the Zaanen-Sawatzky-Allen diagram of the phase-separated (Ga,Mn)As material, consisting of MnAs nanoclusters embedded in GaAs, is determined on the basis of configuration-interaction (CI) cluster-model analysis of their Mn
2p core-level photoemission. The composite material is found to belong to the special class of materials with negative charge-transfer energy (delta). As such, its metallic or insulating/semiconducting behavior depends on the strength of the p-d hybridization (affected by strain) relative to the (size-dependent) p-bandwidth. Whereas internal strain in the embedded clusters counteracts gap opening, a metal-to-semiconductor transition is expected to occur for decreasing cluster size, associated to the opening of a small gap of p-p type (covalent gap). The electronic properties of homogeneous and phase-separated (Ga,Mn)As materials are analyzed, with emphasis on the nature of their metal-insulator transitions.
We have studied the electronic structure of hexagonal MnAs, as epitaxial continuous film on GaAs(001) and as nanocrystals embedded in GaAs, by Mn 2p core-level photoemission spectroscopy. Configuration-interaction analyses based on a cluster model sh
ow that the ground state of the embedded MnAs nanocrystals is dominated by a d5 configuration that maximizes the local Mn moment. Nanoscaling and strain significantly alter the properties of MnAs. Internal strain in the nanocrystals results in reduced p-d hybridization and enhanced ionic character of the Mn-As bonding interactions. The spatial confinement and reduced p-d hybridization in the nanocrystals lead to enhanced d-electron localization, triggering d-d electron correlations and enhancing local Mn moments. These changes in the electronic structure of MnAs have an advantageous effect on the Curie temperature of the nanocrystals, which is measured to be remarkably higher than that of bulk MnAs.
