Direct observations of local electronic states in an Al-based quasicrystal by STEM-EELS.

Abstract

Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery's empirical criteria. However the hybridization effect also stabilize electron system, particularly in Al-transition metal system, in which a lot of quasicrystalline phases were discovered. Therefore, the electronic structures of QCs have not yet been fully understood, whereas their atomic structures have been studied well in terms of configuration entropy by scanning transmission electron microscopy (STEM) [1]. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with STEM, by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [2].jmicro;63/suppl_1/i17-a/DFU069F1F1DFU069F1Fig. 1.Core-loss edges obtained from cluster-centers and cluster-edges. Al L1 (left) Ir O23, Ir N67 (center) and Cu L23 (right). Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67 and Cu L23, all of which reveal different behaviors at the cluster centers and the edges (Fig. 1). All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. On the basis of the calculated DOS and charge-density map, we conclude Al-Ir pairs, which are mainly located at the cluster edges, hybridize their orbitals and Cu atoms are localized at cluster center without hybridization. We analyze a distribution of the hybridized orbitals by Fourier transformation of electron localization function. The distribution seems like a 10-fold standing wave with Fermi wave length. It suggests that the Hume-Rothery mechanism works even when hybridization effect mainly contributes to pseudogap formation. In other words, global distribution of hybridized orbitals is important to stabilize structures whereas usually only local atomic configurations are discussed. Moreover, in the 10-fold standing wave, no distinct peak appears at the cluster centers. It means hybridized orbitals located at cluster centers are unable to contribute to the Hume-Rothery mechanism. It may be the reason why the metallic regions appear at the cluster centers.