The metal -insulator (M -I) transition at about 340 K (T MI ) in TbBaCo 2 O 5.5 has been investigated by X-ray absorption spectra near Tb-L Ш , Ba-L П , and Co-K edges. In the metallic and insulating phases, the highdensity unoccupied bands of Tb-6dσ and Ba-6dσ occupy just above a Fermi level (E F ), while the positive charge-transfer Tb-5d

PACS 61.10.Ht, 71.20.Ps, 71.30.+h X-ray absorption spectra near the absorption edges of Nd, Ba, and Co ions have been taken to study the metal -insulator (M -I) transition occurring at about 360 K (T MI ) in NdBaCo 2 O 5.5 . In the metallic and insulating phases, the electronic density of states (DOS) just above the Fermi level (E F ) consists of the highdensity unoccupied bands of Nd -5dσ and Ba -5dσ. The narrow charge-transfer Co -3d7 (e g )L band and the broad charge-transfer Co -3d(t 2g ) n L m bands occupy positions just below and near E F , respectively, where L represent a hole in the O -2p valence band. The negative charge-transfer Nd -4d n L′ m and Ba4d n L′ m hybridized bands are also just below E F , where L′ represent a hole in the fully occupied 4d band of the Nd and Ba ions. It is suggested that the metallic conductivity above T MI arises from the itinerant (p -d) hybridized holes having an 3d(e g )-symmetry in the top of the O -2p valence band. The structural deformation localized around the Ba ions but not the Nd and Co ions affects the crystal-field splitting of the Co3d orbitals in the CoO polyhedra and the hybridization between the outer orbitals of the cations (Nd, Ba, Co ions) and the O -2p orbitals.

We are developing plasma nanofabrication, namely, nanoand micro scale guided assembly using plasmas. We manipulate nanoand micro objects using electrostatic, electromagnetic, ion drag, neutral drag, and optical forces. The accuracy of positioning the objects depends on fluctuation of position and energy of a fine particle (= each object) in plasmas. Here we evaluate such fluctuations and discuss the mechanism behind them. In the first experiment, we grabbed a fine particle in plasma using an optical tweezers. The fine particle moves in a potential well made by the optical tweezers. This is a kind of Brownian motion and the position fluctuation can be caused by neutral molecule collisions, ion collisions, and fluctuation of electrostatic force. Among theses possible causes, fluctuation of electrostatic force may be main one. In the second experiment, we deduced interaction potential between two fine particles during their Coulomb collision. We found that there exist repulsive and attractive forces between them. The repulsive force is a screened Coulomb one, whereas the attractive force is likely a force due to a shadow effect, a non-collective attractive force. Moreover, we noted that there is a fluctuation of the potential, probably due to fluctuation of electrostatic force. These position and potential energy fluctuations may limit the accuracy of guided assembly using plasmas.

The structural deformations around Sr, Bi, and Ta ions, and the electronic density of states near the Fermi level have been studied in paraelectric and ferroelectric phases of the layer pseudo-perovskite compound, SrBi 2 Ta 2 O 9 (SBT), by means of X-ray absorption spectra (XAS) near the absorption edges of Sr, Bi, and Ta ions. The X-ray absorption near edge structure (XANES) shows that localized largely structural deformations appear around Sr and Ta ions, but not around Bi ion in the vicinity of the ferroelectric to paraelectric structural phase transition. In both phases, there are the charge-transfer (4dL, 4d 2-. This hybridization is induced by an electronic interaction between the cations (Sr, Bi, and Ta ions) and the anions (O ions). The Fermi level is very close to the unoccupied Sr-5pπ and Ta-5d(e g ) bands, but not the unoccupied Bi-6dπ band. In the ferroelectric phase, the hybridization between the Bi ion and the oxygen ions (O2) from the (SrTa 2 O 7 ) 2-blocks considerably reduced and almost disappears. This reduction and disappearance of the hybridization increases the effective charges on O2 in the TaO 6 octahedra, while the (Bi 2 O 2 ) 2+ layers act only as insulators. Thus, the charge-up of O2 contributes a large dielectric permittivity of the pseudo-perovskite (SrTa 2 O 7 ) 2-blocks, in addition to the structural deformation induced near the phase transition at 300 °C. The strong hybridization between the Sr and oxygen ions (O1, O4, and O5) also plays a crucial role to characterize the unusual ferroelectricity in SrBi 2 Ta 2 O 9 family of compounds substituted Sr ions with other ions or vacancies.

No abstract