Y. Sakisaka

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface☆

Abstract High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm−1, 1855 cm−1 (shifting to 1944 cm−1 with increasing coverage) and at 1960 cm−1 (shifting to 2016 cm−1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.

Angle-resolved photoemission study of an in-gap state in TiO2

Abstract An in-gap state formed in the band gap of TiO 2 upon surface oxygen-vacancy doping has been studied by angle-resolved photoemission spectroscopy. The in-gap state with a formal d 1 configuration exhibits a small (1×1) k dispersion, though it never crosses the Fermi level for all the oxygen-vacancy concentration range. We propose that a polaron interaction is responsible for the localization of doped electrons in the in-gap state.

CO on Fe(110): an angle-resolved photoemission study

Abstract CO chemisorption on Fe(100) at 80 K was reinvestigated by angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. At 0.25 L, the CO 5σ peak and the CO 4σ peak are observed at 7.5 and 10.5 eV below the Fermi level, and they shift to 8.3 and 10.7 eV respectively, as the coverage increases up to 3.0 L. In contrast to previous observations for CO on Fe(100) [Phys. Rev. B 27 (1983) 3338] where the 1π − 5σ separation was considerably larger (~1.5 eV), the 1π and 5σ peaks are not observed separately as usually. Their separation is found to be ~0.5 eV. The lower binding-energy peak previously assigned to CO 1π emission is due to oxygen contamination. The present data also indicate that the orientation of the CO axis undergoes a two-stage transition depending on the amount of CO exposure: inclined (⩽ 0.5 L) → upright (~2 L) → inclined (4 L).

Silicon thermal oxidation and its thermal desorption investigated by Si 2p core-level photoemission

Initial thermal oxidation of silicon and thermal desorption of silicon oxide have been investigated by Si 2p core-level photoemission with synchrotron radiation. The surface reaction processes are discussed from the difference in the chemically-shifted components, especially suboxides. On the thermal oxidation, time evolutions of suboxide intensities show a distinct temperature dependence, which is explained by two growth modes for the oxidation: first-order Langmuir-type adsorption mode and two-dimensional island growth mode at oxidation temperatures below and above 650 °C, respectively. On the thermal desorption, the spectrum for the thermal oxide followed by annealing at 1000 °C for 30 s in vacuum, whose thickness is nominally estimated at 3.7 A, are compared with the 3.3 A-thickness thermal oxide without the process of the annealing. The intensities of the suboxide components are considerably different between them, which can be explained by the formation and lateral growth of the voids in the annealed oxide layer.

Effect of surface oxygen vacancies on electronic states of reduced SrTiO3(110) surface

Abstract The electronic structure of reduced SrTiO3(110) surface prepared by annealing in ultrahigh vacuum (UHV), has been studied using photoemission spectroscopy (PES) and scanning tunneling microscopy/spectroscopy (STM/STS). In the PES spectra of reduced SrTiO3(110) surface after annealing in UHV at 800°C, a clear metallic state with a sharp Fermi cut-off and a broad state centered at ∼1.1eV below the Fermi level are observed in the band gap region. A Ti3p->3d resonance enhancement is occurred for the metallic state, but not for the ∼1.1eV state. On the other hand, after the sample was annealed in UHV at 1000°C, it has been shown that the metallic state becomes drastically weak, and that only the ∼1.1eV state is seen in the band gap region. Thereafter, the ∼1.1eV state exhibits a clear Ti3p->3d resonance enhancement. The difference in the spectral feature of these band gap states is directly related to the surface geometric structural change.

Magnetic X-ray absorption fine structure for Ni-Mn alloys.

Magnetic X-ray absorption fine-structure (XAFS) spectra have been measured for Ni-Mn alloys. The magnetic XAFS in the near-edge region (X-ray absorption near-edge structure, XANES) and X-ray magnetic circular dichroism (XMCD) of the Mn and Ni K-edge for Ni(1-x)Mn(x) (x = 0.25, 0.24 and 0.20) show that (i) the local magnetic structure around the Mn atom is quite different from that around the Ni atom, and (ii) the peak intensity in the magnetic XANES of the Mn K-edge depends on the magnetization of the sample in contrast to the Ni K-edge. The Mn K-edge magnetic EXAFS (extended XAFS) for Ni(0.76)Mn(0.24) is also measured. The second and fourth peaks in the Fourier transform are observed to be enhanced in comparison with the non-magnetic EXAFS, indicating that the second- and fourth-shell Ni atoms are replaced by Mn atoms due to heat treatment (atomic ordering). Semi-relativistic theoretical calculation explains the observed magnetic EXAFS.

Electronic structure of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 studied by angle-resolved photoemission spectroscopy

In the photoemission spectra of the κ-(BEDT-TTF)2Cu(NCS)2 surface, two characteristic structures were observed at the binding energies of ~-7.5 eV and ~-11.5 eV, which were attributed to the π- and σ-bonding states, respectively, in comparison with the results of the molecular orbital (MO) calculations. Polarization dependence appearing in the spectra was comprehensively interpreted by the electronic structures calculated for those states, which is in good agreement with the results previously obtained using a scanning tunneling microscope.

GMR in inhomogeneous Ni–Mn alloys of nano-scale

Abstract GMR was investigated in inhomogeneous Ni 1− x Mn x alloys ( x =0.20–0.3) of nano-scale, where ordered single-domain clusters are distributed in disordered Ni-rich or Mn-rich matrices. The GMR depends remarkably on x : GMR for 0.20⩽ x ⩽0.24 is small (Type I) and for 0.26⩽ x ⩽0.30 has a maximum of −3.6% at x =0.27 (Type II). Types I and II are caused, respectively, by ferromagnetic and antiferromagnetic interactions between domains.

Metallic State on Reduced SrTiO3(110) Surface

The electronic structure of reduced SrTiO3(110) surface prepared by annealing in ultrahigh vacuum (UHV), has been studied using photoemission spectroscopy (PES) and scanning tunneling microscopy/spectroscopy (STM/STS). In the photoemission spectra of reduced SrTiO3(110) surface after annealing in UHV at 800°C, a clear metallic state with a sharp Fermi cut-off and a broad state centered at ∼1.1eV below the Fermi level are observed in the band gap region. A Ti3p->3d resonance enhancement is occurred for the metallic state, but not for the ∼1.1eV state. On the other hand, after the sample was annealed in UHV at 1000°C, it has been shown that the metallic state becomes drastically weak, and that only the ∼1.1eV state is seen in the band gap region. Thereafter, the ∼1.1eV state exhibits a clear Ti3p->3d resonance enhancement. The difference in the spectral feature of these band gap states is directly related to the surface geometric structural change.

Measurements and Theoretical Calculations of Magnetic XAFS for Ni‐Mn Alloys in Ordered and Disordered States

We have measured the Mn K‐edge magnetic EXAFS and XANES (K‐XMCD) of Ni0.75Mn0.25 alloys prepared by various annealing times. The 2nd peaks in the Fourier transform attributed to Mn‐Mn pairs in the ordered phase are greatly enhanced in comparison with those in conventional EXAFS. This result suggests that some of Ni atoms in the 2nd shell are replaced by Mn atoms, which have large magnetic moment in comparison with Ni atoms, due to heat‐treatment induced atomic ordering. Relativistic theoretical calculations well explain the observed magnetic EXAFS and K‐XMCD. We estimate that the ratio of the magnetic moments of Mn to that of Ni, which increases as annealing time. It is suggested that the magnetic moment of Mn atom increases with increase of the ordered phase by annealing.