Akio Kimura

A new compact electron spin polarimeter with a high efficiency

We have developed a new compact retarding-potential Mott spin polarimeter and achieved an efficiency of 1.9×10−4 for gold target operating in 25 keV. A novel design of the retarding field electron optics with 0.59 sr collection solid angle for scattered electrons was adopted based on Monte Carlo calculations for the spin-dependent electron scattering process and electron beam ray-tracing calculations. We have combined the new spin polarimeter with an angle-resolved photoelectron spectrometer and measured the spin- and angle-resolved photoelectron spectra and studied the spin-dependent electronic structure of Ni(110) along the ΓS line of its surface Brilluoin zone.

Low-Energy Electronic Structure of the Kondo Insulator YbB12

We have studied the low-energy electronic structure of a Kondo insulator YbB{sub 12} by high-resolution photoemission spectroscopy. A {open_quote}{open_quote}Kondo peak{close_quote}{close_quote} is observed {approximately}25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak. {copyright} {ital 1996 The American Physical Society.}

Surface structure and segregation of ordered Pt3Co(110) induced by oxygen

Abstract We have investigated the surface structure and segregation induced by oxygen adsorption on the ordered Pt3Co(110) alloy using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For the clean Pt3Co(110) surface, two different LEED patterns of 1×1 and 2×1 were observed depending on the annealing temperature, which had different Pt (237eV)/Co (LMM) AES intensity ratios. With increasing oxygen exposure at the substrate temperature of 300–500°C, the 2×1 LEED pattern of the clean Pt3Co(110) surface changed to c(2×4), 1×2, and 1×2+2×1+extra spots patterns, successively, and the Pt/Co AES intensity ratios showed a drastic decrease. Co 2p and O 1s core level spectra by XPS and normal emission valence band spectra by UPS indicated that a Co monoxide overlayer is formed due to a reaction with oxygen at a high-temperature substrate. From these results, it was found that the observed several surface structures are due to the reaction between the segregated Co and adsorbed O atoms to form the stoichiometric CoO-typed overlayer.

Interaction of C60 with Si(111)7×7 and Si(100)2×1 surfaces studied by STM, PES and HREELS: annealing effect

We report here measurements of the valence band spectra, the C 1s core level spectra and the vibrational excitation spectra of a C 60 monolayer (ML) film on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) in combination with scanning tunneling microscopy. 1 ML films are formed after annealing the 5 ML films at 670 K. The bonding states between C 60 molecules and Si substrates are clearly observed at 2.1 eV on both surfaces in the valence band spectra. The shift of binding energies of the molecular orbitals and the C Is core level spectra indicate the charge transfer. The softening of several vibrational modes is observed on the Si(111)7 × 7 and Si(100)2 × 1 surfaces with HREELS. The charge transfer scheme explains the softening well. We discuss the bonding nature in terms of hybridization of C 60 molecular orbitals with the surface states.

Cesium-induced Reconstruction on Si(113)3×2 Surface Studied by Low Energy Electron Diffraction and X-ray Photoelectron Spectroscopy

We have investigated Cs-induced reconstruction on the Si(113)3×2 surface using low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). For Cs deposition at room temperature, the (3×1) LEED pattern was observed for a wide Cs coverage range. At high substrate temperatures, the (3×1), (1×5+2×) and (2×2) phases were observed with increasing Cs deposition time. The relative Cs saturation coverages of (3×1)-Cs at RT and (2×2)-Cs at 300°C were measured from Cs 3d/Si 2p core level XPS intensity ratios. The results are summarized in a phase diagram as a function of the Cs deposition time.

Bonding nature of C60 adsorbed on Si(111)7×7 and Si(100)2×1 surfaces studied by HREELS and PES

We report here the measurements of vibrational excitation spectra, valence band spectra and C Is core level spectra of C 60 molecules adsorbed on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using high-resolution electron energy loss spectroscopy (HREELS) and photoelectron spectroscopy (PES). At a coverage between 1 and 0.25 ML, HREELS and PES measurements show that the interaction is as weak as the van der Waals interaction on both surfaces. At a coverage lower than 0.25 ML, some contradictory results are obtained by HREELS and PES. With HREELS, the softening of several vibrational modes is observed on the Si(111)7 × 7 surface, but no indication of softening is observed on the Si(100)2 × 1 surface. The charge transfer scheme explains the softening well. These results by HREELS indicate that the nature of the bonding is ionic on the Si( 111)7 x 7 surface and of the van der Waals type on the Si(100)2 × 1 surface. In contrast, the bonding states between C 60 molecules and Si substrates are clearly observed at 2.4 eV on both surfaces with PES. The PES measurements indicate that the interaction has a covalent character on the Si(111)7 × 7 surface, and that it is in between covalent and ionic, i.e. not purely ionic or at the covalent limit, on the Si(100)2 x 1 surface. We will discuss the nature of the interaction manifested by HREELS and PES measurements in comparison with the spectra of C 60 molecules adsorbed on metal surfaces.

CESIUM CORE LEVEL BINDING ENERGY SHIFTS AT THE O2/CS/SI(113) SURFACE

Abstract We have studied the cesium core level binding energy shifts at the O2/Cs/Si(113) surface at 150 K using X-ray photo-electron spectroscopy. It was observed that the cesium core levels, such as Cs 3d, 5s and 5p, of theCs/Si(113) surface were shifted towards lower binding energy upon oxidation. However, the Si 2p core level and the valence band of silicon were not shifted and the Cs MNN Auger peaks were drastically shifted in the opposite direction (lower kinetic energy) for the oxygen exposure, at which the negative binding energy shifts of the Cs3d5/2 core level were observed. It was found that the Auger parameter decreased by about 3.7 eV upon oxidation. The negative binding energy shifts of the cesium core levels are thus not due to the shift of the Fermi level position, sample charging effect, and the increase in final state screening. The negative binding energy shift of the Cs3d5/2 core level occurred with the drastic increase in the ionicity of cesium and with the concurrent negative binding energy shift of the O 1s core level at the same exposure. Therefore, the negative binding energy shifts of the Cs3d5/2 core level seem to be due to the Madelung potential by the increasing negative oxygen ions around the positive cesium ion.

Thermal-dependent electronic structure at the interface of C60-adsorbed Si(111)-(7×7) surface

We report here the temperature-dependent measurements of the valence spectra, and the C Is and the Si 2p core-level spectra of the C 60 adsorbed Si(111)-(7 × 7) surface, using photoelectron spectroscopy. At 300 K the valence spectra show the physisorption of most C 60 molecules for a one monolayer C 60 film. After annealing the sample at 1120 K the molecular orbitals disappear due to the breakdown of the C 60 cage. The C Is and Si 2p core-level spectra obtained at the same temperature indicate the progress of SiC formation at the interface between C 60 and the Si(111) surface. After annealing at 1170 K, the formation of SiC islands is confirmed by the binding energies of the peaks observed in the valence and C Is core-level spectra.

Spin-resolved core-level and valence-band photoemission spectroscopy of ferromagnetic MnAs

Abstract Spin-resolved core-level and valence-band photoemission spectroscopy of a MnAs single crystal has been done. The spin-polarization spectra of the Mn 3p and As 3d core levels have structures, indicating spin-polarized photoemission from these states. The minority-spin spectra of Mn 3p and As 3d have stronger intensities at lower binding energy. The Mn 3p majority-spin spectrum has a broader width than the Mn 3p minority-spin spectrum. The spin polarization of valence-band photoemission spectra is enhanced in the Mn 3p–3d photoabsorption region, indicating spin-dependent interference effects.

Electronic structure of Si(113)2 × 2-Cs surface studied by ARUPS

Abstract The electronic structure of aSi(113)2 × 2-Cs surface has been investigated by low energy electron diffraction (LEED) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). The surface-band dispersions were measured along the symmetry axes, Γ-U[33¯2] and Γ-W[1¯10], in the surface Brillouin zone. Three surface states were observed at the binding energy of 0.7 (A 1 ), 0.95 (A 2 ), and 1.65 eV (A 3 ) below the Fermi level at Γ with overall dispersions of about 0.15, 0.5, and 0.2 eV, respectively. The A 2 and A 3 states revealed a2 × 2 periodicity along the [33¯2] direction. Considering the alkali-metal adsorptions on low-index Si surfaces and measured Cs 3d/Si 2p XPS peak intensity ratios, the Cs-saturation coverage of the2 × 2-Cs surface was assigned to about6/8 monolayer. Based on the Cs-saturated adatom + tetramer model, the A 1 and A 2 states are identified as Cs-induced adatom and tetramer states, respectively. The A 3 state has a surface resonance-like character.