Multilateral surface analysis of the CeB6 electron-gun cathode used at SACLA XFEL.

The CeB6(001) single crystal used as a cathode in a low-emittance electron gun and operated at the free-electron laser facility SACLA was investigated using cathode lens electron microscopy combined with X-ray spectroscopy at SPring-8 synchrotron radiation facility. Multilateral analysis using thermionic emission electron microscopy, low-energy electron microscopy, ultraviolet and X-ray photoemission electron microscopy and hard X-ray photoemission spectroscopy revealed that the thermionic electrons are emitted strongly and evenly from the CeB6 surface after pre-activation treatment (annealing at 1500°C for >1 h) and that the thermionic emission intensity as well as elemental composition vary between the central area and the edge of the old CeB6 surface.

Soft X-ray ARPES for three-dimensional crystals in the micrometre region.

An endstation dedicated to angle-resolved photoemission spectroscopy (ARPES) using a soft X-ray microbeam has been developed at the beamline BL25SU of SPring-8. To obtain a high photoemission intensity, this endstation is optimized for measurements under the condition of grazing beam incidence to a sample surface, where the glancing angle is 5° or smaller. A Wolter mirror is used for focusing the soft X-rays. Even at the glancing angle of 5°, the smallest beam spot still having a sufficient photon flux for ARPES is almost round on the sample surface and the FWHM diameter is ∼5 µm. There is no need to change the sample orientation for performing kx - ky mapping by virtue of the electron lens with a deflector of the photoelectron analyzer, which makes it possible to keep the irradiation area unchanged. A partially cleaved surface area as small as ∼20 µm was made on an Si(111) wafer and ARPES measurements were performed. The results are presented.

X-ray Photoemission Spectroscopy Study of Uniaxial Magnetic Anisotropy Induced in a Ni Layer Deposited on a LiNbO3 Substrate.

The competition between magnetic shape anisotropy and the induced uniaxial magnetic anisotropy in the heterojunction between a ferromagnetic layer and a ferroelectric substrate serves to control magnetic domain structures as well as magnetization reversal characteristics. The uniaxial magnetic anisotropy, originating from the symmetry breaking effect in the heterojunction, plays a significant role in modifying the characteristics of magnetization dynamics. Magnetoelastic phenomena are known to generate uniaxial magnetic anisotropy; however, the interfacial electronic states that may contribute to the uniaxial magnetic anisotropy have not yet been adequately investigated. Here, we report experimental evidence concerning the binding energy change in the ferromagnetic layer/ferroelectric substrate heterojunction using X-ray photoemission spectroscopy. The binding energy shifts, corresponding to the chemical shifts, reveal the binding states near the interface. Our results shed light on the origin of the uniaxial magnetic anisotropy induced from the heterojunction. This knowledge can provide a means for the simultaneous control of magnetism, mechanics, and electronics in a nano/microsystem consisting of ferromagnetic/ferroelectric materials.

High-Quality Few-Layer Graphene on Single-Crystalline SiC thin Film Grown on Affordable Wafer for Device Applications.

Graphene is promising for next-generation devices. However, one of the primary challenges in realizing these devices is the scalable growth of high-quality few-layer graphene (FLG) on device-type wafers; it is difficult to do so while balancing both quality and affordability. High-quality graphene is grown on expensive SiC bulk crystals, while graphene on SiC thin films grown on Si substrates (GOS) exhibits low quality but affordable cost. We propose a new method for the growth of high-quality FLG on a new template named "hybrid SiC". The hybrid SiC is produced by bonding a SiC bulk crystal with an affordable device-type wafer and subsequently peeling off the SiC bulk crystal to obtain a single-crystalline SiC thin film on the wafer. The quality of FLG on this hybrid SiC is comparable to that of FLG on SiC bulk crystals and much higher than of GOS. FLG on the hybrid SiC exhibited high carrier mobilities, comparable to those on SiC bulk crystals, as anticipated from the linear band dispersions. Transistors using FLG on the hybrid SiC showed the potential to operate in terahertz frequencies. The proposed method is suited for growing high-quality FLG on desired substrates with the aim of realizing graphene-based high-speed devices.

Visualization of elemental distributions and local analysis of element-specific chemical states of an Arachnoidiscus sp. frustule using soft X-ray spectromicroscopy.

Using soft X-ray (SX) spectromicroscopy, we show maps of the spatial distribution of constituent elements and local analysis of the density of states (DOS) related to the element-specific chemical states of diatom frustules, which are composed of naturally grown nanostructured hydrogenated amorphous silica. We applied X-ray photoemission electron microscopy (X-PEEM) as well as microprobe X-ray fluorescence (µXRF) analysis to characterize the surfaces of diatom frustules by means of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). We successfully demonstrated that SX spectromicroscopy is able to participate in potential observation tools as a new method to spectroscopically investigate diatom frustules.

Spin torque control of antiferromagnetic moments in NiO.

For a long time, there were no efficient ways of controlling antiferromagnets. Quite a strong magnetic field was required to manipulate the magnetic moments because of a high molecular field and a small magnetic susceptibility. It was also difficult to detect the orientation of the magnetic moments since the net magnetic moment is effectively zero. For these reasons, research on antiferromagnets has not been progressed as drastically as that on ferromagnets which are the main materials in modern spintronic devices. Here we show that the magnetic moments in NiO, a typical natural antiferromagnet, can indeed be controlled by the spin torque with a relatively small electric current density (~4 × 107 A/cm2) and their orientation is detected by the transverse resistance resulting from the spin Hall magnetoresistance. The demonstrated techniques of controlling and detecting antiferromagnets would outstandingly promote the methodologies in the recently emerged "antiferromagnetic spintronics". Furthermore, our results essentially lead to a spin torque antiferromagnetic memory.

Development of optical choppers for time-resolved measurements at soft X-ray synchrotron radiation beamlines.

Two types of optical choppers for time-resolved measurements at synchrotron radiation soft X-ray beamlines have been developed. One type uses an air-spindle-type rotation mechanism with a two-stage differential pumping system to maintain the ultra-high vacuum of the X-ray beamline, and the other uses a magnetic bearing. Both can be installed at the soft X-ray beamlines at SPring-8, greatly improving the accessibility of pump-and-probe spectroscopy. The combination of X-ray chopper and pump-and-probe photoemission electron microscope at SPring-8 provides drastic improvements in signal-to-noise ratio and resolution compared with techniques using high-voltage gating of channel plate detectors. The choppers have the capability to be used not only at synchrotron radiation facilities but also at other types of soft X-ray and VUV beamlines.

Wide-angle display-type retarding field analyzer with high energy and angular resolutions.

Deployments of spherical grids to obtain high energy and angular resolutions for retarding field analyzers (RFAs) having acceptance angles as large as or larger than ±45° were explored under the condition of using commercially available microchannel plates with effective diameters of approximately 100 mm. As a result of electron trajectory simulations, a deployment of three spherical grids with significantly different grid separations instead of conventional equidistant separations showed an energy resolving power (E/ΔE) of 3200 and an angular resolution of 0.6°. The mesh number of the wire mesh retarding grid used for the simulation was 250. An RFA constructed with the simulated design experimentally showed an E/ΔE of 1100 and an angular resolution of 1°. Using the RFA and synchrotron radiation of 900 eV, photoelectron diffraction (PED) measurements were performed for single-crystal graphite. A clear C 1s PED pattern was observed even when the differential energy of the RFA was set at 0.5 eV. Further improvement of the energy resolution was theoretically examined under the assumption of utilizing a retarding grid fabricated by making a large number of radially directed cylindrical holes through a partial spherical shell instead of using a wire mesh retarding grid. An E/ΔE of 14 500 was predicted for a hole design with a diameter of 60 µm and a depth of 100 µm. A retarding grid with this hole design and a holed area corresponding to an acceptance angle of ±7° was fabricated. An RFA constructed with this retarding grid experimentally showed an E/ΔE of 1800. Possible reasons for the experimental E/ΔE lower than the theoretical values are discussed.

Investigation of continuous changes in the electric-field-induced electronic state in Bi(1-x)Ca(x)FeO(3-δ).

Amongst the most interesting phenomena in correlated oxide systems are the doping-driven competitions between energetically similar ground states found in, e.g., high-Tc superconductors and colossal magnetoresistance manganites. It has recently been reported that doped multiferroics also exhibit this generic concept of phase competition. Here, we employ photoelectron emission microscopy (PEEM) to demonstrate evidence of systematic changes in the electronic structure of Bi(1-x)Ca(x)FeO(3-δ) treated by electrically controlled hole carrier doping, the outcome of which clearly correlates with the local modulation of electronic conductivity observed in the same material.

Low-temperature catalyst activator: mechanism of dense carbon nanotube forest growth studied using synchrotron radiation.

The mechanism of the one-order-of-magnitude increase in the density of vertically aligned carbon nanotubes (CNTs) achieved by a recently developed thermal chemical vapor deposition process was studied using synchrotron radiation spectroscopic techniques. In the developed process, a Ti film is used as the underlayer for an Fe catalyst film. A characteristic point of this process is that C2H2 feeding for the catalyst starts at a low temperature of 450°C, whereas conventional feeding temperatures are ∼800°C. Photoemission spectroscopy using soft and hard X-rays revealed that the Ti underlayer reduced the initially oxidized Fe layer at 450°C. A photoemission intensity analysis also suggested that the oxidized Ti layer at 450°C behaved as a support for nanoparticle formation of the reduced Fe, which is required for dense CNT growth. In fact, a CNT growth experiment, where the catalyst chemical state was monitored in situ by X-ray absorption spectroscopy, showed that the reduced Fe yielded a CNT forest at 450°C. Contrarily, an Fe layer without the Ti underlayer did not yield such a CNT forest at 450°C. Photoemission electron microscopy showed that catalyst annealing at the conventional feeding temperature of 800°C caused excess catalyst agglomeration, which should lead to sparse CNTs. In conclusion, in the developed growth process, the low-temperature catalyst activation by the Ti underlayer before the excess Fe agglomeration realised the CNT densification.

Microscopically-tuned band structure of epitaxial graphene through interface and stacking variations using Si substrate microfabrication.

Graphene exhibits unusual electronic properties, caused by a linear band structure near the Dirac point. This band structure is determined by the stacking sequence in graphene multilayers. Here we present a novel method of microscopically controlling the band structure. This is achieved by epitaxy of graphene on 3C-SiC(111) and 3C-SiC(100) thin films grown on a 3D microfabricated Si(100) substrate (3D-GOS (graphene on silicon)) by anisotropic etching, which produces Si(111) microfacets as well as major Si(100) microterraces. We show that tuning of the interface between the graphene and the 3C-SiC microfacets enables microscopic control of stacking and ultimately of the band structure of 3D-GOS, which is typified by the selective emergence of semiconducting and metallic behaviours on the (111) and (100) portions, respectively. The use of 3D-GOS is thus effective in microscopically unlocking various potentials of graphene depending on the application target, such as electronic or photonic devices.

Orbital-specific tunability of many-body effects in bilayer graphene by gate bias and metal contact.

Graphene, a 2D crystal bonded by π and σ orbitals, possesses excellent electronic properties that are promising for next-generation optoelectronic device applications. For these a precise understanding of quasiparticle behaviour near the Dirac point (DP) is indispensable because the vanishing density of states (DOS) near the DP enhances many-body effects, such as excitonic effects and the Anderson orthogonality catastrophe (AOC) which occur through the interactions of many conduction electrons with holes. These effects renormalize band dispersion and DOS, and therefore affect device performance. For this reason, we have studied the impact of the excitonic effects and the AOC on graphene device performance by using X-ray absorption spectromicroscopy on an actual graphene transistor in operation. Our work shows that the excitonic effect and the AOC are tunable by gate bias or metal contacts, both of which alter the Fermi energy, and are orbital-specific.

Capability of insulator study by photoemission electron microscopy at SPring-8.

The observation method of photoemission electron microscopy (PEEM) on insulating samples has been established in an extremely simple way. Surface conductivity is induced locally on an insulating surface by continuous radiation of soft X-rays, and Au films close to the area of interest allow the accumulated charges on the insulated area to be released to ground level. Magnetic domain observations of a NiZn ferrite, local X-ray absorption spectroscopy of sapphire, high-resolution imaging of a poorly conducting Li0.9CoO2 film surface, and Au pattern evaporation on a fine rock particle are demonstrated. Using this technique, all users' experiments on poorly conducting samples have been performed successfully at the PEEM experimental station of SPring-8.