Akiko Fukushima

Development of soft x-ray time-resolved photoemission spectroscopy system with a two-dimensional angle-resolved time-of-flight analyzer at SPring-8 BL07LSU

We have developed a soft x-ray time-resolved photoemission spectroscopy system using synchrotron radiation (SR) at SPring-8 BL07LSU and an ultrashort pulse laser system. Two-dimensional angle-resolved measurements were performed with a time-of-flight-type analyzer. The photoemission spectroscopy system is synchronized to light pulses of SR and laser using a time control unit. The performance of the instrument is demonstrated by mapping the band structure of a Si(111) crystal over the surface Brillouin zones and observing relaxation of the surface photo-voltage effect using the pump (laser) and probe (SR) method.

High resolution soft X-ray absorption spectroscopy for the chemical state analysis of Mn

Abstract Mn L 2,3 X-ray absorption spectra of various manganese compounds were measured using a synchrotron radiation facility. Very high-resolution spectra were obtained. Sharp lines, which were not found before, were observed, and reproducibility was checked using a different beam line in the synchrotron radiation facility. Difference in spectral line shape was observed as the difference of quantum yield method. An unknown sample was measured and the specimen was concluded to be a mixture of two different oxidation states of Mn. The effects on the change of spectral profiles are discussed.

Anisotropy and dissipation in the mixed state of high temperature superconductors

Abstract Dissapative behavior of the flux line system in the mixed state of high temperature superconductors is studied. Difference in the magnitude of anisotropy between YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 CaCu 2 O 8+ y is manifest in the angular dependence of dissipation. An anomalous enhancement of disspation at high current densities is found for a narrow range of field angle near to the basal plane direction. Dissipation processed in the parallel and nearly parallel field configurations are discussed in the light of flux dynamics under the influence of strong pinning potential intrinsic to the layered structure.

Is pressure dependence of Tc through change in carrier density

Abstract The pressure dependence of the Hall coefficient is studied for YBa 2 Cu 4 O 8 , YBa 2 Cu 3 O 7 , and Bi 2 Sr 2 CaCu 2 O 8 . The Hall coefficient decreases by ∼8–12%/GPa for these cuprates. The anomalous temperature dependence of the Hall coefficient is found to be intimately related with the occurrence of superconductivity.

High-resolution three-dimensional spin- and angle-resolved photoelectron spectrometer using vacuum ultraviolet laser light

We describe a spin- and angle-resolved photoelectron spectroscopy (SARPES) apparatus with a vacuum-ultraviolet (VUV) laser (hν = 6.994 eV) developed at the Laser and Synchrotron Research Center at the Institute for Solid State Physics, The University of Tokyo. The spectrometer consists of a hemispherical photoelectron analyzer equipped with an electron deflector function and twin very-low-energy-electron-diffraction-type spin detectors, which allows us to analyze the spin vector of a photoelectron three-dimensionally with both high energy and angular resolutions. The combination of the high-performance spectrometer and the high-photon-flux VUV laser can achieve an energy resolution of 1.7 meV for SARPES. We demonstrate that the present laser-SARPES machine realizes a quick SARPES on the spin-split band structure of a Bi(111) film even with 7 meV energy and 0.7(∘) angular resolutions along the entrance-slit direction. This laser-SARPES machine is applicable to the investigation of spin-dependent electronic states on an energy scale of a few meV.

Photocarrier-injected electronic structure of VO2∕TiO2:Nb

We study the effect of photocarrier injection on the electronic structure of VO2∕TiO2:Nb thin films using photoemission spectroscopy. The results indicate that the valence band and core electronic states of VO2 shift systematically to lower binding energy upon photocarrier injection, consistent with doping in a rigid-band picture. The shift in binding energy, and its saturation, follows the known photovoltage behavior. In addition, the V 3d states near EF exhibit a redistribution/transfer of spectral weight, similar to the temperature dependent insulator to metal transition in VO2. The study provides evidence for hole-doping induced electronic structure changes due to the photocarriers in VO2.

Band Structure of Sr0.5Ba0.5Nb2O6 Thin Film Probed by Soft-X-Ray Emission Spectroscopy

The electronic structure of ferroelectric Sr0.5Ba0.5Nb2O6 thin film has been studied by soft-X-ray emission spectroscopy (SXES). In the valence band region, O 2p and Nb 4d partial densities of states are observed in the O 1s and Nb 4p SXES spectra. The energy position of the Nb 4d state overlaps with that of the O 2p state. This finding indicates that the O 2p state strongly hybridizes with the Nb 4d state in the valence band.

Band structure of TiO2-Doped yttria-stabilized zirconia probed by soft-X-ray spectroscopy

The electronic structure of TiO2-doped yttria-stabilized zirconia (YSZ) has been studied by soft-X-ray emission spectroscopy (SXES) and X-ray absorption spectroscopy (XAS). The valence band is mainly composed of the O 2p state. The O 1s XAS spectrum exhibits the existence of the Ti 3d unoccupied state under the Zr 4d conduction band. The intensity of the Ti 3d unoccupied state increases with increasing TiO2 concentration. The energy separation between the top of the valence band and the bottom of the Ti 3d unoccupied state is in accord with the energy gap, as expected from dc-polarization and total conductivity measurements.

Microstructured thin films and multilayers of superconductor and ferromagnetic metal

Abstract We have investigated superconducting properties of thin films and multilayers consisting of superconductor/ferromagnet composite systems. Niobium thin films deposited on porous alumina exhibit a structure in the resistive transition curve near T c at a field that is comparable to the matching field. A series of niobium/iron multilayers with different iron layer thicknesses show superconducting properties characteristic of decoupled layers.

Electronic Structure of Ce1-xNdxO2-δ Probed by Soft-X-Ray Spectroscopy

The electronic structure of Nd3+-doped CeO2 (Ce1-xNdxO2) was studied by X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES). The XAS spectra show that the intensity of the conduction band, which is mainly composed of the O 2p state hybridized with the unoccupied Ce 4f state, decreases with increasing Nd3+ concentration. The RPES spectra show that the Ce 4f partial density of states in the valence band decreases with increasing Nd3+ concentration. These findings indicate that the hybridization effect between the Ce 4f and O 2p states decreases with increasing Nd3+ concentration.