Kazuo Soda

Performance of a very high resolution soft x-ray beamline BL25SU with a twin-helical undulator at SPring-8

We report on the excellent performance of a newly constructed soft x-ray helical undulator beamline BL25SU of SPring-8 for photon energies 500–1800 eV. The full beamline was designed to perform very high resolution soft x-ray spectroscopy of solids with using high brilliance, highly circularly polarized undulator radiation. The grazing incidence monochromator employs varied-line-spacing plane gratings which operate in convergent light from a spherical mirror and focuses monochromatic light onto the exit slit. A resolving power in excess of 15 000 was measured at 540 and 870 eV for a grating with a central groove density of 600 lines/mm from the photoemission spectra of Au. A resolving power of more than 20 000 is estimated near 870 eV for a grating with a central groove density of 1000 lines/mm. A photon flux of more than 1×1011 photons/s/100 mA/0.02% b.w. is supplied onto the sample between 500 and 1800 eV with very low amount of higher-order light. The low heat load from the twin-helical undulator gives...

Twin helical undulator beamline for soft X-ray spectroscopy at SPring-8

A very high resolution soft X-ray beamline, BL25SU, has been designed and is under construction at SPring-8. Completely right or left circularly polarized light is supplied on a common axis of a newly designed twin helical undulator. A helicity modulation up to 10 Hz can be performed using five kicker magnets. The fundamental radiation covers the region 0.5-3 keV. Higher-order radiation is rather weak on the axis. A monochromator with varied-line-spacing plane gratings is installed to cover the region below 1.5 keV. A very high resolution beyond 10(4) is expected for the whole energy region.

La-doped EuO: A rare earth ferromagnetic semiconductor with the highest Curie temperature

We report the fabrication of single-crystalline La-doped EuO thin films with a Curie temperature (TC) of about 200 K, the highest among rare-earth compounds without transition metals. From first-principle band calculation and x-ray diffraction measurement, the observed increase in TC cannot be explained only by the increase in hybridization intensity due to lattice contraction and the increase in up-spin electrons of the Eu 5d state caused by the electron doping. Hybridization between the Eu 4f and donor states and/or Ruderman–Kittel–Kasuya–Yoshida interaction mediated by the doped La 5d state is a possible origin of the increase in TC.

F-Center Formation at Highly Excited Triplet States of Self-Trapped Excitons in KCl

Measurements have been made of the creation of the F centers by the optical excitation of the self-trapped excitons with a light pulse from a tunable dye laser. It is found that the ratio of the F -center concentration created by the light pulse to the triplet exciton concentration at the incidence of the light pulse depends on the integrated photon flux \(\varPhi\) as \(a\{1-\exp(-b\varPhi)\}\), where a and b are constants. Kinetic models are developed for treating the non-radiative processes of the highly excited self-trapped exciton, and the efficiency of the F -center creation from the highly excited triplet exciton was determined using the experimentally obtained parameters a and b . It is found that the efficiency is almost constant between 1.77 and 2.08 eV above the lowest triplet exciton and becomes higher above 2.58 eV. The dependence of the efficiency on energy is discussed in terms of existing models of the defect formation.

Discovery of the Last Remaining Binary Platinum‐Group Pernitride RuN2

The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.

Frenkel Defect Formation and Intersystem Crossing at Highly-Excited States of the Triplet Self-Trapped Exciton in NaCl

Measurements were made of the absorption change at the F band and of the singlet luminescence in NaCl, induced by photoexcitation of the lowest state of the triplet self-trapped excitons into excited states higher than the lowest state by 1.7 eV ∼2.8 eV. It is found that excitation with photons below 2.5 eV causes both the F -center creation and the σ-luminescence, while excitation with photons above 2.5 eV causes only the σ-luminescence. These results indicate that non-radiative transition from the higher excited states to the lowest state of the self-trapped exciton is selective. The cause of the selectiveness is discussed. The yield of the F center creation at the higher excited state of the self-trapped exciton was found to be about 10 -2 . The stability of the created F centers in NaCl is found to be lower than in other alkali chlorides.

SAMRAI: A novel variably polarized angle-resolved photoemission beamline in the VUV region at UVSOR-II

A novel variably polarized angle-resolved photoemission spectroscopy beamline in the vacuum-ultraviolet (VUV) region has been installed at the UVSOR-II 750 MeV synchrotron light source. The beamline is equipped with a 3 m long APPLE-II type undulator with horizontally/vertically linear and right/left circular polarizations, a 10 m Wadsworth type monochromator covering a photon energy range of 6-43 eV, and a 200 mm radius hemispherical photoelectron analyzer with an electron lens of a +/-18 degrees acceptance angle. Due to the low emittance of the UVSOR-II storage ring, the light source is regarded as an entrance slit, and the undulator light is directly led to a grating by two plane mirrors in the monochromator while maintaining a balance between high-energy resolution and high photon flux. The energy resolving power (hnu/Deltahnu) and photon flux of the monochromator are typically 1 x 10(4) and 10(12) photons/s, respectively, with a 100 microm exit slit. The beamline is used for angle-resolved photoemission spectroscopy with an energy resolution of a few meV covering the UV-to-VUV energy range.

Soft X-ray beamline for spectroscopy of solids at SPring-8

The performance of the first soft X-ray beamline BL25SU at SPring-8 is described. A grating monochromator is found to realize very good energy resolution. This is demonstrated by the experimental photoabsorption spectra of the Ar 2p(∼250 eV) and N2 1 s (∼400 eV) edges, which show the quality competitive to or better than the best results reported to date. In addition, the excellent performance is augmented by the high photon flux of about 1011 photons/s100 mA−10.02%−1 b.w., low degree of higher order light (less than 1%) and high circular polarization at the experimental stations.

Orientation of the Optical Dipole Moments of the Lowest-to-Higher-States Transitions of Self-Trapped Excitons in Alkali Chlorides

Measurements have been made of the dichroic changes, induced by polarized laser-light irradiation, of the specimens populated with the self-trapped excitons in the π-luminescence and in the optical absorption due to the lowest self-trapped excitons in NaCl, KCl and RbCl. The degree of polarization was determined as a function of the photon energy of the laser light and was analyzed by taking it into account that the electron transitions from the lowest a 1 g orbital to the b 1 u , b 2 u and b 3 u orbitals overlap each other. The peak positions and the half widths of each transition were determined, and it is shown that the electron orbitals of the self-trapped excitons are in the order of b 2 u , b 3 u , b 1 u and b 3 u * from lower energy. It is also pointed out that the width of the band due to the transition to the b 3 u orbital is nearly the same or even larger than that to the b 1 u orbital. These results are compared with the optical absorption of the F 2 center. The implication of the results on th...

Thermoelectric properties of Heusler-type off-stoichiometric Fe2V1+xAl1−x alloys

We report the thermoelectric properties of Heusler-type off-stoichiometric Fe2V1+xAl1�x alloys. Due to the off-stoichiometric effect, which is the substitution of V/Al atoms with Al/V atoms, semiconductor-like electric resistivity behavior in Fe2VAl is changed to metallic behavior in Fe2V1+xAl1�x alloys and both positive and negative absolute Seebeck coefficients are drastically increased. The maximum thermoelectric power factor of Fe2V1+xAl1�x alloys is 4.3 × 10 �3 (x = �0.03: p-type) and 6.8 × 10 �3 WmK �2 (x = 0.05: n-type) with a peak temperature in the range 300–600 K, exceeding the values of previously reported Fe-based Heusler alloys as well as those of available thermoelectric materials such as Bi-Te semiconductors. Based on x-ray diffraction and photoemission spectroscopy results, it is thought that the maintenance of the Heusler-type (L21) crystal structure and the modification of the electronic structure due to the offstoichiometry could explain the large thermoelectric power factor and high peak temperature in Fe2V1+xAl1�x alloys.