M. Oura

Performance of a Highly Stabilized and High-resolution Beamline BL17SU for Advanced Soft X-ray Spectroscopy at SPring-8

A new soft x‐ray beamline BL17SU (RIKEN) has been constructed at SPring‐8. The beamline consists of two branches with each varied‐line‐spacing‐plane‐grating‐monochromator. Both monochromators perform high energy resolution (E/ΔE > 10,000) between 0.2 and 1 keV. One of the monochromator achieves high energy stability of 10 meV over a period of half a day.

Recoil Effect of Photoelectrons in the Fermi Edge of Simple Metals

High energy resolution photoelectron spectroscopy of conduction electrons in the vicinity of the Fermi edge in Al and Au at excitation energies of 880 and 7940 eV was carried out using synchrotron radiation. For the excitation energy of 7940 eV, the observed Fermi energy of Al shows a remarkable shift to higher binding energy as compared with that of Au, with accompanying broadening. This is due to the recoil effect of the emitted photoelectrons. The observed spectra are well reproduced by a simple model of Bloch electrons based on the isotropic Debye model.

Construction and performance of a figure-8 undulator

The figure-8 undulator is a novel insertion device adopted as a light source at the soft x-ray photochemistry beamline of the Super Photon ring-8GeV. It was designed to produce linearly polarized soft x rays between 100 and 5000 eV with low on-axis power density. Magnetic fields of the constructed device were measured with compensation for the planar Hall effect using two different methods. After magnetic-field corrections, it was found that the first-harmonic intensity and the on-axis power density calculated using the measured fields were almost the same as ideal ones. After installation in the storage ring, a power-distribution pattern was observed by a fluorescent screen and it was found that the shape corresponded to expectation. The quantity of fluctuation of the photon-beam axis was estimated by measuring spectra at various observation angles and it was found to be small compared to the photon-beam size. In addition, it was also found that the measured spectrum at the gap of 77 mm agreed well with ...

Evidence for a correlated insulator to antiferromagnetic metal transition in CrN.

We investigate the electronic structure of chromium nitride (CrN) across the first-order magnetostructural transition at T(N)∼286  K. Resonant photoemission spectroscopy (PES) shows a gap in the 3d partial density of states at the Fermi level and an on-site Coulomb energy U∼4.5  eV, indicating strong electron-electron correlations. Bulk-sensitive high-resolution (6 meV) laser PES reveals a clear Fermi edge indicating an antiferromagnetic metal below T(N). Hard x-ray Cr 2p core-level PES shows T-dependent changes across T(N) which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. Electrical resistivity confirms an insulator above T(N) (E(g)∼70  meV) becoming a disordered metal below T(N). Thus, CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magnetostructural transition.

Time-resolved HAXPES at SACLA: probe and pump pulse-induced space-charge effects

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends time-resolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulse-induced vacuum space-charge effects on the V 1s emission of VO2 and the Ti 1s emission of SrTiO3. The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.

Degree of circular polarization of soft X-rays emitted from a multi-polarization-mode undulator characterized by means of magnetic circular dichroism measurements.

The degree of circular polarization of soft X-rays emitted from the multi-polarization-mode undulator of BL17SU at SPring-8 has been deduced by means of magnetic circular dichroism in core-level X-ray absorption spectroscopy for a ferrimagnetic Gd-Fe-Co amorphous thin film. The results of reference measurements performed using well characterized undulator radiation of BL25SU at SPring-8 have also been utilized. The degrees of circular polarization were presumed for all the available operational modes and were compared with simple theoretical calculations. It was found that the calculated degrees of circular polarization were validated by the measurements reasonably well and will be useful in further experiments at BL17SU.

Correlative multielectron processes in K-shell photoionization of Ca, Ti and V in the energy range of 8-35 keV

The Kalpha satellite spectra arising from the correlative multielectron processes accompanying K-shell photoionization of Ca, Ti and V were measured using a broad range crystal spectrometer. Multiconfiguration Dirac-Fock calculations were performed to interpret the observed,x-ray energies and the spectral line shape., The calculations agree fairly well with the experimental results. The variation of the KalphaL(1) satellite and the Kalpha(1,2)(h) hypersatellite intensities was measured as a function, of excitation energy in the range of 8-35 keV. The KalphaL(1)/Kalpha(1,2) intensity ratio for each,element is already saturated in our energy range, and the asymptotic values of 1.58 +/- 0.08, 1.26 +/- 0.06 and 0.97 +/- 0.05% for Ca, Ti. and V were determined. By combining the present results and the previously measured values for various elements, we have found a Z(-3.5) scaling law for the KalphaL(1)/Kalpha(1,2) intensity ratio, The measured Kalpha(1,2)(h)/Kalpha(1,2) for each element, is found to increase smoothly from its onset and shows a long saturation range extending up to at least 25 keV above the threshold for Ti. The evolution of the Kalpha(1,2)(h)/Kalpha(1,2) intensity ratio is compared with the analytic Thomas model and with the theoretical calculation based on the screened hydrogenic model. (Less)

Characterization of spectroscopic photoemission and low energy electron microscope using multipolarized soft x rays at BL17SU/SPring-8

Spectroscopic photoemission and low energy electron microscope (SPELEEM) improved its performance after installation at BL17SU/SPring-8, where a multipolarization-mode undulator is employed to produce circularly and linearly polarized soft x rays. This undulator enables us to study the domain structures of ferromagnetic and antiferromagnetic materials by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism. SPELEEM is used to study light elements (C, N, and O), 3d transition-metal elements and 4f rare earth elements, utilizing a wide range of photon energies. The two cylindrical mirrors adopted in front of SPELEEM ensure an illumination area of 14 x 14 microm(2) on the samples. The lateral resolution of a secondary electron photoemission electron microscope image is estimated to be better than 85 nm, whereas the energy resolution of the instrument is better than 0.4 eV.

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films.

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ∼1 and ∼10  nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)↔Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.

Front-end XY-slits assembly for the SPring-8 undulator beamlines

A front-end XY-slits assembly has been designed for the SPring-8 undulator beamlines. This assembly can handle the high heat flux from the undulator, its grazing-incidence L-shaped configuration employing an enhanced heat-transfer technology.