Effect of Anisotropic Hybridization in YbAlB_{4} Probed by Linear Dichroism in Core-Level Hard X-Ray Photoemission Spectroscopy.

We have probed the crystalline electric-field ground states of pure |J=7/2,J_{z}=±5/2⟩ as well as the anisotropic c-f hybridization in both valence fluctuating systems α- and ß-YbAlB_{4} by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between α- and ß-YbAlB_{4} was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of c-f hybridization, which may be a key to understanding a heavy Fermi liquid state in α-YbAlB_{4} and a quantum critical state in ß-YbAlB_{4}.

Electronic and Thermoelectric Properties of Layered Oxychalcogenides (BiO)Cu Ch ( Ch = S, Se, Te).

We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)Cu Ch ( Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperature-dependent electrical resistivity ρ( T). PE spectra are explained by calculating the electronic properties using the generalized-gradient approximation method. PE spectra and ρ( T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)Cu Ch ( Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.

Polarized hard X-ray photoemission system with micro-positioning technique for probing ground-state symmetry of strongly correlated materials.

An angle-resolved linearly polarized hard X-ray photoemission spectroscopy (HAXPES) system has been developed to study the ground-state symmetry of strongly correlated materials. The linear polarization of the incoming X-ray beam is switched by a transmission-type phase retarder composed of two diamond (100) crystals. The best value of the degree of linear polarization was found to be -0.96, containing a vertical polarization component of 98%. A newly developed low-temperature two-axis manipulator enables easy polar and azimuthal rotations to select the detection direction of photoelectrons. The lowest temperature achieved was 9 K, offering the chance to access the ground state even for strongly correlated electron systems in cubic symmetry. A co-axial sample monitoring system with long-working-distance microscope enables the same region on the sample surface to be measured before and after rotation. Combining this sample monitoring system with a micro-focused X-ray beam by means of an ellipsoidal Kirkpatrick-Baez mirror (25 µm × 25 µm FWHM), polarized valence-band HAXPES has been performed on NiO for voltage application as resistive random access memory to demonstrate the micro-positioning technique and polarization switching.

Response-time improved hydrothermal-method-grown ZnO scintillator for soft x-ray free-electron laser timing-observation.

For pump and probe experiments in x-ray free-electron laser (XFEL) facilities, accurate timing synchronization between short-wavelength femtosecond pulses from XFELs and short optical pulses from other light sources is required. For this purpose, the response time of a hydrothermal-method-grown ZnO is improved by over one order of magnitude via intentional iron ion doping. The fluorescence rise- and decay-time constants are measured to be less than 10 and 100 ps, respectively. Owing to its intense fluorescence even for single pulse XFEL excitation, the timing jitter of the soft x-ray pulse and timing electronics are evaluated to be less than 70 ps.

Ultra-fast switching of light by absorption saturation in vacuum ultra-violet region.

Advances in free electron lasers producing high energy photons [Nat. Photonics 2(9), 555-559 (2008)] are expected to open up a new science of nonlinear optics of high energy photons. Specifically, lasers of photon energy higher than the plasma frequency of a metal can show new interaction features because they can penetrate deeply into metals without strong reflection. Here we show the observation of ultra-fast switching of vacuum ultra-violet (VUV) light caused by saturable absorption of a solid metal target. A strong gating is observed at energy fluences above 6J/cm2 at wavelength of 51 nm with tin metal thin layers. The ratio of the transmission at high intensity to low intensity is typically greater than 100:1. This means we can design new nonlinear photonic devices such as auto-correlator and pulse slicer for the VUV region.