Junpei Azuma

Surface photovoltage effect and its time dependence in GaAs–GaAsP superlattice studied with combination of synchrotron and laser radiation

The surface photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL) were measured by core-level photoelectron spectroscopy with the combination of synchrotron radiation and laser. It was found that the SPV effect in the SL is remarkably suppressed as compared with that in a bulk GaAs. The difference in the temporal profile of the SPV between SL and bulk samples was observed in microsecond range. The suppression of the SPV effect in the negative electron affinity surfaces of the SL was also observed. It is concluded that the SL with a high-doping surface layer is suitable for the spin-polarized electron source without the SPV effect.

Probing shallow electron traps in cerium-doped Gd3Al2Ga3O12 scintillators by UV-induced absorption spectroscopy

From measuring absorption spectra of cerium-doped Gd3Al2Ga3O12 (Ce:GAGG) and undoped GAGG crystals at low temperatures under UV-light irradiation, we find that they exhibit a broad band at around 12000 cm−1. This band is enhanced by high-temperature annealing under a hydrogen atmosphere. On the basis of present experimental results, the UV-induced band is assigned to shallow electron traps of defect complexes associated with oxygen vacancies. The UV-induced band completely disappears with Mg2+ codoping. We conclude that the Mg2+ codoping has the effect of inhibiting the formation of shallow electron traps, which realizes a faster scintillation response of Ce:GAGG.

Photoelectron Spectroscopic Study of Electronic Structures of L-Cysteine

The valence band structure of L -cysteine films was investigated by ultraviolet photoelectron spectroscopy (UPS) in the photon energy range of 40–100 eV. Smooth thin films were produced by vacuum evaporation and characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Raman scattering spectroscopy. The photon energy dependence of the UPS spectra indicates that electronic states close to the valence band maximum may be attributed to sulfur, while the other states in the higher-binding-energy region may be attributed to carbon, nitrogen, and oxygen. Molecular orbital calculation was performed on seven possible geometries of L -cysteine. The observed UPS spectrum is in good agreement with the simulated one in terms of the C3 geometry of L -cysteine. It is concluded that the sulfur-originated electronic state is located at the valence-band maximum, indicating that cysteine is one of the useful and promising materials in future bioelectronics.

Ultrafast Time Dependence of Surface Photo-Voltage Effect on p-Type GaAs(100) Surface

The ultrafast surface photo-voltage (SPV) effect on the p -type GaAs(100) surface has been investigated by using the time-resolved valence-band photoemission spectroscopy with the Ti:sapphire regenerative amplifier. The rise of the SPV effect has been observed on the p -type GaAs(100). The time scale of the SPV rise depends on the dopant levels, indicating the important role of the surface electric field due to the band bending. On the other hand, the rise of the SPV effect on n -type GaAs(100) is faster than the experimental time resolution of 430 fs. It is considered that the SPV rise should be caused by the transport process of the photo-excited electrons and holes in the n - and p -type GaAs(100) surfaces, respectively.

Ultrafast Decay of Surface Photo-Voltage Effect on n-type GaAs(100) Surface

Surface photo-voltage (SPV) effect on n -type GaAs(100) has been investigated with a newly developed time-resolved photoemission spectroscopy, in which the sample surface is excited by one-photon of the pump pulse and the photoemission is measured by two-photon excitation of the probe pulse. Ultrafast decay of the SPV was found in a several picosecond range at 300 and 100 K, corresponding to the hot-electron lifetime in GaAs. The slow SPV decay was also observed in a few hundred picoseconds, due to the thermalized electrons.

Topological phase transitions driven by magnetic phase transitions in Fe(x)Bi2Te3 (0≤x≤0.1) single crystals.

We propose a phase diagram for Fe(x)Bi2Te3 (0≤x≤0.1) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic to antiferromagnetic gives rise to topological phase transitions, where the paramagnetic topological insulator of Bi2Te3 turns into a band insulator with ferromagnetic-cluster glassy behavior around x∼0.025, and it further evolves to a topological insulator with valence-bond glassy behavior, which spans over the region from x∼0.03 up to x∼0.1. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

Photoelectron spectroscopic study on the electronic structures of the dental gold alloys and their interaction with L-cysteine

The valence electronic structures of the dental gold alloys, type 1, type 3, and K14, and their interaction with L-cysteine have been studied by ultraviolet photoelectron spectroscopy with synchrotron radiation. It was found that the electronic structures of the type-1 and type-3 dental alloys are similar to that of polycrystalline Au, while that of the K14 dental alloy is much affected by Cu. The peak shift and the change in shape due to alloying are observed in all the dental alloys. It is suggested that the new peak observed around 2 eV for the L-cysteine thin films on all the dental alloys may be due to the bonding of S 3sp orbitals with the dental alloy surfaces, and the Cu–S bond, as well as the Au–S and Au–O bonds, may cause the change in the electronic structure of the L-cysteine on the alloys.

Development of the Experimental System for Time‐ and Angle‐resolved Photoemission Spectroscopy

Experimental system for the time‐ and angle‐resolved photoemission spectroscopy have been constructed at BL13 in SAGA Light Source, in order to study the electronic non‐equilibrium in the surface layer of laser‐excited materials The experimental system is very useful for photoemission spectroscopy in the wide temporal and angular ranges. The time‐ and angle‐resolved photoemission spectra can be obtained with using the gate electronics for the MCP detector of the photoemission spectrometer. The gated MCP detector is synchronized with the laser pulse from Ti:sapphire regenerative amplifier with the repetition frequency of 10 to 300 kHz. The time‐window of the gated MCP detector can be changed between 10 nano‐ and 160 micro‐second. The time‐resolved measurement in pico‐second region can be performed with using the pump‐probe technique which uses fundamental, second and third harmonics from the Ti:sapphire laser as the excitation source. Using these systems, we can perform the time‐ and angle‐resolved photoem...

Valence-Band Structures of Quasi-One-Dimensional Crystals C5H10NH2PbX3 [X = I, Br]

X-ray and ultra-violet photoemission spectra have been measured to study the valence-band structures of piperidinium trihaloplumbates C 5 H 10 NH 2 PbX 3 [X = I, Br], which have natural nano-scale one-dimensional structures. The valence-band spectra of lead halides, which have the three-dimensional structures, are also reported for comparison. The partial densities of states related to the constituent ions and molecules were obtained with the comparison between the XPS and UPS. Empirical LCAO calculations were also carried out to analyze the experimental results. The quantum-well-like electronic structure in C 5 H 10 NH 2 PbX 3 [X = I, Br] is proposed.

Two Types of Self-Trapped Excitons in a Quasi-One-Dimensional Crystal Piperidinium Tribromoplumbate

Photo-excited states of piperidinium tribromoplumbate (C 5 H 10 NH 2 PbBr 3 ) single crystals were investigated by spectroscopic methods. The strong anisotropy was observed in polarized reflection ...