Hiroyuki Okazaki

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (lambda = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-x Ti x O2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

The creation of SiV⁻, GeV⁻, and SnV⁻ are presented by the atmospheric annealing in the argon flow. Compared to high‐pressure annealing, in which gas cannot flow, atmospheric annealing with an inert gas flow not only causes less degradation of the sample surface but also has the advantage of reducing equipment cost and preparation time. Excessive annealing time has been shown to reduce the amount of centers created. The optimal annealing time that maximizes formations depends on the type of diamond sample and the implanted ions. Furthermore, inspired by the split‐vacancy structure of the group IV–V centers, atmospheric pre‐annealing at 600 °C to increase the amount of di‐vacancy is demonstrated, followed by annealing at 1800 °C for 1 min. A shorter duration of high‐temperature annealing is expected to qualitatively reduce stress and deterioration of the crystallinity of the diamond sample.

In this work, we performed transient reflection measurements using the pump–probe protocol to investigate the influence of lattice defects induced by 380 keV argon (Ar) ions on the phonon dynamics of interlayer shearing (E2g1) and in-plane carbon stretching (E2g2) modes in highly oriented pyrolytic graphite (HOPG). The frequency and decay rate of E2g1- and E2g2-mode phonons were changed significantly at the fluence of 1×1014 ions/cm2. This indicated that significant structural changes in HOPG due to 380 keV Ar-ion irradiation were expressed as changes in the frequency and decay rate of phonons. In HOPG irradiated with 380 keV Ar+ at the fluence of 1×1014 ions/cm2, our measurement results indicated that compressive and tensile stresses contributed to in-plane and interlayer graphite structures, respectively.

X-ray absorption spectroscopy measurements were performed for the C K-edge of Pt nanoparticles on Ar⁺-irradiated carbon supports in order to elucidate the origin of improved catalyst performance after the introduction of vacancies into the carbon support. We observed a change in the electronic structure at the interface between the Pt nanoparticles and the carbon support after vacancy introduction, which is in good agreement with theoretical results. The results indicated that vacancy introduction resulted in a drastic change in the Pt–C interactions, which likely affected the d-band center of the Pt nanoparticles and led to the enhancement of the oxygen reduction reaction in catalysts.

CsBi _{4− x} Pb _x Te ₆ is synthesized and the superconductivity associated with the structural transition from Pb substitution is studied. Photoemission spectroscopy measurements are performed in order to elucidate the relationship between the electronic structure and the occurrence of the superconductivity. When Bi is substituted with Pb, an electron doping-like change in the electronic structure is directly observed which is contrary to the naive expectation of hole doping. This observation is consistent with band structure calculations and appears to be a unique characteristic of CsBi _{4− x} Pb _x Te ₆ because of the dissociation of Bi dimers upon Pb substitution. These results indicate that it may be possible to control the electron and hole doping via manipulating the Bi dimers through Pb substitution.

The crystalline phases that comprise the bulk microstructure of an isotropic Ga-containing Nd-Fe-B sintered magnet are identified by in situ elevated-temperature x-ray diffraction for temperatures between 25 and 800°C. Comparing the temperature-dependent volume fractions of the crystalline phases with the coercivity as a function of annealing temperature reveals that the large coercivity enhancements are remarkably well-correlated with the existence of a crystalline Nd₆(Fe,Ga)₁₄ phase. Scanning soft x-ray absorption microscopy measurements are used to directly observe the distribution of Fe and Nd in anisotropic as-sintered and optimally-annealed samples and demonstrate that the surface microstructure is similar to bulk. Fe L₃-edge x-ray magnetic circular dichroism measurements are used to directly map the demagnetization process in the fractured surfaces of anisotropic as-sintered and optimally-annealed samples. In the as-sintered sample, the demagnetization curves of compositionally different regions all have the the same shape but with a maximum magnetic signal that only depends on the Fe content within the probing depth of the x-ray absorption measurements. In the optimally-annealed sample, the demagnetization curves from compositionally different regions have rather different shapes and the exposed Nd₂Fe₁₄B grains without any grain boundary coverage have much lower coercivity than those with. This provides evidence that the grain boundary phase that forms upon annealing is able to protect the Nd₂Fe₁₄B grains from magnetization reversal by exchange-decoupling the Nd₂Fe₁₄B grains.