小西 健久

Hard X-ray Photoemission spectra (HAXPES) for Al films on Si substrates were measured, and thickness dependence of the Al and Si core level spectra was obtained. A quantitative analysis of the plasmon-loss processes and the continuous background was performed. Such loss process can be quantitatively understood using a HAXPES analysis, especially for the intensity analysis of the core levels. Si 1 s electrons excite extrinsic Al plasmons and continuous loss features as far as they propagate within the Al film. The results give us important information and enable us to separate the intrinsic, extrinsic and their interference effects in plasmon-loss processes. Even for the high energy excitation inherent to HAXPES, the interference effect between intrinsic and extrinsic plasmon-losses is shown to be not negligible. A stronger contribution from loss features than from the main peaks should be considered for quantitative analysis.

Solid-solution-type Au–Pt alloy nanoparticles (NPs) were prepared from the nanoclusters of each metal using the polymer-conjugated fusion growth method. The elemental mapping analysis showed that the mixing state of the...

The excitonic insulator Ta2NiSe5 experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps ~ 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of ~0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Nivalence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc ~ 8 GPa. Superconductivity with a maximum Tsc ~ 1.2K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

Fusion growth from each nanocluster applying the random coil-globule transition of a thermoresponsive polymer was explored for alloy nanoparticle (NP) formation for the first time. The fusion growth of bimetallic Au-Pt alloy NPs was examined at 100, 150, and 200 degrees C via the structural collapse caused by the transition of a thermoresponsive polymer, poly(N-isopropylacrylamide). A significant temperature dependence was revealed in the formation path, even in the narrow range of reaction temperatures studied here. Structural investigations for the products were performed using transmission electron microscopy, energy-dispersive X-ray spectrometry, elemental mapping, X-ray absorption fine structure, and X-ray diffraction. At 100 degrees C, Au NPs slightly alloyed with Pt atoms rather than form neat Au NPs. The element ratio was determined to be Au:Pt = 12:1 or less of Pt atoms. Small Pt particles coated the surface of the central NPs. A nonmetallic component of Pt atoms was mainly detected in the product solution on the basis of the number of unoccupied Pt 5d orbitals. Aggregates comprising small particles several nanometers in size and void parts were generated at 150 degrees C. All of the characterization results indicate that the hydrothermal condition at 200 degrees C led to the production of Au-Pt alloy NPs having a solid-solution-type structure. The reaction path of the fusion growth of the bimetallic NPs is discussed on the basis of the effect of the temperature applied during the reaction.

The origin of successive phase transitions observed in the layered perovskite α-Sr₂CrO₄is studied by the density-functional-theory-based electronic structure calculation and mean-field analysis of the proposed low-energy effective model. We find that, despite the fact that the CrO₆octahedron is elongated along the <italic>c</italic>-axis of the crystal structure, the crystal-field level of nondegenerate 3<italic>d_xy</italic>orbitals of the Cr ion is lower in energy than that of doubly degenerate 3<italic>d_y</italic><inline-formula id="if1"><italic>_z</italic></inline-formula>and 3<italic>d_x</italic><inline-formula id="if2"><italic>_z</italic></inline-formula>orbitals, giving rise to the orbital degrees of freedom in the system with a 3<italic>d</italic>²electron configuration. We show that the higher (lower) temperature phase transition is caused by the ordering of the orbital (spin) degrees of freedom.

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

Using the band structure calculation and mean-field analysis of the derived three-chain Hubbard model with phonon degrees of freedom, we discuss the origin of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide Ta2NiSe5. We show that the Bose-Einstein condensation of excitonic electron-hole pairs cooperatively induces the instability of the phonon mode at momentum q→0 in the quasi-one-dimensional Ta-NiSe-Ta chain, resulting in the structural phase transition of the system. The calculated single-particle spectra reproduce the deformation of the band structure observed in the angle-resolved photoemission spectroscopy experiment. © 2013 American Physical Society.

We make the electronic structure calculations of Ta2NiSe 5 known as a candidate for excitonic insulators where the semiconducting or semi-metallic ground state becomes unstable against the coherent formation of excitons. We use the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction U is taken into account (GGA+U). We find that the material has a very simple quasi-one-dimensional (1D) electronic structure which relates to the formation of excitons. From the calculated partial density of states, band dispersion and spatial distribution of charge densities, we find in the first approximation that the conduction band is configured by the Ta 1D chain and the valence band is configured by the Ni-Se 1D chain.

We have synthesized nickel by means of pulsed laser ablation. A nickel disc was used for ablation with the focused output of fundamental harmonic from Nd:YAG laser. X-ray diffraction result shows that the synthesized nanoparticles are of pure metallic nickel with a face-centred cubic structure and the average particle size is 35 nm. The extended X-ray absorption fine structure (EXAFS) studies of pure nickel foil and the synthesized nanoparticles show similar structures. The position of the main peak is same in these nanoparticles with reference to the nickel foil. The only difference was observed in the reduction of the amplitude. The nearest-neighbour distance is similar as for pure nickel foil. The Debye–Waller factor is also similar. There is no trace of oxide and hydroxide in the EXAFS data, suggesting that the synthesized nanoparticles contain only nickel metal.

We study possible methods how to obtain imaging of magnetic atoms by use of photoelectron diffraction (PD). We have proposed a method to apply Daimon effect where forward focusing PD peaks are rotated we use the difference of the PD intensities for different circular polarization but -φ is used for the - circular polarization only in the data handling. This technique allows us to obtain atomic image only of spin polarized atoms. In this work we further apply this technique to ferromagnetic and antiferromagnetic surface layers with in-plane magnetization adsorbed on a nonmagnetic layer. © 2010 IOP Publishing Ltd.

The spin-reorientation transition of thin Au/Co/Au films, grown in-situ on W(110), is studied in XMCD and EXAFS experiments. At 300 K, for in-situ grown Co on a Au(111) film, the dominant easy magnetization direction was found to be in the surface plane, for the uncapped Co/Au bilayers. This is a novel observation, in terms of easy magnetization direction for low thickness Co on Au. After capping with Au, a sizeable out-of-plane magnetization is observed below a thickness of four atomic Co layers. When the spin-reorientation transition occurs because of Au capping, a 5 thin Co layer undergoes structural changes of lattice parameters Δa/a -1.2 % and Δc/c +6.6 %. The observation of structural changes which accompany the spin reorientation transition, contradicts previous work on Co/Au(111), and allows to quantify the magnetoelastic energy contribution, connected with the presence of the Co/Au interface. © 2009 IOP Publishing Ltd.