Kentaro Sato

Managing the scarcity of chemical elements

The issues associated with the supply of rare-earth metals are a vivid reminder to all of us that natural resources are limited. Japans Element Strategy Initiative is a good example of a long-term strategy towards the sustainable use of scarce elements.

Dependence of exciton transition energy of single-walled carbon nanotubes on surrounding dielectric materials

Abstract We theoretically investigate the environmental effect for optical transition energies of single-walled carbon nanotubes (SWNTs), by calculating the exciton transition energies of SWNTs. The static dielectric constants used in the exciton calculation can be expressed as a function of the dielectric constants of the surrounding material and that of the SWNT, in which the static and dynamic dielectric constants of the SWNT represent the screening effect of core electrons and the valence π electrons, respectively. The calculated results reproduce the environmental effect of the experimental transition energies for various surrounding materials and for various diameters of SWNTs.

Dielectric constant model for environmental effects on the exciton energies of single wall carbon nanotubes

The excitonic optical transition energies Eii of single wall carbon nanotubes, that are modified by surrounding materials around the tubes (known as the environmental effect), can be reproduced by defining a dielectric constant κ which depends on the subband index, nanotube diameter, and exciton size. The environmental effects on excitons can be recognized on a plot of the functional form of κ simply by the different linear slopes obtained for different samples. This treatment should be very useful for calculating Eii for any type of nanotube environment, hence providing an accurate assignment of many nanotube (n,m) chiralities.

High-energy spin and charge excitations in electron-doped copper oxide superconductors

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the Γ-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.

Spin–Orbit Interaction in Single Wall Carbon Nanotubes: Symmetry Adapted Tight-Binding Calculation and Effective Model Analysis

Energy band for single wall carbon nanotubes with spin–orbit interaction is calculated using non-orthogonal tight-binding method. A Bloch function with spin degree of freedom is introduced to adapt the screw symmetry of nanotubes. The energy gap opened by spin–orbit interaction for armchair nanotubes, and the energy band splitting for chiral and zigzag nanotubes are evaluated quantitatively. Spin polarization direction for each split band is shown to be parallel to the nanotube axis. The energy gap and the energy splitting depend on the diameter and chirality in an energy scale of sub-milli-electron volt. An effective model for reproducing the low energy band structure shows that the two mechanism of the band modification, shift of the energy band in two dimensional reciprocal lattice space, and, effective Zeeman energy shift, are relevant. The effective model explains well the energy gap and splitting for more than 300 nanotubes within the diameter between 0.7 to 2.5 nm.

Dynamic behavior of two-laser-induced bubbles in water

Two bubbles are simultaneously generated in water by focusing a Q‐switched ruby laser. The dynamic behavior of the induced bubbles is investigated by means of high‐speed photography. Consequently it is found that the bubble–bubble interaction is significantly influenced not only by the relative size of bubbles but also by the mutual distance between them. When two bubbles are axisymmetrically produced one behind the other near a rigid wall, we can observe a very interesting phenomenon, bubble splitting followed by bubble pinching.

Synthesis of F0 contours using generation process model parameters predicted from unlabeled corpora: application to emotional speech synthesis

Abstract A corpus-based method of generating fundamental frequency (F0) contours from text was developed for Japanese. Instead of directly predicting F0 values, the method predicts command values of the F0 contour generation process model using binary decision trees. Since the model controls the F0 movement in word or in longer units, sudden undulations, unlikely in natural utterances, can be avoided even in the case of erroneous prediction. The method includes a scheme of extracting the model commands from given F0 contours, which makes it possible to prepare the corpora for training the binary decision trees automatically. Since accuracy of the extracted model commands in the training corpora is crucial for the method, constraints are applied on the location of commands. Although the method can generate any speaking styles if the corpora of the styles are available, this paper is aimed at realizing three types of emotional speech (anger, joy, and sadness) besides calm speech. The mismatches between the predicted and target contours for angry speech were similar to those for calm speech. Synthesis of emotional speech was then conducted. Phoneme durations were predicted in a similar corpus-based method, and segmental features were generated using an HMM-based speech synthesizer. A perceptual experiment was conducted for the synthesized speech, and the result indicated that anger could be conveyed well by the developed method. The result was less satisfactory for joy and sadness.

Phonon self-energy corrections to nonzero wave-vector phonon modes in single-layer graphene.

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G(⋆) Raman feature at 2450 cm(-1) to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

Localized Modes in Tantalum Hydrides Studied by Neutron Inelastic Scattering

The local vibration spectra of hydrogen atoms dissolved in tantalum have been studied by the energy-loss scattering of pulsed neutrons. The observed time-of-flight spectra show split peaks of the local modes near 120 and 175 meV, which are almost invariant with changing concentration (H/Ta=0.1, 0.5 and 0.7) and configuration of hydrogen at 20° and 85°C. From comparison with previous neutron results on niobium and vanadium hydrides, it is shown that the energies of the split local modes are nearly independent of the metal-hydrogen distance, and the relative intensity and the FWHM of the high energy mode increase in the disordered phase at 85°C in comparison with the ordered phase at 20°C.