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203 results on '"Sogawa T."'

1. Surface Carrier Dynamics on Semiconductor Studied with Femtosecond Core-Level Photoelectron Spectroscopy Using Extreme Ultraviolet High-Order Harmonic Source

Author

Sogawa T., Tateno K., Gotoh H., Nishikawa T., Nakano H., Kato K., Tsunoi T., and Oguri K.

Subjects
Physics, QC1-999
Abstract

We have used a femtosecond time-resolved core-level surface PES system based on the 92-eV harmonic source to study the surface carrier dynamics that induces the transient SPV on semiconductor surfaces. We clarified the temporal evolution of the transient SPV characterized by the time of the photo-generated carrier separation and recombination. This result demonstrates the potential of this technique for clarifying the initial stage of the surface carrier dynamics after photoexcitation.

Published
2013
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2. Decay and Frequency Shift of Inter and Intravalley Phonons in Graphene -Dirac Cone Migration-

Author

Sasaki, K., Kato, K., Tokura, Y., Suzuki, S., and Sogawa, T.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

By considering analytical expressions for the self-energies of intervalley and intravalley phonons in graphene, we describe the behavior of D, 2D, and D$'$ Raman bands with changes in doping ($\mu$) and light excitation energy ($E_L$). Comparing the self-energy with the observed $\mu$ dependence of the 2D bandwidth, we estimate the wavevector $q$ of the constituent intervalley phonon at $\hbar vq\simeq E_L/1.6$ ($v$ is electron's Fermi velocity) and conclude that the self-energy makes a major contribution (60%) to the dispersive behavior of the D and 2D bands. The estimation of $q$ is based on an image of shifted Dirac cones in which the resonance decay of a phonon satisfying $q > \omega/v$ ($\omega$ is the phonon frequency) into an electron-hole pair is suppressed when $\mu < (vq-\omega)/2$. We highlight the fact that the decay of an intervalley (and intravalley longitudinal optical) phonon with $q=\omega/v$ is strongly suppressed by electron-phonon coupling at an arbitrary $\mu$. This feature is in contrast to the divergent behavior of an intravalley transverse optical phonon, which bears a close similarity to the polarization function relevant to plasmons., Comment: 11 pages, 5 figures

Published
2012
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3. Pseudospin for Raman D Band in Armchair Graphene Nanoribbons

Author

Sasaki, K., Kato, K., Tokura, Y., Suzuki, S., and Sogawa, T.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

By analytically constructing the matrix elements of an electron-phonon interaction for the $D$ band in the Raman spectra of armchair graphene nanoribbons, we show that pseudospin and momentum conservation result in (i) a $D$ band consisting of two components, (ii) a $D$ band Raman intensity that is enhanced only when the polarizations of the incident and scattered light are parallel to the armchair edge, and (iii) the $D$ band softening/hardening behavior caused by the Kohn anomaly effect is correlated with that of the $G$ band. Several experiments are mentioned that are relevant to these results. It is also suggested that pseudospin is independent of the boundary condition for the phonon mode, while momentum conservation depends on it., Comment: 25 pages, 5 figures

Published
2011
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4. Polarization Dependence of Optical Transitions in Graphene Nanoribbons

Author

Sasaki, K., Kato, K., Tokura, Y., Sogawa, T., and Saito, R.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

The universality of $k$-dependent electron-photon and electron-phonon matrix elements is discussed for graphene nanoribbons and carbon nanotubes. An electron undergoes a change in wavevector in the direction of broken translational symmetry, depending on the light polarization direction. We suggest that this phenomenon originates from a microscopic feature of chirality., Comment: 4 pages, 2 figures

Published
2011

5. Theory of optical transitions in graphene nanoribbons

Author

Sasaki, K., Kato, K., Tokura, Y., Oguri, K., and Sogawa, T.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Matrix elements of electron-light interactions for armchair and zigzag graphene nanoribbons are constructed analytically using a tight-binding model. The changes in wavenumber ($\Delta n$) and pseudospin are the necessary elements if we are to understand the optical selection rule. It is shown that an incident light with a specific polarization and energy, induces an indirect transition ($\Delta n=\pm1$), which results in a characteristic peak in absorption spectra. Such a peak provides evidence that the electron standing wave is formed by multiple reflections at both edges of a ribbon. It is also suggested that the absorption of low-energy light is sensitive to the position of the Fermi energy, direction of light polarization, and irregularities in the edge. The effect of depolarization on the absorption peak is briefly discussed., Comment: 11 pages, 7 figures

Published
2011
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6. Charged exciton emission at 1.3 $\mu$m from single InAs quantum dots grown by metalorganic chemical vapor deposition

Author

Cade, N. I., Gotoh, H., Kamada, H., Tawara, T., Sogawa, T., Okamoto, H., and Nakano, H.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

We have studied the emission properties of self-organized InAs quantum dots (QDs) grown in an InGaAs quantum well by metalorganic chemical vapor deposition. Low-temperature photoluminescence spectroscopy shows emission from single QDs around 1300 nm; we clearly observe the formation of neutral and charged exciton and biexciton states, and we obtain a biexciton binding energy of 3.1 meV. The dots exhibit an s-p shell splitting of approximately 100 meV, indicating strong confinement., Comment: 3 pages, 3 figures, submitted APL

Published
2005
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9. Effect of electron-hole interaction on electron spin relaxation in GaAs/AlGaAs quantum wells at room temperature

Author

Gotoh, H., Ando, H., Sogawa, T., Kamada, H., Kagawa, T., and Iwamura, H.

Subjects
Quantum wells -- Research, Relaxation (Nuclear physics) -- Research, Physics
Abstract

An analysis of the effects of electron-hole interactions on the electron spin relaxation in a GaAs/AlGaAs quantum well at room temperature is presented. It was established that the electron-hole exchange interaction should contribute to the spin relaxation even at room temperature, as well as to the D'yakonov-Perel process.

Published
2000

15. Aberrant expression of microRNAs and the miR-1/MET pathway in canine hepatocellular carcinoma

Author

Lai, Y.-C., primary, Ushio, N., additional, Rahman, M. M., additional, Katanoda, Y., additional, Ogihara, K., additional, Naya, Y., additional, Moriyama, A., additional, Iwanaga, T., additional, Saitoh, Y., additional, Sogawa, T., additional, Sunaga, T., additional, Momoi, Y., additional, Izumi, H., additional, Miyoshi, N., additional, Endo, Y., additional, Fujiki, M., additional, Kawaguchi, H., additional, and Miura, N., additional

Published
2018
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18. Growth and characterization of GaP nanowires on Si substrate.

Author

Zhang, G., Tateno, K., Sogawa, T., and Nakano, H.

Subjects
TRANSMISSION electron microscopy, EPITAXY, CRYSTAL growth, CHEMICAL inhibitors, NANOWIRES, PARTICLES (Nuclear physics), NANOSTRUCTURED materials, SEMICONDUCTORS
Abstract

The integration of III-V semiconductor materials with Si technology is of great interest for optoelectronic integration circuits. We have studied the growth and structural aspects of GaP nanowires (NWs) grown on Si substrate in a metalorganic vapor phase epitaxy system. Au colloid particles dispersed on Si substrate were used as catalysts to conduct the NW growth. The growth temperature considerably affected the growth rate and shape of GaP NWs. The growth rate showed a maximum value of 14.69 nm/s at 480 °C. When growth temperature increased the radial growth on NW sides was enhanced and the NWs therefore exhibited a tapering shape. GaP NWs with a uniform diameter could be grown at a growth temperature as low as 420 °C using a two-temperature process. The NW diameter could be well controlled by using size-selective Au colloid particles. The growth rate dependence showed that the thin NWs grew more slowly than thick ones and the V/III source ratio had a significant effect on the growth rate dependence. An analysis of the GaP/Si interface by transmission electron microscopy indicated that the NWs were epitaxially grown on the Si(111) substrate. Based on these experimental results, the growth mechanism of the GaP NWs on Si was discussed. [ABSTRACT FROM AUTHOR]

Published
2008
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19. Growth and characterization of GaP nanowires on Si substrates

Author

Tateno, K., Sogawa, T., and Nakano, H.

Subjects
Epitaxy -- Research, Gallium compounds -- Structure, Gallium compounds -- Optical properties, Phosphorus compounds -- Structure, Phosphorus compounds -- Optical properties, Transmission electron microscopes -- Observations, Physics
Abstract

The growth and structural aspects of GaP nanowires (NWs) grown on Si substrate in a metalorganic vapor phase epitaxy system are studied. The analysis of the GaP/Si interface by transmission electron microscopy has shown that the NWs are epitaxially grown on the Si(111) substrate.

Published
2008

31. Picosecond spectral dynamics of gain-switched quantum-well lasers and its dependence on quantum-well structures.

Author

Sogawa, T. and Arakawa, Y.

Subjects
*LASERS, *QUANTUM wells, *STREAK cameras, *GALLIUM arsenide, *PICOSECOND pulses
Abstract

Discusses a study which measured time-resolved spectra of gain-switched gallium arsenide/aluminum gallium arsenide quantum-well (QW) lasers by a streak camera with a monochromator to investigate the effects of QW structure on picosecond lasing dynamics. Relation of QW to pulse forms and spectral dynamics; Overflow effect of carriers outside QW; Experimental results.

Published
1990
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