Kentaro Sato

Massachusetts Institute of Technology, Cambridge, MA, United States

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Publications (13)33.96 Total impact

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    ABSTRACT: In this work we use ressonant raman spectroscopy to study optical and electronic proprieties of twisted bilayer graphene focusing in the G-band enhanced effect, this one has a dependence of the angle and laser energy is in resonance.
    Frontiers in Optics; 10/2013
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    ABSTRACT: The G-band Raman intensity is calculated for twisted bilayer graphene as a function of laser excitation energy based on the extended tight binding method. Here we explicitly consider the electron-photon and electron-phonon matrix elements of twisted bilayer graphene to calculate the resonance Raman intensity. The G-band Raman intensity is sensitive to the laser excitation energy and the twisting angle between the layers as a result of folding the electronic energy band structure. The Van Hove energy singularity, which is an electron transition energy between the conduction and valence bands, depends on n−m of the twisting vector (n,m). The relative intensity of the G band as a function of twisting vectors is presented, which should be useful for the experimental identification of the twisting angle.
    Physical review. B, Condensed matter 09/2012; 86(12). · 3.77 Impact Factor
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    ABSTRACT: In this letter, we present a step towards understanding the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS(2), WSe(2), oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications.
    Scientific Reports 01/2012; 2:1017. · 5.08 Impact Factor
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    ABSTRACT: Bernal (ABA stacking order) and rhombohedral (ABC) trilayer graphene (3LG) are characterized by Raman spectroscopy. From a systematic experimental and theoretical analysis of the Raman modes in both of these 3LGs, we show that the G band, G' (2D) band, and the intermediate-frequency combination modes of 3LGs are sensitive to the stacking order of 3LG. The phonon wavevector q, that gives the double resonance Raman spectra is larger in ABC than ABA, which is the reason why we get the different Raman frequencies and their spectral widths for ABA and ABC 3LG. The weak electron-phonon interaction in ABC-stacked 3LG and the localized strain at the boundary between ABC- and ABA-stacked domains are clearly reflected by the softening of the G mode and the G' mode, respectively.
    ACS Nano 10/2011; 5(11):8760-8. · 12.03 Impact Factor
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    ABSTRACT: The double resonance Raman spectra of the overtone of the out-of-plane tangential optical (oTO) phonon and of combinations of the LO, ZO, and ZA phonons with one another are calculated for bilayer graphene. In the case of the bilayer graphene, these Raman peaks are observed in the energy region between 1600 and 1800 cm-1. We obtain results for both the fixed q=0 and the dispersive q=2k peaks of the overtones of the oTO phonon of bilayer graphene. We calculate the double resonance Raman spectra of the combination modes coming from the LO, iTO, LA, and iTA phonons in bilayer graphene. The calculated Raman peaks are compared with the experimental results.
    Physical review. B, Condensed matter 07/2011; 84(3). · 3.77 Impact Factor
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    ABSTRACT: The measured optical transition energies Eii of single-wall carbon nanotubes are compared with bright exciton energy calculations. The Eii differences between experiment and theory are minimized by considering first, a diameter/chiral angle-dependent dielectric constant and second, a diameter/exciton size-dependent dielectric constant (k). In our description, k is composed of the screening contributions from the tube, represented by ktube, and from the environment, represented by kenv. We discuss the main aspects of each approach and show that in the first case, different k dependencies are obtained for (E^S11, E^S22, E^M11) relative to (E^S33, E^S44) which is understood as follows: A changing environment changes the k diameter dependence for (E^S11, E^S22, E^M11), but for (E^S33, E^S44) the environmental effects are minimal. We show that in order to achieve a single dependence for all Eii, the exciton's size should be taken into account, as considered in the second approach. The resulting calculated exciton energies reproduce experimental Eii values within |50| meV for a diameter range (0.7< dt
    03/2011;
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    ABSTRACT: 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.
    Journal of the Physical Society of Japan 01/2009; 78(7). · 2.09 Impact Factor
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    ABSTRACT: Electron energy-loss spectra of single double-walled carbon nanotubes (DWCNTs) were compared with calculated joint density of states (jDOSs) obtained by a simple tight-binding (STB) and an extended tight-binding (ETB) method. From the comparisons, interband transition energies of ETB calculations show better agreement with peak positions of the experimental spectra than those of STB results. From a further comparison among calculated jDOS, real and imaginary parts of a dielectric function and a loss function Im[-1/epsilon], it was confirmed that the peak energies in a spectrum of single DWCNTs are almost equal to those of the optical absorption spectrum epsilon(2).
    Journal of electron microscopy 08/2008; 57(4):129-32. · 1.31 Impact Factor
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    ABSTRACT: The exiton transition energies of single wall carbon nanotubes which are observed in resonance Raman, photo-absorption, and photoluminescense spectroscopies, depend on the surrouding materials (environmental effect). The environmental effect can be explained by screeining of the excitonic states by the dielectric materials. We calculate the transition energies for many different (n,m) carbon nanotubes up to 4eV and to 3nm in diameter. The calculated results are compare with many experimental data with different conditions for samples. The energy shift for the exciton transition energies can be explained by a fitting parameter of static dielectric constants of surrouding materials. However we will show that the effective dielectric constant has a unique, type, metallicity, diameter, and energy dependence of the dielectric constants in order to reproduce the exciton energies for the wide range of diameter and excitation energies. By analyzing the data, we will give a simple formula for the dielectric constants for carbon nanotubes themselves and the surrounding materials as a function of chirality and diameter of single wall carbon nanotubes.
    03/2008;
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    ABSTRACT: We report the peculiar behavior of the G band Raman intensity, which is dependent on the metallicity of single-wall carbon nanotubes SWCNTs. In the metallic SWCNTs, the G band intensity was enhanced relative to the G band intensity, while the G band intensity was suppressed in the semiconducting SWCNTs. Resonance Raman spectroscopy using laser energies of E laser = 2.41, 1.96, 1.58, and 1.165 eV showed these features on the metal-enriched and semiconducting-enriched SWCNT samples that had been selectively sepa-rated by the nitronium ions. The metallicity dependence was explained theoretically by calculating the reso-nance Raman intensity within the extended tight-binding calculations. The calculated results confirm that the G band intensity of the metallic SWCNTs is stronger than that for the semiconducting SWCNTs because the electron-phonon matrix elements for the TO phonon at the K point is larger for metallic SWCNTs and the resonance window for E 33 S is larger than that for E 11 M .
    Physical review. B, Condensed matter 11/2007; · 3.77 Impact Factor
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    ABSTRACT: We theoretically investigate the dependence of exciton transition energies on dielectric constant of surrounding materials. We make a simple model for the relation between dielectric constant of environment and a static dielectric constant describing the effects of electrons in core states, $\sigma$ bonds and surrounding materials. Although the model is very simple, calculated results well reproduce experimental transition energy dependence on dielectric constant of various surrounding materials. Comment: 5pages, 4 figures
    Chemical Physics Letters 04/2007; · 2.15 Impact Factor
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    ABSTRACT: Using extended tight binding framework, the exciton states and exciton-phonon interaction are calculated for understanding optical properties of single wall carbon nanotubes. Resonance Raman intensity for first and second order Raman processes are calculated as a function of (n,m) with use of exciton wavefunctions. Chirality, type and diameter dependence of Raman intensity is now fully given. In particular, the dark exciton plays an important role for second-order, intervalley, resonance Raman processes. Although the exciton-phonon interaction is not so different from the electron-phonon interaction, the optical absorption (emission) is enhanced significantly by the localized exciton wavefunctions.References: J. Jiang et al, Phys. Rev. B, in press.
    03/2007;
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    ABSTRACT: We present calculated Raman intensity of radial breathing modes (RBM), and other first and second order Raman signals as a function of (n,m) with exciton wavefunctions. Because of strong k dependent electron-phonon and electron-photon matrix elements, the Raman intensity shows (2n+m) family pattern. Within the extended tight binding calculation, we make exciton Kataura-plot for RBM. The Raman intensity is enhanced by localized wavefunction of the bright exciton which decreases with increasing energy and diameter. We will further discuss disorder induced D-band Raman intensity with some experimental results.
    03/2006;

Publication Stats

87 Citations
33.96 Total Impact Points

Institutions

  • 2012
    • Massachusetts Institute of Technology
      • Department of Electrical Engineering and Computer Science
      Cambridge, MA, United States
  • 2007–2012
    • Tohoku University
      • Department of Physics
      Sendai, Kagoshima-ken, Japan
  • 2011
    • Nanyang Technological University
      • School of Physical and Mathematical Sciences
      Singapore, Singapore