arXiv subjects

Seiji Kojima

Publications and source records attributed to Seiji Kojima.

4 recordsLinked to original sources

arXiv2026

Continuum model for the terahertz dielectric response of glasses

Boson peak dynamics in glasses produce a robust crossover in the terahertz (THz) dielectric response that standard Debye or Lorentz models do not capture. We develop a continuum description of this THz response, coupling an infrared-effective charge fluctuation spectrum to a frequency-dependent shear modulus, and apply it to glycerol glass. The model reproduces the measured complex dielectric function and the nearly linear infrared light-vibration coupling around the boson peak, and it highlights the dominant role of transverse shear dynamics.

cond-mat.dis-nn↗

arXiv2025

Relationship between the boson peak and first sharp diffraction peak in glasses

Boson peak (BP) dynamics refers to the universal excitation in the terahertz region of glass. In this study, the universal dynamics of BP were quantitatively evaluated in various glassy materials based on the heterogeneous elasticity theory (HET), and the determinants of BP were successfully extracted. A strong correlation was observed between the maximum possible coarse-graining wavenumber, which is a determinant of the BP in the HET, and the first sharp diffraction peak (FSDP) wavenumber, which is a characteristic index of the medium-range order in glasses. The results indicate that the behaviour of BP in glass can be quantitatively understood in the following two steps. First, the FSDP representing the largest structural correlation in glass is dominantly used to determine the unit size of the elastic modulus heterogeneity, and second, the magnitude of the elastic modulus fluctuation is used to determine the frequency and intensity of the BP.

cond-mat.dis-nn↗

arXiv2019

Detection of boson peak and fractal dynamics of disordered system using terahertz spectroscopy

Disordered systems exhibit universal excitation, referred to as the boson peak, in the terahertz region. Meanwhile, the so-called fracton is expected to appear in the nanoscale region owing to the self-similar structure of monomers in polymeric glasses. We demonstrate that such excitations can be detected using terahertz spectroscopy. For the interaction between terahertz light and the vibrational density of states of the fractal structure, we formulate an infrared light-vibration coupling coefficient for the fracton region. Accordingly, we show that information concerning fractal and fracton dimensions appears in the exponent of the absorption coefficient. Finally, using terahertz time-domain spectroscopy and low-frequency Raman scattering, we experimentally observe these universal excitations in a protein lysozyme system that has an intrinsically disordered and self-similar nature in a single supramolecule. These findings are applicable to disordered and polymeric glasses in general and will be key to understanding universal dynamics of disordered systems by terahertz light.

cond-mat.mtrl-sci↗

arXiv2017

Boson peak dynamics of glassy glucose studied by integrated terahertz-band spectroscopy

We performed terahertz time-domain spectroscopy, low-frequency Raman scattering, and Brillouin light scattering on vitreous glucose to investigate the boson peak (BP) dynamics. In the spectra of α(ν)/ν2 [α(ν) is the absorption coefficient], the BP is clearly observed around 1.1 THz. Correspondingly, the complex dielectric constant spectra show a universal resonancelike behavior only below the BP frequency. As an analytical scheme, we propose the relative light-vibration coupling coefficient (RCC), which is obtainable from the combination of the far-infrared and Raman spectra. The RCC reveals that the infrared light-vibration coupling coefficient CIR(ν) of the vitreous glucose behaves linearly on frequency which deviates from Taraskin's model of CIR(ν) = A + Bν2 [S. N. Taraskin et al., Phys. Rev. Lett. 97, 055504 (2006)]. The linearity of CIR(ν) might require modification of the second term of the model. The measured transverse sound velocity shows an apparent discontinuity with the flattened mode observed in the inelastic neutron scattering study [N. Violini et al., Phys. Rev. B 85, 134204 (2012)] and suggests a coupling between the transverse acoustic and flattened modes.

cond-mat.mtrl-sci↗