Hidekatsu Suzuura

Electron lifetime due to optical-phonon scattering in a graphene sheet

We study scattering between electrons and optical phonons in a graphene sheet. Electron-phonon interaction is derived for zone-center and zone-boundary phonons within the framework of the effective-mass equations. We calculate the electron lifetime due to phonon emission and absorption. The inter-valley scattering induced by the zone-boundary phonons turned out to be dominant in high-field transport.

Applicability of continuum absorption in semiconductor quantum wells to absolute absorption-strength standards

The absolute strength of two-dimensional continuum absorption in semiconductor quantum wells (QWs) was investigated for application to an absorption-strength standard. We presented an equation for the continuum-state absorbance in a QW, showing that the absorbance near the band edge is essentially described by a universal value proportional to the fine-structure constant with correction terms for band-mixing effects and Coulomb enhancement effects. By using k·p equations with the established band parameters, we calculated the absorbance with quantitative estimations of the correction terms in typical semiconductor QWs. The results indicate that the absorbance of the QW is applicable to an absorption-strength standard.

Chirality Dependence of the Energy Splitting Between Bright and Dark Excitons in Semiconducting Carbon Nanotubes

We have calculated the chirality dependence of exciton energy splitting in semiconducting carbon nanotubes (CNs) by using the k⋅p scheme within the Hartree‐Fock approximation. The exciton fine structure is highly dependent on the structure of CNs showing a clear family pattern similar to that in the bright‐exciton levels. The bright excitons have higher energy than the dark one, while the splitting is quite small and there exist CNs in which the bright exciton becomes the lowest level in the fine structures. In contrast, the inter‐valley excitons always have higher energy than the bright and dark ones.

Aharonov-Bohm effect on impurity-bound excitons in semiconducting carbon nanotubes

We study the Aharonov-Bohm effect on exciton absorption spectra in semiconducting carbon nanotubes with an impurity in an effective-mass theory. In the presence of an impurity potential, there are two impurity-bound exciton states, one is dark and the other is bright. The dark impurity-bound exciton becomes brightened with increasing a magnetic flux through the cross section. The energy level of the bright impurity-bound exciton shows a blueshift in the magnetic flux dependence for a long-range impurity; in contrast, it shows a redshift for a short-range impurity, due to an enhancement of the coupling between intra- and inter-valley excitons. This suggests that the magnetic flux dependence can distinguish the range of the impurity potential and the strength of the inter-valley coupling of excitons.

Stability of exciton states and screening effects in doped carbon nanotubes

Excitonic effects and the absorption spectra of doped semiconducting nanotubes are studied in an effective-mass theory by taking into consideration static and dynamic screening. The dynamic screening effects give rise to the enhancement of the exciton binding energy and makes the excitons in carbon nanotubes more stable than static screening approximation.

Fano effect on dynamical conductivity for perpendicular polarization in double-wall carbon nanotubes

The dynamical conductivity of double-wall carbon nanotubes for perpendicularly polarized light to the tube axis is studied by taking into account screening effects, exciton effects and depolarization effects within an effective-mass theory. The exciton peak of the semiconducting inner tube has an asymmetric line shape due to the coupling with a continuum state of the outer tube, indicating the Fano effect. The Fano coupling strength can be tuned by varying the inter-wall distance.

Effects of inter‐wall screening and anti‐screening on excitons in double‐walled carbon nanotubes

Exciton properties of double‐wall carbon nanotubes are studied in the static screened Hartree‐Fock approximation within an effective‐mass theory. Inter‐wall screening effects suppress the band gap energy and the binding energy of exciton states. As a result, the exciton energy levels are redshifted from those in the single‐wall carbon nanotube with the same diameter. However, because of anti‐screening effects, the behavior of excited excitons is qualitatively different from that of the ground exciton. The energy shift of the ground exciton has little dependence on the tube diameter, while that of the excited excitons decreases with the increase of the diameter, following the band gap shift.

Numerical study on spin Hall effect for the Rashba model in a dirty limit

We study the impurity scattering in the spin Hall effect. The spin Hall conductance is numerically calculated by the tight-binding model with the Rashba spin-orbit coupling. In a dirty limit, the spin Hall conductance is proportional to the square of the elastic scattering time as long as the diffusive conduction is realized.