Shota Ono

Tuning the electrical resistivity of semiconductor thin films by nanoscale corrugation

Department of Applied Physics, Graduate School of Engineering, HokkaidoUniversity, Sapporo. 060-8628 JapanE-mail: shota-o@eng.hokudai.ac.jpE-mail: shima@eng.hokudai.ac.jpAbstract. The low-temperature electrical resistivity of corrugated semiconductorfilms is theoretically considered. Nanoscale corrugation enhances the electron-electronscattering contribution to the resistivity, resulting in a stepwise resistivity developmentwith increasing corrugation amplitude. The enhanced electron scattering is attributedto the curvature-induced potential energy that affects the motion of electrons confinedto a thin curved film. Geometric conditions and microscopic mechanism of the stepwiseresistivity are discussed in detail.

Low-Temperature Resistivity Anomalies in Periodic Curved Surfaces

Effects of periodic curvature on the electrical resistivity of corrugated semiconductor films are theoretically considered. The presence of a curvature-induced potential affects the motion of electrons confined to the thin curved film, resulting in a significant resistivity enhancement at specific values of two geometric parameters: the amplitude and period of the surface corrugation. The maximal values of the two parameters in order to observe the corrugation-induced resistivity enhancement in actual experiments are quantified by employing existing material constants.

TOMBO: All-electron mixed-basis approach to condensed matter physics

Abstract TOMBO is a computer code for calculating the electronic structure of systems that consist both of core and valence electrons and nuclei, based on density-functional theory. It is based on an all-electron mixed-basis approach, in which the Kohn–Sham (KS) wave function is expressed by a linear combination of plane-waves and atomic-orbitals. This approach can describe both spatially localized and extended orbitals, which enables us to perform all-electron calculations with high accuracy from isolated clusters to periodic crystals. The present paper describes a theory of the all-electron mixed-basis approach, as well as input variables and benchmark tests in TOMBO . The algorithm for accelerating the computational time that is needed to solve the KS equation is also presented.

Ab initio molecular dynamics simulation study of successive hydrogenation reactions of carbon monoxide producing methanol

Doing ab initio molecular dynamics simulations, we demonstrate a possibility of hydrogenation of carbon monoxide producing methanol step by step. At first, the hydrogen atom reacts with the carbon monoxide molecule at the excited state forming the formyl radical. Formaldehyde was formed after adding one more hydrogen atom to the system. Finally, absorption of two hydrogen atoms to formaldehyde produces methanol molecule. This study is performed by using the all-electron mixed basis approach based on the time dependent density functional theory within the adiabatic local density approximation for an electronic ground-state configuration and the one-shot GW approximation for an electronic excited state configuration.

Heterojunction of single-walled capped carbon nanotube and zinc phthalocyanine with high energy conversion efficiency

We propose a heterojunction of capped carbon nanotube (CNT) and zinc phthalocyanine as a solar cell. The charge separation mechanism at the interface is investigated using the density functional theory, through an analysis of the spatial profile of the Kohn-Sham wave functions. Estimated energy conversion efficiency of the capped CNT-based solar cells is quite larger than that of C60-based ones, offering an essential idea to overcome the difficulty to increase the efficiency.

Anomalous enhancement in the infrared phonon intensity of a one-dimensional uneven peanut-shaped C60 polymer

A one-dimensional (1D) uneven peanut-shaped C(60) polymer formed from electron-beam (EB)-induced polymerization of C(60) molecules showed an anomalous increase in two characteristic infrared (IR) peak intensities, which are respectively due to the radial and tangential motion of the 1D polymer, when compared to the IR peaks of pristine C(60) films. This anomaly was analyzed on the basis of the vibrational van Hove singularity (VHS), using an extended thin-shell elastic model fully considering the effects of periodic radius modulation inherent to the 1D uneven peanut-shaped C(60) polymer. We succeeded in explaining the enhancement in the tangential peak intensity by VHS, whereas the origin to cause that in the radial peak intensity is still unclear.

Nonequilibrium phonon dynamics beyond the quasiequilibrium approach

The description of nonequilibrium states of solids in a simplified manner is a challenge in the field of ultrafast dynamics. Here, the phonon thermalization in solids through the three-phonon scatterings is investigated by solving the Boltzmann transport equation (BTE). The numerical solution of the BTE shows that the transverse acoustic and longitudinal acoustic (LA) phonon temperatures are not well-defined during the relaxation, indicating the breakdown of the quasiequilibrium approximation. The development of hot and cold phonons and the backward energy flow from low to high energy phonons are observed in the initial and final stage of the relaxation, respectively. A minimal model is presented to relate the latter with the power-law decay of the LA phonon energy.

Geometry dependence of electronic and energetic properties of one-dimensional peanut-shaped fullerene polymers.

In the present study, we investigate different types of 1D peanut-shaped fullerene polymers (PSFPs) using density functional theory to understand the electronic states and the energetic stability of curved carbon nanomaterials. We generated 53 different models of the 1D PSFPs by means of the generalized Stone-Wales transformations and performed structural optimization for each model. Band structures of the 1D PSFPs exhibit either metallic or semiconducting property according to the geometrical structures. We find that the energetic stability of the 1D PSFPs depends on the geometry: the more octagon and pentagon-octagon pairs (heptagons and hexagon-heptagon pairs) in their geometrical structures, the more stable (unstable) the 1D PSFPs.

Phonon Dispersion and Electron--Phonon Interaction in Peanut-Shaped Fullerene Polymers

We reveal that the periodic radius modulation peculiar to one-dimensional (1D) peanut-shaped fullerene (C 60 ) polymers exerts a strong influence on their low-frequency phonon states and their interactions with mobile electrons. The continuum approximation is employed to show the zone-folding of phonon dispersion curves, which leads to fast relaxation of a radial breathing mode in the 1D C 60 polymers. We also formulate the electron–phonon interaction along the deformation potential theory, demonstrating that only a few set of electron and phonon modes yields a significant magnitude of the interaction relevant to the low-temperature physics of the system. The latter finding gives an important implication for the possible Peierls instability of the C 60 polymers suggested in the earlier experiment.

Effect of one-dimensional superlattice potentials on the band gap of two-dimensional materials

Using the tight-binding approach, we analyze the effect of a one-dimensional superlattice (1DSL) potential on the electronic structure of black phosphorene and transition metal dichalcogenides. We observe that the 1DSL potential results in a decrease of the energy band gap of the two-dimensional (2D) materials. An analytical model is presented to relate the decrease in the direct-band gap to the different orbital characters between the valence band top and conduction band bottom of the 2D materials. The direct-to-indirect gap transition, which occurs under a 1DSL potential with an unequal barrier width, is also discussed.