Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

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Electronic excitations: density-functional versus many-body Green's-function approaches

Recently discovered carbon nanotubes have exhibited many unique material properties including very high thermal conductivity. Strong sp 2 bonding configurations in carbon network and nearly perfect self-supporting atomic structure in nanotubes give unusually high phonon-dominated thermal conductivity along the tube axis, possibly even surpassing that of other carbon-based materials such as diamond and graphite (in plane). In this chapter, we explore theoretical and experimental investigations for the thermal-transport properties of these materials.

Unusually High Thermal Conductivity of Carbon Nanotubes

Hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing non-carbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogue-a monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as optoelectronic nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices. This review gives an introduction to the rich BN nanotube/nanosheet field, including the latest achievements in the synthesis, structural analyses, and property evaluations, and presents the purpose and significance of this direction in the light of the general nanotube/nanosheet developments.

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Boron Nitride Nanotubes and Nanosheets

Modeling hardness of polycrystalline materials and bulk metallic glasses

Carbon nanotubes are unique tubular structures of nanometer diameter and large length/diameter ratio. The nanotubes may consist of one up to tens and hundreds of concentric shells of carbons with adjacent shells separation of ∼0.34 nm. The carbon network of the shells is closely related to the honeycomb arrangement of the carbon atoms in the graphite sheets. The amazing mechanical and electronic properties of the nanotubes stem in their quasi-one-dimensional (1D) structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young’s modulus and tensile strength, which makes them preferable for composite materials with improved mechanical properties. The nanotubes can be metallic or semiconducting depending on their structural parameters. This opens the ways for application of the nanotubes as central elements in electronic devices including field-effect transistors (FET), single-electron transistors and rectifying diodes. Possibilities for using of the nanotubes as high-capacity hydrogen storage media were also considered. This report is intended to summarize some of the major achievements in the field of the carbon nanotube research both experimental and theoretical in connection with the possible industrial applications of the nanotubes.

Carbon nanotubes: properties and application

This article reviews recent progresses in first-principles studies on graphene/hexagonal boron nitride (h-BN) heterostructures. The experimental demonstration of the usefulness of hexagonal boron nitride as a substrate for graphene devices has attracted considerable interest of graphene/h-BN heterostructure systems. More complicated graphene/h-BN heterostructures have also been fabricated in recent years. In parallel with the experimental progress, first-principles studies have also revealed interesting properties of these heterostructures. The structural and electronic properties of the graphene/h-BN heterostructures are discussed here based on the recent first-principles results.

First-Principles Study on Graphene/Hexagonal Boron Nitride Heterostructures

Electronic structure of C60-encapsulating semiconducting carbon nanotube

Calcul des energies de quasiparticules d'agregats de Sodium et de Potassium en utilisant le modele jellium-sphere blackground (fond continu de la sphere de Jellium) pour les niveaux de cœur d'ions positifs, et l'approximation GW de Hedin pour les energies des electrons de valence. Dans le calcul de l'interaction de Coulomb ecrantee, une nouvelle methode generalisee pour approximer le spectre d'excitation des systemes interactifs a N-corps est introduite. Les potentiels d'ionisation et les energies de quasiparticules calcules sont nettement ameliores comparativement aux calculs bases sur la theorie de la fonctionnelle de la densite locale de Hohenberg-Kohn-Sham

Quasiparticle energies in small metal clusters

We study the electronic structure of C 78 and that of C 78 -graphite cointercalation compound First we show the electronic structure of five isomers of C 78  Their geometric structures have been optimized by an empirical model potential and their electronic structure has been calculated using the tight-binding model The C 2 v -symmetry C 78 , a major isomer experimentally extracted, is found to have a considerably deep lowest unoccupied state Using this C 2 v C 78 , we design a stage-1 C 78 -graphite cointercalation compound Its electronic structure calculated with the tight-binding model shows that the deep lowest unoccupied state of the C 2 v C 78 causes charge transfer from a graphite sheet to C 78  Although the material is formed with only carbon atoms, C 78 -graphite cointercalation compound is the hole doped graphite intercalation compound

Electronic Structure of C78 and C78-Graphite Cointercalation Compound.

The adsorption process of hydrogen atom on nitrogen-doped carbon nanotube (CNT) and its effects on the electronic properties are investigated using the first-principles density-functional methods. As possible hydrogen adsorption sites, three different positions are considered to discuss adsorption energies of the hydrogen atom on N-doped (10,0) CNTs. It is found that the favorable hydrogen adsorption site on N-doped (10,0) CNT is not on top of the nitrogen atom but on top of carbon atoms next to the nitrogen atom. Interestingly, it is found that the impurity state induced by doping with nitrogen atom is shifted from conduction-band minimum to valence-band maximum by the adsorption of the hydrogen atom.

https://iopscience.iop.org/article/10.1088/1742-6596/302/1/012006/pdf

Susumu Saito

Papers

First-Principles Study on Graphene/Hexagonal Boron Nitride Heterostructures

Electronic structure of C60-encapsulating semiconducting carbon nanotube

Quasiparticle energies in small metal clusters

Electronic Structure of C78 and C78-Graphite Cointercalation Compound.

Structure and stability of hydrogen atom adsorbed on nitrogen-doped carbon nanotubes