Search

Layered electrides, as typified by ${\mathrm{Ca}}_{2}\mathrm{N}$, are unconventional quasi-two-dimensional metals with low work functions, in which the conduction electrons are localized between the cation layers as well as outside the surface. We have investigated the electronic structure of the interface between a layered electride and another material, using graphene on ${\mathrm{Ca}}_{2}\mathrm{N}$ as an example. Our first-principles calculation shows that a graphene layer on ${\mathrm{Ca}}_{2}\mathrm{N}$ remains flat and is doped to an extremely high density of $n=5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ with its Fermi level (${E}_{F}$) aligned with the logarithmically divergent van Hove singularity (VHS) at a saddle point of graphene's ${\ensuremath{\pi}}^{*}$ band. This finding shows that graphene/${\mathrm{Ca}}_{2}\mathrm{N}$ is an ideal testing ground for the exploration of the many-body ground states, most notably superconducting states ($p+ip$ wave, $d$ wave, and $f$ wave), predicted to appear when ${E}_{F}$ is close to a VHS. The work function decreases abruptly upon monolayer attachment but reverts to that of ${\mathrm{Ca}}_{2}\mathrm{N}$ upon bilayer attachment. This peculiar behavior is explained in terms of the distinctive electronic structures of the constituent materials and their bonding.

We present first-principles density functional calculations that clarify the electronic properties of carbon defects in hexagonal boron nitride $(h$-BN) monolayers under biaxially applied strains. We find that strain can control the ionization energies of both donor and acceptor states. Furthermore, we also find that strain can lead to the dramatic change in conduction channel properties of donor states due to the interchange of the conduction-band-minimum state with the nearly-free-electron state. We also report the simulated scanning tunneling microscopy (STM) images of carbon defects in $h$-BN monolayers for experimental identification of those defects. We show that the STM images strongly reflect distinctive spatial distributions of local density of states around carbon defects depending on the substitution sites and thereby they could be identified by using STM experiments.

We investigate the electronic properties of boron-carbon-nitrogen (B-C-N) analogues of a recently proposed carbon kagome lattice [Chen et al., Phys. Rev. Lett. 113, 085501 (2014)]. The B-C-N kagome lattices are constructed by replacing the carbon zigzag chains of the carbon kagome lattice with boron nitride zigzag chains. We use calculations of phonon dispersion curves to demonstrate the thermodynamic stabilities of the BCN and ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattices. The B-C-N kagome lattices are wide-band gap semiconductors although the band gaps of the BCN and ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattices are increased and reduced, respectively, compared with the carbon case. The reduction of the band gap is found to be caused by a direct to indirect gap transition in the ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattice.

We propose an alternative structure of TiO2 anatase that has a higher energy oxygen p-like valence band maximum than the pristine TiO2 anatase and thus has a much better alignment with the water splitting levels. This alternative structure is unique when considering a large subspace of possible structural distortions of TiO2 anatase. We propose two routes towards this state and argue that one of them might have been realized in the recently discovered so-called black TiO2., Comment: 7 pages, 9 figures

We perform first-principles total-energy calculations to investigate stabilities and electronic properties of hexagonal boron nitride ($h$-BN) bilayers under biaxial tensile strains. The possible stacking patterns of $h$-BN bilayers are investigated in detail. We show that the interlayer distances between two layers in $h$-BN bilayers can be changed under applied strains, and furthermore, they can decrease and increase depending on the stacking patterns of $h$-BN bilayers. We find that the band gaps are tunable by applying strains. We also find that tensile strains can give rise to a transformation from an indirect- to a direct-gap semiconductor in the case of the most stable stacking bilayer. These results indicate the high importance of $h$-BN bilayers as future electronic and optoelectronic device materials.

On the basis of density-functional theory, we study electronic structures of thin carbon nanotube bundles. We find that nearly-free-electron (NFE) states, which are distributed in intra- and intertube regions, are present at $3\ensuremath{\sim}4\mathrm{eV}$ above the Fermi level. In the intertube region the NFE state of the nanotube bundles are strongly modified from that of the isolated nanotube. We also find that mixing of the NFE state with ${\mathrm{sp}}^{2}$ orbitals at atomic sites is decisive in a peculiar behavior of their charge distribution. Importance of the NFE state in transport properties is discussed.

We study the electronic structure of the superconducting body-centered-orthorhombic fulleride ${\mathrm{Ba}}_{4}{\mathrm{C}}_{60}$ using the local-density approximation in the density-functional theory. It is found that Ba states are strongly hybridized with ${\mathrm{C}}_{60}$ states through pentagons, explaining the observed lattice-constant differences, $agbgc.$ Due to this hybridization and to a rather low symmetry of the lattice causing the splitting of states, the ${t}_{1g}$-derived conduction band partially occupied by two electrons is found to be relatively wide. Fermi surfaces also show noncubic nature and contain dual quasiplanar sheets parallel to the $a\ensuremath{-}b$ plane in the momentum space, giving rise to a quasinesting vector parallel to the c axis.

We have studied the electronic structure of the superconducting fullerides ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ using the local-density approximation in the density functional theory. Their conduction-band profiles are found to be quantitatively very similar to each other although the lattice constant of ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ is definitely smaller than that of ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}.$ The density of states at Fermi level of ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ is slightly larger than that of ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}.$ In addition, ${\mathrm{C}}_{60}$ states are found to be hybridized not only with Ba states but also with K (Rb) states. These results are in sharp contrast to ${A}_{3}{\mathrm{C}}_{60}$ superconductors $(A=\mathrm{K}$ and Rb). The hybridization is expected to play an important role in their superconducting properties since carriers are found to be not only on ${\mathrm{C}}_{60}$ but also around K (Rb) sites as well as Ba sites.

We report on the electronic structure and energetic stabilities of two-dimensional ${\mathrm{C}}_{60}$ polymers, in both tetragonal and rhombohedral phases, studied by using the local-density approximation in the framework of the density-functional theory. Owing to hybrid networks of ${\mathrm{sp}}^{2}$-like (threefold coordinated) and ${\mathrm{sp}}^{3}$-like (fourfold coordinated) carbon atoms, the electronic structure of these phases is considerably different from that of face-centered-cubic (fcc) ${\mathrm{C}}_{60}.$ Both systems are found to be elemental semiconductors having small indirect gaps. Furthermore, since the interlayer distance between adjacent polymerized planes for both phases is small, these systems are found to have three-dimensional electronic structures. From structural optimizations under the experimental lattice parameters, we reveal energetic high stabilities of these phases. In particular, the tetragonal phase is found to be considerably more stable in energy than the fcc phase. Its high stability is caused by the formation of intercluster bonds whose energy gain is larger than the energy loss due to the distortion of the carbon networks of ${\mathrm{C}}_{60}$ units upon polymerization.

We study solids of ${\mathrm{Si}}_{20}$ fullerenes, ${\mathrm{Si}}_{46}$ and ${\mathrm{Na}}_{2}$${\mathrm{Ba}}_{6}$${\mathrm{Si}}_{46}$, in the framework of density-functional theory. The electronic structure of ${\mathrm{Si}}_{46}$ is remarkably different from that of the diamond Si lattice due to its pentagonal network. The valence-band top becomes deeper, resulting in a narrow valence-band width and a wide fundamental gap. Also, another gap appears within the valence bands. In the Na- and Ba-codoped phase, Ba states show strong hybridization with ${\mathrm{Si}}_{46}$ states giving a very high Fermi-level density of states, which should be of essential importance for the superconductivity observed in ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{Ba}}_{\mathit{y}}$${\mathrm{Si}}_{46}$.

We perform a systematic first-principles study of energetics and electronic properties of chiral carbon nanotubes (CNTs) in the density-functional theory. It is found that chiral CNTs possess slightly twisted ground-state geometries. Moderate-diameter CNTs show twisting-dependent electronic properties well classified by their chiral indices, while the electronic structures of small-diameter CNTs possess sizable but individually different twisting dependences, leading to metal-semiconductor transitions in some CNTs. The CNT having the widest fundamental gap is predicted to be the twisted (4,3) CNT.

We study superlattices with alternate stacking of graphene and boron nitride monolayers. We propose several candidate stacking sequences of the superlattices, and optimize their geometries based on the energetics in the framework of the density functional theory. From the total energies of the superlattices with the candidate stacking sequences, we identify the most stable stacking sequence. The atomic configuration of the superlattice with the most stable stacking sequence is found to have the shortest B-C distance among all the optimized superlattice geometries, indicating a strong interaction between the carbon and boron atoms. We also study the electronic structure of the superlattices in detail. It is revealed that the most stable structure exhibits metallic electronic properties.

We study the energetics, electronic structures, and electron-phonon couplings of a one-dimensional potassium-doped ${\mathrm{C}}_{60}$ chain encapsulated in a boron nitride nanotube using the framework of the density-functional theory. We demonstrate that the reaction of potassium doping is exothermic and the resulting material is one-dimensional metal where conducting electrons are only in the ${\mathrm{C}}_{60}$ chain. Interestingly, the Fermi-level density of states has a peculiar pressure dependence and can be larger than those in the three-dimensional alkali-doped fullerene compounds, indicating the possibility of various phase transitions. We also discuss the electron-phonon couplings and the possibility of superconductivity.

We report electronic and geometric structures of four main candidates for C 84 obtained by using quantitatively reliable models. From a comparison to the recent photoemission spectra (PES), we show that a rather round fullerene with D 2 symmetry proposed by Achiba et al. is the most abundant C 84 cluster in the graphite-arc soot. Other C 84 clusters with T d and D 6h symmetries are also expected to be present. The previously proposed rather that D 2 -symmetry C 84 is found to be unfavorable. The electronic structure of D 2 C 76 is also obtained and the egreement with PES is found to be excellent

We have calculated the electronic structure of the ${\mathrm{C}}_{70}$ cluster of the ${\mathit{D}}_{5\mathit{h}}$ geometry by the local-density approximation. The electronic levels obtained show excellent agreement with photoemission experiments, which provides a further strong support for this geometry. The bunching of levels is observed at several energies, including the region around the highest-occupied state and the lowest-unoccupied state, showing the possibility of superconductivity in doped solid ${\mathrm{C}}_{70}$. The total-energy calculation suggests that the bond-length alternation found in ${\mathrm{C}}_{60}$ is also present in ${\mathrm{C}}_{70}$. The calculated binding energy per atom of the ${\mathrm{C}}_{70}$ cluster is found to be slightly larger than that of the ${\mathrm{C}}_{60}$ cluster. This implies the importance of dynamical effects on the relative abundance of fullerenes.

Microscopic total-energy electronic-structure calculations for K{sub {ital x}}C{sub 60} show that solid C{sub 60} weakly bonded via van der Waals forces is transformed upon potassium doping into a strongly condensed {ital ionic} {ital metal} in which both Madelung and kinetic energies contribute to its large cohesive energy and bulk modulus. We also find that K doping induces lattice contraction which results in nonrigid energy-band modification. The Fermi level for K{sub 3}C{sub 60} is found to be located close to a peak of the density of states.

We study boron-doped single-walled carbon nanotubes by using first-principles methods based on the density functional theory. The total energy, band structure, and density of states are calculated. From the formation energy of boron-doped nanotubes with different diameters, it is found that a narrower tube needs a smaller energy cost to substitute a carbon atom with a boron atom. By using the result of different doping rates in the (10,0) tube, we extrapolate the result to low boron density limit and find that the ionization energy of the acceptor impurity level should be approximately 0.2 eV. Furthermore, we discuss the doping rate dependence of the density of states at the Fermi level, which is important to realize superconductivity.

Structures for realizing hole-doped $\mathrm{Mg}{\mathrm{B}}_{2}$ without appealing to chemical substitutions are proposed. These structures that consist of alternating $\mathrm{Mg}{\mathrm{B}}_{2}$ and graphene layers have small excess energy compared to bulk graphite and $\mathrm{Mg}{\mathrm{B}}_{2}$. Density functional theory based first-principles electronic structure calculations show significant charge transfer from the $\mathrm{Mg}{\mathrm{B}}_{2}$ layer to graphene, resulting in effectively hole-doped $\mathrm{Mg}{\mathrm{B}}_{2}$ and electron-doped graphene. Substantial enhancement in the density of states at the Fermi level and significant in-plane lattice expansion of the proposed structures are predicted. These structures combines three important factors, namely, hole doping, high density of states at the Fermi level, and in-plane lattice expansion, that are favorable for a strong electron-phonon coupling.

We study the two-dimensionally polymerized rhombohedral phase of ${\mathrm{C}}_{60}$ by using the local-density approximation in the density-functional theory. The electronic structure of this hybrid solid carbon of ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ C atoms is found to be essentially three dimensional with a narrow fundamental gap due to a strong interlayer interaction, which also stabilizes the system considerably. Possible modifications of this elemental semiconductor are also discussed.

We study the electronic structure of the carbon nanotube theoretically by the first-principles techniques using the local-density approximation (LDA) with the many-body correction in the GW approximation. We find that the (9,0) tube is gapful irrespective of naive expectation from the graphene band structure. All of the $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\sigma}$ hybridization effect, lattice relaxation effect, and many-body effect due to electron interaction enhance the band gap, and the value is as large as $0.17\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ when taking into account all effects. For the $(n,0)$ nanotubes with $n=9$, 12, 15, and 18, the LDA gap is found to range from $0.08\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. These sizable gap values obtained by the most reliable methods to date shed light on the classification of carbon nanotubes by their electronic transport properties.

We study the electronic structure of carbon nanotubes theoretically by first-principles techniques. Geometry is optimized with the local-density approximation (LDA) in density functional theory, and many-body effects between electrons are taken into account within the $\mathrm{GW}$ approximation. We find that the (5,0) tube is metallic even at the $\mathrm{GW}$ level, being different from the tight-binding result. The (6,0) tube is also confirmed to be metallic. The $\mathrm{GW}$ correction to LDA is found to be small in metallic tubes. The (7,0) tube is semiconducting, in which the $\mathrm{GW}$ correction considerably increases the gap. On the other hand, the $\mathrm{GW}$ correction is small in graphene, suggesting that the density functional theory gives a reasonable description of large nanotubes.

Using density functional theory, we calculate the charge redistribution incurred upon forming multiwall carbon nanotubes, or by sandwiching initially isolated single-wall nanotubes between graphene layers. In these systems, we observe a significant charge transfer between the \ensuremath{\pi} electron system of the tube walls and a newly formed interlayer state. We discuss the direction of charge flow in terms of the interlayer hybridization and work function differences in the composite systems.

We study an unusual class of carbon structures, based on rigidly interconnected segments of graphite. The resulting foamlike systems cover the structural phase space extending from hexagonal diamond to graphite. Related to the most stable phases of carbon, these hybrid systems show an unusually high structural stability at low mass densities. Our density-functional calculations indicate that carbon foam is metallic, stable, and structurally rigid.

We report first-principles total-energy electronic-structure calculations that provide energetics of encapsulation of ${\mathrm{C}}_{60}$ in nanotubes consisting of boron and nitrogen atoms and electronic structures of resulting ``BNC peapods.'' We find that the encapsulating process is exothermic for (10,10) and (9,9) nanotubes. The energy gain upon the encapsulation is larger than that for carbon peapods, so that the BN nanotube is a promising candidate to accommodate the fullerenes inside. The material is predicted to be a new form of ternary semiconductor with interesting structural hierarchy.

To clarify the microscopic formation process of ${\mathrm{C}}_{60}$ and other fullerenes, we study the geometries and energetics of small carbon clusters and the reaction between carbon clusters using the long-range transferable tight-binding model parametrized by Omata et al. (Omata TB), the local-density-approximation (LDA) in the framework of the density-functional theory, and the constant-temperature molecular dynamics combined with Omata-TB (Omata TBMD). From the LDA geometry-optimization study, we find that the binding energy per atom of the ${\mathrm{C}}_{10}$ ring is $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$/atom larger than that of the ${\mathrm{C}}_{10}$ chain. This energetic preference of a ring to a chain in ${\mathrm{C}}_{10}$ is most prominent among all ${\mathrm{C}}_{n}$ clusters studied $(5\ensuremath{\leqslant}n\ensuremath{\leqslant}17)$. Moreover, the study of $sp$-hybridized small carbon clusters with Omata TBMD reveals that ${\mathrm{C}}_{10}$ is the smallest stable ring at the temperature of $2000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which can explain the high abundance of ${\mathrm{C}}_{10}$ in the experimental $\mathrm{C}_{n}{}^{\ensuremath{-}}$ mass spectra. From the remarkable stability of the ${\mathrm{C}}_{10}$ ring as well as from its high abundance, it is considered that the $sp$-hybridized ${\mathrm{C}}_{10}$ ring should play a role of major constituent units of larger clusters and fullerenes. Therefore we perform various sets of simulations of reactions between carbon clusters possessing the C atoms in multiples of 10, ${\mathrm{C}}_{10m}+{\mathrm{C}}_{10n}$ ($m+n=2,3,4,5,6$, $m\ensuremath{\geqslant}n\ensuremath{\geqslant}1$), at several temperatures with Omata TBMD. As a result, it is found that, in most cases studied, ${\mathrm{C}}_{20}$ and ${\mathrm{C}}_{30}$ clusters possess the $s{p}^{2}$-hybridized planar geometries, while ${\mathrm{C}}_{40}$, ${\mathrm{C}}_{50}$, and ${\mathrm{C}}_{60}$ take the $s{p}^{2}$-hybridized ``fullerenelike'' closed-cage geometries. These ${\mathrm{C}}_{40}$, ${\mathrm{C}}_{50}$, and ${\mathrm{C}}_{60}$ cages can be formed at as low as $1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is in good accord with the experimental temperature of fullerene formation. In a few cases, even the ${\mathrm{C}}_{30}$ cluster is found to take the cage structure. These results are in good accord with the experimental results of the gas ion chromatography. The straightforward growth process from the $sp$-hybridized ring to the $s{p}^{2}$-hybridized plane and that from the $s{p}^{2}$-hybridized plane to the $s{p}^{2}$-hybridized fullerenelike cage revealed in the present study are considered to be the main road of the formation of fullerenes. Finally, the study of structural stabilities of cage geometries obtained through the reaction between a fullerenelike cage $({\mathrm{C}}_{40})$ or a symmetrical fullerene (${D}_{5h}$ ${\mathrm{C}}_{50}$ or ${I}_{h}$ ${\mathrm{C}}_{60}$) and a carbon cluster (${\mathrm{C}}_{10}$ or ${\mathrm{C}}_{12}$) at various temperatures with Omata TBMD indicates that ${\mathrm{C}}_{2n}$ fullerenelike cages larger than ${\mathrm{C}}_{60}$ tend to decay into ${\mathrm{C}}_{2n\ensuremath{-}2}$ through the ${\mathrm{C}}_{2}$ loss process at the higher rate than ${\mathrm{C}}_{60}$ or smaller fullerenelike cages. Therefore this ${\mathrm{C}}_{2}$ loss process is considered to be one of the most important processes leading to the extreme abundance of ${\mathrm{C}}_{60}$.

We report first-principles total-energy electronic-structure calculations that provide energetics and electronic structures of double-walled BN nanotubes, $(n,0)$@(15,0) and $(m,0)$@(20,0). We find that the most favorable double-walled nanotubes studied here are (7,0)@(15,0) and (12,0)@(20,0) in which the interwall distances are about $3 \AA{}.$ The electronic energy bands around the Fermi energy depend interestingly on the tube radii due to the hybridization between $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ states. We also find that the nearly-free-electron states of the nanotubes induce peculiar charge redistribution in the interwall region.

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