Yoshiyuki Kawazoe

Yoshiyuki Kawazoe
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Yoshiyuki Kawazoe

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Physics - Materials Science (24)
Physics - Mesoscopic Systems and Quantum Hall Effect (11)
Physics - Strongly Correlated Electrons (2)
Quantum Physics (1)
Physics - Chemical Physics (1)
Physics - Statistical Mechanics (1)

Publications Authored By Yoshiyuki Kawazoe

As a nonclassical crystal growth mode, oriented attachment (OA) plays an increasingly important role in materials science and nanotechnology, and has shown great significance in the development of crystal growth theory. The OA process usually involves the oriented self-assembly of primary nanoparticles and conversion to single crystals or pseudocrystals by interface fusion (Fig. 1a). Read More

Emergent Dirac fermion states underlie many intriguing properties of graphene, and the search for them constitute one strong motivation to explore two-dimensional (2D) allotropes of other elements. Phosphorene, the ultrathin layers of black phosphorous, has been a subject of intense investigations recently, and it was found that other group-Va elements could also form 2D layers with similar puckered lattice structure. Here, by a close examination of their electronic band structure evolution, we discover two types of Dirac fermion states emerging in the low-energy spectrum. Read More

Two-dimensional (2D) topological insulator (TI) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure topology identified by the $Z_2$ number as protected by time-reversal symmetry (TRS). 2D TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. Read More

Graphene, a two dimensional (2D) carbon sheet, acquires many of its amazing properties from the Dirac point nature of its electronic structures with negligible spin-orbit coupling. Extending to 3D space, graphene networks with negative curvature, called Mackay-Terrones crystals (MTC), have been proposed and experimentally explored, yet their topological properties remain to be discovered. Based on the first-principle calculations, we report an all-carbon MTC with topologically non-trivial electronic states by exhibiting node-lines in bulk. Read More

The lattice relaxation in oxygen-deficient Ta2O5 is investigated using first-principles calculations. The presence of a charge-neutral oxygen vacancy can result in a long-ranged lattice relaxation which extends beyond 18 {\AA} from the vacancy site. The lattice relaxation has significant effects on the vacancy formation energy as well as the electronic structures. Read More

We report observation of 90-degree ferroelectric domain structures in transmission electron microscopy (TEM) of epitaxially-grown films of PbTiO3. Using molecular dynamics (MD) simulations based on first-principles effective Hamiltonian of bulk PbTiO3, we corroborate the occurance of such domains showing that it arises as metastable states only in cooling simulations (as the temperature is lowered) and establish characteristic stability of 90-degree domain structures in PbTiO3. In contrast, such domains do not manifest in similar simulations of BaTiO3. Read More

Based on the first principles calculation combined with quasi-harmonic approximation, in this work we focus on the analysis of temperature dependent lattice geometries, thermal expansion coefficients, elastic constants and ultimate strength of graphene and graphyne. For the linear thermal expansion coefficient, both graphene and graphyne show a negative region in the low temperature regime. This coefficient increases up to be positive at high temperatures. Read More

To study temperature dependent elastic constants, a new computational method is proposed by combining continuum elasticity theory and first principles calculations. A Gibbs free energy function with one variable with respect to strain at given temperature and pressure was derived, hence the full minimization of the Gibbs free energy with respect to temperature and lattice parameters can be put into effective operation by using first principles. Therefore, with this new theory, anisotropic thermal expansion and temperature dependent elastic constants can be obtained for crystals with arbitrary symmetry. Read More

We study the magnitude of zero-point vibration in one-component crystals. For the crystals whose constituent atoms share the same bonding geometry, we prove the existence of a characteristic temperature, T0, at which the magnitude of zero-point vibrations equals to that of the excited vibrations. Within the Debye model T0 is found to be ~1/3 of the Debye temperature. Read More

In order to study a magnetic principle of carbon based materials, multiple spin state of zigzag edge modified graphene molecules are analyzed by the first principle density functional theory to select suitable modification element. Radical carbon modified C64H17 shows that the highest spin state is most stable, which arises from two up-spin's tetrahedral molecular orbital configuration at zigzag edge. In contrast, oxygen modified C59O5H17 show the lowest spin state to be most stable due to four spins cancellation at oxygen site. Read More

In order to explain room-temperature ferromagnetism of graphite-like materials, this paper offers a new magnetic counting rule of radical carbon zigzag edge nano graphene. Multiple spin state analysis based on a density function theory shows that the highest spin state is most stable. Energy difference with next spin state overcomes kT=2000K suggesting a room-temperature ferromagnetism. Read More

Using first-principle density functional theory, we investigated the hydrogen storage capacity of Li functionalized adamantane. We showed that if one of the acidic hydrogen atoms of adamantane is replaced by Li/Li+, the resulting complex is activated and ready to adsorb hydrogen molecules at a high gravimetric weight percent of around ~ 7.0 %. Read More

Several experiments have recently found room-temperature ferromagnetism in graphite-like carbon based materials. This paper offers a model explaining such ferromagnetism by using an asymmetric nano-graphene. Our first typical model is C48H24 graphene molecule, which has three dihydrogenated (-CH2) zigzag edges. Read More

Room temperature ferromagnetic materials composed only by light elements like carbon, hydrogen and/or nitrogen, so called carbon magnet, are very attractive for creating new material categories both in science and industry. Recently several experiments suggest ferromagnetic features at a room temperature, especially in graphite base materials. This paper reveals a mechanism of such ferromagnetic features by modeling nanometer size asymmetric graphene molecule by using both a semi-empirical molecular orbital method and a first principle density function theory. Read More

Recent experiments indicate room-temperature ferromagnetism in graphite-like materials. This paper offers multiple spin state analysis applied to asymmetric graphene molecule to find out mechanism of ferromagnetic nature. First principle density functional theory is applied to calculate spin density, energy and atom position depending on each spin state. Read More

Recent experiments indicate room-temperature ferromagnetism in graphite like materials. This paper offers an multiple spin state analysis to find out the origine of ferromagnetism in case of nano meter size graphene molecule.First principle density function theory calculation (DFT-GGA with 631-G basis set) is applied to nano meter size asymmetric graphene fifteen molecules. Read More

Specific forms of the exchange correlation energy functionals in first-principles density functional theory-based calculations, such as the local density approximation (LDA) and generalized-gradient approximations (GGA), give rise to structural lattice parameters with typical errors of -2% and 2%. Due to a strong coupling between structure and polarization, the order parameter of ferroelectric transitions, they result in large errors in estimation of temperature dependent ferroelectric structural transition properties. Here, we employ a recently developed GGA functional of Wu and Cohen [Phys. Read More

The stability of atomic intercalated boron nitride K4 crystal structures, XBN (X=H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ga, Ge, As, Se, Br, Rb or Sr) is evaluated by the geometric optimization and frozen phonon calculations based on the first principles calculations. NaBN, MgBN, GaBN, FBN and ClBN are found to be stable. NaBN, GaBN, FBN and ClBN are metallic, whereas MgBN is semiconducting. Read More

The stability of atomic intercalated carbon $K_{4}$ crystals, XC$_{2}$ (X=H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ga, Ge, As, Se, Br, Rb or Sr) is evaluated by geometry optimization and frozen phonon analysis based on first principles calculations. Although C $K_{4}$ is unstable, NaC$_{2}$ and MgC$_{2}$ are found to be stable. It is shown that NaC$_{2}$ and MgC$_{2}$ are metallic and semi conducting, respectively. Read More

The geometric and electronic structures of NaN, CuN, and AgN metal clusters are systematically studied based on the density functional theory over a wide range of cluster sizes 2=Read More

Recent scanning tunneling spectroscopy (STM) experiments display images with star and ellipsoidal like features resulting from unique geometrical arrangements of a few adsorbed hydrogen atoms on graphite. Based on first-principles STM simulations, we propose a new model with three hydrogen atoms adsorbed on the graphene sheet in the shape of an equilateral triangle with a hexagon ring surrounded inside. The model reproduces the experimentally observed starlike STM patterns. Read More

We use molecular dynamics simulations to understand the mechanisms of polarization switching in ferroelectric BaTiO$_3$ achieved with external electric field. For tetragonal and orthorhombic ferroelectric phases, we determine the switching paths, and show that polarization rotation through intermediate monoclinic phases (a) facilitates switching at low fields (b) is responsible for a sharp anisotropy in polarization switching. We develop understanding of this through determination of detailed electric field-temperature phase diagrams, that are fundamental to technological applications based on electromechanical and switching response of ferroelectrics. Read More

A newly developed fast molecular-dynamics method is applied to BaTiO3 ferroelectric thin-film capacitors with short-circuited electrodes or under applied voltage. The molecular-dynamics simulations based on a first-principles effective Hamiltonian clarify that dead layers (or passive layers) between ferroelectrics and electrodes markedly affect the properties of capacitors, and predict that the system is unable to hop between a uniformly polarized ferroelectric structure and a striped ferroelectric domain structure at low temperatures. Simulations of hysteresis loops of thin-film capacitors are also performed, and their dependence on film thickness, epitaxial constraints, and electrodes are discussed. Read More

We report a simple safe and attractive pedagogic demonstration with magnetic compasses that facilitates an intuitive understanding of the concept that ferromagnetism and ferroelectricity do not result from dipole-dipole interactions alone. Phonon dispersion relations were calculated for a 3-dimensional simple-cubic dipole crystal and a 2-dimensional square-lattice dipole crystal. The latter calculation corresponds to the compass demonstration that confirm the antiferro ground state structure. Read More

Using first-principles density functional calculations, we study the possible phases of CeMnNi$_{4}$ and show that the ground state is ferromagnetic. We observed the hexagonal phase to be lowest in energy whereas experimentally observed cubic phase lies slightly higher in energy. We optimized the structure in both phases and in all different magnetic states to explore the possibility of the structural and magnetic phase transitions at ground state. Read More

Thermoelectric properties of a nanocontact made of two capped single wall nanotubes (SWNT) are calculated within the tight-binding approximation and Green's function method. It is found that semiconducting nanotubes can have high Seebeck coefficient very near the actual Fermi energy. This in turn leads to very high figures of merit easily exceeding one. Read More

Valley lines on total-energy surfaces for the zone-center distortions of free-standing and in-plane strained SrTiO3 are investigated with a newly developed first-principles structure optimization technique [Jpn. J. Appl. Read More

Surprisingly large spontaneous electric dipole moments recently observed in homonuclear niobium clusters below 100 K (Moro el. al. Science 300, 1265 (2003)) are explained using first-principles electronic structure calculations. Read More

Hund's multiplicity rule is investigated for the carbon atom using quantum Monte Carlo methods. Our calculations give an accurate account of electronic correlation and obey the virial theorem to high accuracy. This allows us to obtain accurate values for each of the energy terms and therefore to give a convincing explanation of the mechanism by which Hund's rule operates in carbon. Read More

An ab initio structure optimization technique is newly developed to determine the valley line on a total-energy surface for zone-center distortions of ferroelectric perovskite oxides and is applied to barium titanate BaTiO3 and lead titanate PbTiO3. The proposed technique is an improvement over King-Smith and Vanderbilt's scheme [Phys. Rev. Read More

An exchange-correlation energy functional $ E_{\mathrm xc} $ and the resultant exchange-correlation potential $ v_{\mathrm xc}({\bf r}) $ in density-functional theory are proposed using orbital-dependent coupling-constant-averaged pair correlation functions, $ {\bar{g}}^{\sigma \sigma'}({\bf r, r'})$ for electronic structure calculations of atoms, molecules, and solids. These orbital-dependent $ {\bar{g}}^{\sigma \sigma'}({\bf r, r'})$ fulfill the symmetric property, the Pauli principle and the sum rules. In the limit of uniform density $ {\bar{g}}^{\sigma \sigma'}({\bf r, r'})$ are reduced to the very accurate analogues of the electron liquid that are obtained from an interpolation between long- and short-range correlations involving the exchange corrections. Read More

The nonlinear capacitance in doped nanotube junctions is calculated self consistently. It decreases as a function of the applied bias when the latter becomes larger than the pseudogap of the nanotube. For this device, one can deduce a relaxation time of about 0. Read More