arXiv subjects

Yoshiyuki Kawazoe

Publications and source records attributed to Yoshiyuki Kawazoe.

48 records · Page 3Linked to original sources

arXiv2009

Geometrical indications of adsorbed hydrogen atoms on graphite producing starlike and ellipsoidal features in scanning tunneling microscopy images

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. Additionally, we confirm that an ortho-hydrogen pair is the configuration corresponding to the ellipsoidal images. These calculations reveal that when the hydrogen pairs are in the same orientation, they are energetically more stable.

cond-mat.mtrl-sci↗

arXiv2008

Polarization Rotation, Switching and E-T phase diagrams of BaTiO$_3$: A Molecular Dynamics Study

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.

cond-mat.mtrl-sci↗

arXiv2008

Fast Molecular-Dynamics Simulation for Ferroelectric Thin-Film Capacitors Using a First-Principles Effective Hamiltonian

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.

cond-mat.mtrl-sci↗

arXiv2006

Ferroics: magnetic-compass lattice and optical phonon dispersions of dipolar crystals

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. A sum rule for the three optical phonon frequencies is discussed. A mathematical non-analyticity in the longitudinal optical phonon are also illustrated.

cond-mat.mtrl-sci↗

arXiv2005

Thermoelectric properties of a nanocontact

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. Modifying the properties by shifting the chemical potential can be achieved by doping or application of a (nano-)gate voltage. The presence of impurities near the contact also strongly modify the properties and this can be taken advantage of, in order to design devices with high thermopower and figures of merit.

cond-mat.mtrl-sci↗

arXiv2005

Ab-inito study on different phases of ferromagnetic CeMnNi4

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. We do not find any phase transitions between the magnetic and non-magnetic phases. The calculated structural, magnetic properties of cubic phase are in excellent agreement with experiments. Further, we do not observe half metallic behavior in any of the phases. However, the cubic phase does have fewer density of states for down-spin component giving a possibility of forming half metallic phase artificially, introducing vacancies, and disorder in lattice.

cond-mat.mtrl-sci↗

arXiv2005

ab initio Study of Strain-Induced Ferroelectricity in SrTiO3

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. Phys. vol.43 (2004), p.6785]. The results of numerical calculations confirmed that the ferroelectricity is induced, and the Curie temperature is increased, by applying biaxial compressive or tensile strains. Along the distortion, strong nonlinear coupling between the soft- and hard-modes is demonstrated.

cond-mat.mtrl-sci↗

arXiv2004

Origin of spontaneous electric dipoles in homonuclear niobium clusters

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. The calculated moments for Nb(n) (n <= 15) closely follow the experimental data in which large dipole moments are seen for n = 11-14. We establish that the dipoles are strongly correlated with the geometrical asymmetry of the clusters. The magnitude of the dipole moment is roughly proportional to the spread in the principal moments of inertia and its direction tends to align along the axis of the largest principal moment. Charge deformation densities reveal directional, partially covalent bonds that enhance the formation of asymmetric geometries. Classical simulations of the deflection of a cluster in a molecular beam reveal that the electronic dipole may persist at higher temperatures, but is masked by the rotational dynamics of the cluster.

cond-mat.mtrl-sci↗

arXiv2004

Interpretation of Hund's multiplicity rule for the carbon atom

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. We find that the energy gain in the triplet with respect to the singlet state is due to the greater electron-nucleus attraction in the higher spin state, in accordance with Hartree-Fock calculations and studies including correlation. The method used here can easily be extended to heavier atoms.

cond-mat.str-el↗

arXiv2004

Ab initio Study of Valley Line on a Total-Energy Surface for Zone-Center Distortions of Ferroelectric Perovskite Oxides BaTiO3 and PbTiO3

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. B 49, p.5828 (1994)] of evaluating total energy as a function of the amplitude of atomic displacements. The results of numerical calculations show that total energy can be expressed as a fourth-order function of the amplitude of atomic displacements in BaTiO3 but not in PbTiO3.

cond-mat.mtrl-sci↗

arXiv2003

Nonlinear charging, and transport times in doped nanotubes junctions

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 0.1 femtosecond. Because of its negative differential resistance (NDR), a switching time of less than a fs can also be deduced.

cond-mat.mtrl-sci↗

arXiv2003

Exchange-correlation energy functional constructed from orbital-dependent coupling-constant-averaged pair correlation functions

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}}^{σσ'}({\bf r, r'})$ for electronic structure calculations of atoms, molecules, and solids. These orbital-dependent $ {\bar{g}}^{σσ'}({\bf r, r'})$ fulfill the symmetric property, the Pauli principle and the sum rules. In the limit of uniform density $ {\bar{g}}^{σσ'}({\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. The major contribution of $ v_{\mathrm xc}({\bf r}) $ is given in the form of the Coulomb interaction with the exchange-Coulomb hole around an electron. The present theory not only guarantees local charge neutrality, but also reproduces the exact asymptotic form of the exchange potential, $ v_{\mathrm x}({\bf r}) = - e^2 / r $ for finite systems. The present method of dealing with correlations, if properly applied to finite systems, can give even the asymptotic form of the correlation potential $ v_{\mathrm c}({\bf r}) $ of order $ r^{-4} $ as well as the van der Waals potential of order $ r^{-6} $ for large r.

cond-mat.str-el↗