arXiv subjects

Hiroaki Nakamura

Publications and source records attributed to Hiroaki Nakamura.

At least 37 records · Page 2Linked to original sources

arXiv2018

Temperature dependence of mobility of conducting polymer polyaniline with secondary dopant

The conductivity $σ$ and carrier density $n$ of the conducting polymer polyaniline were investigated by changing the concentration $x$ of a secondary dopant, meta-cresol. We found that $σ$ changes by four orders of magnitude within the $x$-range of 1-10 %, while $n$, as estimated from the Hall measurements, shows a weak dependence on $x$ in the region of 2% <$x$< 50%. These results suggest that $σ$ can be enhanced by the change in the mobility $μ$. We analyzed the temperature dependence of $μ$ by not only the combination of two different types of scattering mechanism, but also by the polaron hopping model. The experimental data of $μ(T)$ can be explained well by the latter model with reasonable fitting parameter values of a small-polaron binding energy and a longitudinal optical phonon frequency.

cond-mat.mtrl-sci↗

arXiv2015

Polylogarithmic analogue of the Coleman-Ihara formula, I

The Coleman-Ihara formula expresses Soule's $p$-adic characters restricted to $p$-local Galois group as the Coates-Wiles homomorphism multiplied by $p$-adic $L$-values at positive integers. In this paper, we show an analogous formula that $\ell$-adic polylogarithmic characters for $\ell=p$ restrict to the Coates-Wiles homomorphism multiplied by Coleman's $p$-adic polylogarithms at any roots of unity of order prime to $p$.

math.NT↗

arXiv2012

Current-induced cooling phenomenon in a two-dimensional electron gas under a magnetic field

We investigate the spatial distribution of temperature induced by a dc current in a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We numerically calculate the distributions of the electrostatic potential phi and the temperature T in a 2DEG enclosed in a square area surrounded by insulated-adiabatic (top and bottom) and isopotential-isothermal (left and right) boundaries (with phi_{left} < phi_{right} and T_{left} =T_{right}), using a pair of nonlinear Poisson equations (for phi and T) that fully take into account thermoelectric and thermomagnetic phenomena, including the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. We find that, in the vicinity of the left-bottom corner, the temperature becomes lower than the fixed boundary temperature, contrary to the naive expectation that the temperature is raised by the prevalent Joule heating effect. The cooling is attributed to the Ettingshausen effect at the bottom adiabatic boundary, which pumps up the heat away from the bottom boundary. In order to keep the adiabatic condition, downward temperature gradient, hence the cooled area, is developed near the boundary, with the resulting thermal diffusion compensating the upward heat current due to the Ettingshausen effect.

cond-mat.mes-hall↗

arXiv2012

Formation and Classification of Amorphous Carbon by Molecular Dynamics Simulation

By using molecular dynamics simulation, formation mechanisms of amorphous carbon in particular sp${}^3$ rich structure was researched. The problem that reactive empirical bond order potential cannot represent amorphous carbon properly was cleared in the transition process from graphite to diamond by high pressure and the deposition process of amorphous carbon thin films. Moreover, the new potential model which is based on electron distribution simplified as a point charge was developed by using downfolding method. As a result, the molecular dynamics simulation with the new potential could demonstrate the transition from graphite to diamond at the pressure of 15 GPa corresponding to experiment and the deposition of sp${}^3$ rich amorphous carbon.

cond-mat.mtrl-sci↗

arXiv2010

Molecular Dynamics Simulation of Chemical Vapor Deposition of Amorphous Carbon: Dependence on H/C Ratio of Source Gas

By molecular dynamics simulation, the chemical vapor deposition of amorphous carbon onto graphite and diamond surfaces was studied. In particular, we investigated the effect of source H/C ratio, which is the ratio of the number of hydrogen atoms to the number of carbon atoms in a source gas, on the deposition process. In the present simulation, the following two source gas conditions were tested: one was that the source gas was injected as isolated carbon and hydrogen atoms, and the other was that the source gas was injected as hydrocarbon molecules. Under the former condition, we found that as the source H/C ratio increases, the deposition rate of carbon atoms decreases exponentially. This exponential decrease in the deposition rate with increasing source H/C ratio agrees with experimental data. However, under the latter molecular source condition, the deposition rate did not decrease exponentially because of a chemical reaction peculiar to the type of hydrocarbon in the source gas.

cond-mat.mtrl-sci↗

arXiv2010

Hybrid Simulation between Molecular Dynamics and Binary Collision Approximation Codes for Hydrogen injection onto Carbon Materials

Molecular dynamics (MD) simulation with modified Brenner's reactive empirical bond order (REBO) potential is a powerful tool to investigate plasma wall interaction on divertor plates in a nuclear fusion device. However, MD simulation box's size is less than several nm for the performance of a computer. To extend the size of the MD simulation, we develop a hybrid simulation code between MD code using REBO potential and binary collision approximation (BCA) code. Using the BCA code instead of computing all particles with a high kinetic energy for every step in the MD simulation, considerable computation time is saved. By demonstrating a hydrogen atom injection on a graphite by the hybrid simulation code, it is found that the hybrid simulation code works efficiently in a large simulation box.

cond-mat.mtrl-sci↗

arXiv2010

Extension of the simulation code ACAT to treat real atomic positions

We have investigated plasma-surface interactions with molecular dynamics (MD) simulations. It, however, is high cost computation and is limited to simulations for materials of nanometer order. In order to overcome the limitation, a complementary model based on binary collision approximation (BCA) can be established. We employed a BCA-based simulation code ACAT and extended to handle any structure involving crystalline and amorphous. The extended code, named "ACaT", stores all positions of projectile and target atoms and velocities of recoil atoms, so it can be combined with the MD code. It also holds the potential to reproduce channeling phenomena. Thus it is expected to be useful for evaluation of channeling effects.

cond-mat.mtrl-sci↗

arXiv2009

Incident angle dependence of reactions between graphene and hydrogen atom by molecular dynamics simulation

Incident angle dependence of reactions between graphene and hydrogen atoms are obtained qualitatively by classical molecular dynamics simulation under the NVE condition with modified Brenner reactive empirical bond order (REBO) potential. Chemical reaction depends on two parameters, i.e., polar angle $θ$ and azimuthal angle $ϕ$ of the incident hydrogen. From the simulation results, it is found that the reaction rates strongly depend on polar angle $θ$. Reflection rate becomes larger with increasing $θ$, and the $θ$ dependence of adsorption rate is also found. The $θ$ dependence is caused by three dimensional structure of the small potential barrier which covers adsorption sites. $ϕ$ dependence of penetration rate is also found for large $θ$.

physics.plasm-ph↗

arXiv2009

Quantum Nernst Effect in a Bismuth Single Crystal

We calculate the phonon-drag contribution to the transverse (Nernst) thermoelectric power $S_{yx}$ in a bismuth single crystal subjected to a quantizing magnetic field. The calculated heights of the Nernst peaks originating from the hole Landau levels and their temperature dependence reproduce the right order of magnitude for those of the pronounced magneto-oscillations recently reported by Behnia et al [Phys.Rev.Lett. 98, 166602 (2007)]. A striking experimental finding that $S_{yx}$ is much larger than the longitudinal (Seebeck) thermoelectric power $S_{xx}$ can be naturally explained as the effect of the phonon drag, combined with the well-known relation between the longitudinal and the Hall resistivity $rho_{xx}>>|rho_{yx}|$ in a semi-metal bismuth. The calculation that includes the contribution of both holes and electrons suggests that some of the hitherto unexplained minor peaks located roughly at the fractional filling of the hole Landau levels are attributable to the electron Landau levels.

cond-mat.other↗

arXiv2009

Fundamental relation between longitudinal and transverse conductivities in the quantum Hall system

We investigate the relation between the diagonal ($σ_{xx}$) and off-diagonal ($σ_{xy}$) components of the conductivity tensor in the quantum Hall system. We calculate the conductivity components for a short-range impurity potential using the linear response theory, employing an approximation that simply replaces the self-energy by a constant value $-i \hbar /(2 τ)$ with $τ$ the scattering time. The approximation is equivalent to assuming that the broadening of a Landau level due to disorder is represented by a Lorentzian with the width $Γ= \hbar /(2 τ)$. Analytic formulas are obtained for both $σ_{xx}$ and $σ_{xy}$ within the framework of this simple approximation at low temperatures. By examining the leading terms in $σ_{xx}$ and $σ_{xy}$, we find a proportional relation between $\mathrm{d}σ_{xy}/\mathrm{d}B$ and $B σ_{xx}^2$. The relation, after slight modification to account for the long-range nature of the impurity potential, is shown to be in quantitative agreement with experimental results obtained in the GaAs/AlGaAs two-dimensional electron system at the low magnetic-field regime where spin splitting is negligibly small.

cond-mat.mes-hall↗

arXiv2008

Quantum Nernst effect in a bismuth single crystal

We report a theoretical calculation explaining the quantum Nernst effect observed experimentally in a bismuth single crystal. Generalizing the edge-current picture in two dimensions, we show that the peaks of the Nernst coefficient survive in three dimensions due to a van Hove singularity. We also evaluate the phonon-drag effect on the Nernst coefficient numerically. Our result agrees with the experimental result for a bismuth single crystal.

cond-mat.mes-hall↗

arXiv2008

Energy Current with Multi--body Interaction using Dirac Delta Function

Energy density and energy flux was introduced along Takesue's method. Particle energies were localized at particle positions using Dirac delta function. The energy density was connected with the energy flux by continuity equation. New method was proposed to deal with multi--body interaction. Consequently, the energy current between particles could be calculated even if the multi--body interaction appeared. Moreover, An application to the molecular dynamics simulation of hydrogen adsorption on a graphene with modified Brenner reactive empirical bond order potential was demonstrated.

cond-mat.stat-mech↗

arXiv2007

Molecular Dynamics Simulation of Plasma Surface Interaction

New interlayer intermolecular potential model was proposed and it represented ``ABAB'' staking of graphite. Hydrogen atom sputtering on graphite surface was investigated using molecular dynamics simulation. In the initial short time period, maintaining the flat structure of graphenes, hydrogen atoms brought about the difference interaction process in each incident energy. The first graphene often adsorbed 5 eV hydrogen atoms and reflected almost all of 15 eV hydrogen atoms. The hydrogen atoms which were injected at 30 eV penetrated into the inside of the graphite surface and were adsorbed between interlayer. The desorption of C2H2 on the clear graphite surface was observed in only the case incident at 5 eV. The animation of the MD simulation and radial distribution function indicated that the graphenes were peeled off one by one at regular interval. In common to the incident energy, the yielded molecules often had chain structures terminated by hydrogen atoms. The erosion yield increased compared with the case of no interlayer intermolecular force.

cond-mat.mtrl-sci↗

arXiv2007

Hydrogen Adsorption of Back Side of Graphene

We studied the interaction between a single hydrogen atom and a single graphene using classical molecular dynamics simulation with modified Brenner REBO potential. Three interactions, which are adsorption, reflection, penetration, were observed. Overhang structure appears and creates an adsorption site on the backside of the graphene. It is considered that backside adsorption occurs under the two conditions that an incident hydrogen atom should have incident energy which is larger than the potential barrier of a hexagonal hole of the graphene and that after the hydrogen atom passes through the graphene, it does not keep its kinetic energy to be trapped by the adsorption site. The conditions explained that as the incident energy increased, the incident point of the backside adsorption shifted to the periphery of a hexagonal hole of the graphene in the simulation. Moreover, when a hexagonal hole of the graphene was expanded by the hydrogen atom incidence to the periphery of the hexagonal hole, its potential barrier was reduced.

cond-mat.mtrl-sci↗

arXiv2007

Molecular Dynamics Simulation of the Hydrogen Isotope Sputtering of Graphite

We used a molecular dynamics simulation with the modified Brenner reactive empirical bond order potential to investigate the erosion of a graphite surface due to the incidence of hydrogen, deuterium, and tritium atoms. Incident particles cause pressure on the graphite surface, and the chemical bond between graphene layers then generates heat to erode the graphite surface. We evaluated the speed of surface destruction by calculating the pseudo-radial distribution function. The speed of surface destruction due to incident hydrogen isotopes was higher than that due to hydrogen atoms. The surface destruction increased exponentially and its decay time constant was a power function of the incident energy. We measured the erosion yield, which indicated a steady state for the graphite erosion. The erosion yield flux in the steady state increased linearly with the incident energy. The erosion yield flux was almost independent of the type of incident particle, and the erosion yield start time was smaller for hydrogen isotopes than for hydrogen atoms.

cond-mat.mtrl-sci↗

arXiv2007

Quasi-bound states in continuum

We report the prediction of quasi-bound states (resonant states with very long lifetimes) that occur in the eigenvalue continuum of propagating states for a wide region of parameter space. These quasi-bound states are generated in a quantum wire with two channels and an adatom, when the energy bands of the two channels overlap. A would-be bound state that lays just below the upper energy band is slightly destabilized by the lower energy band and thereby becomes a resonant state with a very long lifetime (a second QBIC lays above the lower energy band).

quant-ph↗

arXiv2007

Some properties of the resonant state in quantum mechanics and its computation

The resonant state of the open quantum system is studied from the viewpoint of the outgoing momentum flux. We show that the number of particles is conserved for a resonant state, if we use an expanding volume of integration in order to take account of the outgoing momentum flux; the number of particles would decay exponentially in a fixed volume of integration. Moreover, we introduce new numerical methods of treating the resonant state with the use of the effective potential. We first give a numerical method of finding a resonance pole in the complex energy plane. The method seeks an energy eigenvalue iteratively. We found that our method leads to a super-convergence, the convergence exponential with respect to the iteration step. The present method is completely independent of commonly used complex scaling. We also give a numerical trick for computing the time evolution of the resonant state in a limited spatial area. Since the wave function of the resonant state is diverging away from the scattering potential, it has been previously difficult to follow its time evolution numerically in a finite area.

quant-ph↗

arXiv2007

Chemical Reaction between Single Hydrogen Atom and Graphene

We study chemical reaction between a single hydrogen atom and a graphene, which is the elemental reaction between hydrogen and graphitic carbon materials. In the present work, classical molecular dynamics simulation is used with modified Brenner's empirical bond order potential. The three reactions, that is, absorption reaction, reflection reaction and penetration reaction, are observed in our simulation. Reaction rates depend on the incident energy of the hydrogen atom and the graphene temperature. The dependence can be explained by the following mechanisms: (1) The hydrogen atom receives repulsive force by pi-electrons in addition to nuclear repulsion. (2) Absorbing the hydrogen atom, the graphene transforms its structure to the ``overhang'' configuration such as sp-3 state. (3) The hexagonal hole of the graphene is expanded during the penetration of the hydrogen atom.

cond-mat.other↗