Searcharxiv⌕ Search

arXiv subjects

Hiroaki Nakamura

Publications and source records attributed to Hiroaki Nakamura.

54 records · Page 3Linked to original sources
arXiv2007

Non-Abelian gauge field theory of the spin-orbit interaction and a perfect spin filter

We point out that the Rashba and Dresselhaus spin-orbit interactions in two dimensions can be regarded as a Yang-Mills non-Abelian gauge field. The physical field generated by the gauge field gives the electron wave function a spin-dependent phase which is frequently called the Aharonov-Casher phase. Applying on an AB ring this non-Abelian field together with the usual vector potential, we can make the interference condition completely destructive for one component of the spin while completely constructive for the other component of the spin over the entire energy range. This enables us to construct a perfect spin filter.

quant-ph↗
arXiv2006

Gap-mediated magnetization of a pseudo-one-dimensional system with a spin-orbit interaction

We argue that a pseudo-one-dimensional electron gas is magnetized when a voltage bias is applied with the Fermi level tuned to be in the energy gap generated by a spin-orbit interaction. The magnetization is an indication of spin-carrying currents due to the spin-orbit interaction. The origin of the magnetization, however, is essentially different from the "spin accumulation" in two-dimensional systems with spin-orbit interactions.

cond-mat.mes-hall↗
arXiv2006

Microscopic Energy Current Field with Multi--body Force in Hamiltonian System

Microscopic energy current can be derived from microscopic energy field localized only in particle positions. If the energy current is applied to classical molecular dynamics, it is expected to become a new information. However, multi-body force except two-body force causes obscurity when multi-body interaction energy is localized. In the present study, our new method enables to determine the localization of the multi--body interaction energy. We obtain the energy current between particles by the law of the multi--body force corresponding to Newton's third law on two--body force.

cond-mat.stat-mech↗
arXiv2006

Molecular Dynamics Simulation of Sputtering Process of Hydrogen and Graphene Sheets

To clarify the yielding mechanism of small hydrocarbon molecules in chemical sputtering between hydrogen and graphene sheets, we made classical molecular dynamics simulation with modified Brenner's REBO potential which we proposed to deal with chemical reaction. As the simulation model, we prepared more realistic physical system, which is composed of 160 incident hydrogen atoms and ten graphene multilayers, than our previous model. From the present work, we found the following fact: breaking the covalent bonds between carbon atoms by hydrogen does not play an important role during destruction process of graphene structure, but momentum transfer from incident hydrogen to graphene causes to destroy graphene structure. Moreover, it is found that almost all fragments of graphene sheets form chain-shaped molecules, and that yielded hydrocarbon molecules are composed of carbon chain and single hydrogen-atom.

cond-mat.mtrl-sci↗
arXiv2006

Supramolecule Structure for Amphiphilic Molecule by Dissipative Particle Dynamics Simulation

Meso-scale simulation of structure formation for AB-dimers in solution W monomers was performed by dissipative particle dynamics (DPD) algorithm. As a simulation model, modified Jury Model was adopted Jury, S. et al. "Simulation of amphiphilic mesophases using dissipative particle dynamics," Phys. Chem. Chem. Phys. vol.1(1999) pp. 2051-2056, which represents mechanics of self-assembly for surfactant hexaethylene glycol dodecyl ether (C12E6) and water(H2 O). The same phase diagram as Jury's result was obtained. We also found that it takes a longer time to form the hexagonal phase (H1) than to form the lamellar phase (Lalpha).

cond-mat.soft↗
arXiv2006

Phase Diagram for Self-assembly of Amphiphilic Molecule C12E6 by Dissipative Particle Dynamics Simulation

In a previous study, dissipative particle dynamics simulation was used to qualitatively clarify the phase diagram of the amphiphilic molecule hexaethylene glycol dodecyl ether (C12E6). In the present study, the hydrophilicity dependence of the phase structure was clarified qualitatively by varying the interaction potential between hydrophilic molecules and water molecules in a dissipative particle dynamics (DPD) simulation using the Jury model. By varying the coefficient of the interaction potential $x$ between hydrophilic beads and water molecules as x=-20, 0, 10, and 20, at a dimensionless temperature of T=0.5 and a concentration of amphiphilic molecules in water of phi=50% the phase structures grew to lamellar (x=-20), hexagonal (x=0), and micellar (x=10) phases. For x=20, phase separation occurs between hydrophilic beads and water molecules.

cond-mat.soft↗
arXiv2006

Dependence of self-assembled amphiphile structure on interaction between hydrophilic groups

In a previous study (Comp. Phys. Com.(2005) Vol.169, 139-143), we clarified the dependence of the phase structure on the hydrophilicity of an amphiphilic molecule by varying the interaction potential between the hydrophilic molecule and water(aAA) in a dissipative particle dynamics (DPD) simulation using the Jury model. In the present paper, we perform another DPD simulation using the previous model to investigate the dependence of the interaction potential between adjacent hydrophilic groups on the phase structure. By varying the coefficient of the interaction potential between adjacent hydrophilic groups aAA (aAA=15,25,40, and 250) at a dimensionless temperature of T=0.5 and a concentration of amphiphilic molecules in water of phi=50%, hexagonal (aAA=14,25,40) and micellar (aAA=250) phases were observed. In comparison with the previous results, the dependence of the A-B dimer's shape on aAA was determined to be weaker than that on aAA. Therefore, it is concluded that the solvent water W plays an important role in aggregation of the A-B dimers.

cond-mat.soft↗
arXiv2005

Quantum Nernst Effect

We report our recent predictions on the quantum Nernst effect, a novel thermomagnetic effect in the quantum Hall regime. We assume that, when the chemical potential is located between a pair of neighboring Landau levels, edge currents convect around the system. This yields theoretical predictions that the Nernst coefficient is strongly suppressed and the thermal conductance is quantized. The present system is a physical realization of the non-equilibrium steady state.

cond-mat.mes-hall↗
arXiv2005

Quantum Nernst Effect

It is theoretically predicted that the Nernst coefficient is strongly suppressed and the thermal conductance is quantized in the quantum Hall regime of the two-dimensional electron gas. The Nernst effect is the induction of a thermomagnetic electromotive force in the $y$ direction under a temperature bias in the $x$ direction and a magnetic field in the $z$ direction. The quantum nature of the Nernst effect is analyzed with the use of a circulating edge current and is demonstrated numerically. The present system is a physical realization of the non-equilibrium steady state.

cond-mat.mes-hall↗
arXiv2000

Design and optimization of tapered structure of near-field fiber probe based on FDTD simulation

The finite-difference time-domain method was employed to simulate light propagation in tapered near-field fiber probes with small metal aperture. By conducting large-volume simulations, including tapered metal-cladding waveguide and connected optical fiber waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double-tapered probe was reproduced and was ascribed to the shortening of the cutoff region and the efficient coupling to the guiding mode of the optical fiber. The dependence of the efficiency on the tapered structure parameters was also examined.

physics.optics↗
arXiv2000

Rigidity of Orientationally Ordered Domains of Short Chain Molecules

By molecular dynamics simulation, discovered is a strange rigid-like nature for a hexagonally packed domain of short chain molecules. In spite of the non-bonded short-range interaction potential (Lennard-Jones potential) among chain molecules, the packed domain gives rise to a resultant global moment of inertia. Accordingly, as two domains encounter obliquely, they rotate so as to be parallel to each other keeping their overall structures as if they were rigid bodies.

cond-mat.soft↗
arXiv1998

Physical Model of Nernst Element

Generation of electric power by the Nernst effect is a new application of a semiconductor. A key point of this proposal is to find materials with a high thermomagnetic figure-of-merit, which are called Nernst elements. In order to find candidates of the Nernst element, a physical model to describe its transport phenomena is needed. As the first model, we began with a parabolic two-band model in classical statistics. According to this model, we selected InSb as candidates of the Nernst element and measured their transport coefficients in magnetic fields up to 4 Tesla within a temperature region from 270K to 330K. In this region, we calculated transport coefficients numerically by our physical model. For InSb, experimental data are coincident with theoretical values in strong magnetic field.

cond-mat.mtrl-sci↗
arXiv1997

Peltier Current Lead Experiments with a Thermoelectric Semiconductor near 77 K

Peltier current lead was proposed to reduce heat leak from the current lead. The temperature of the hot side of semiconductors was kept to be room temperature and the liquid nitrogen was used to cool the system in the experiment. The experiment confirmed the principle of the Peltier current lead, and the reduction of the heat leak is calculated to be 30 % for the liquid helium system and 40 % for the liquid nitrogen system. We also proposed a new current lead system which is composed of semiconductors and high temperature superconducting material (HTS). This idea bases on the functionally gradient material (FGM), and the HTS is connected to the semiconductor directly. The temperature of the hot side of semiconductor is kept to be the liquid nitrogen temperature, the temperature of HTS can be expected to be lower than 77 K. Therefore, we can expect high current capacity of the HTS and/or high stability of the HTS. We use BiSb as a N-type semiconductor and BiTe as a P-type semiconductor in the experiment, and the temperature of the cold side of the semiconductor is 73 K in this experiment.

cond-mat.mtrl-sci↗
arXiv1997

Transport Coefficients of InSb in a Strong Magnetic Field

Improvement of a superconducting magnet system makes induction of a strong magnetic field easier. This fact gives us a possibility of energy conversion by the Nernst effect. As the first step to study the Nernst element, we measured the conductivity, the Hall coefficient, the thermoelectric power and the Nernst coefficient of the InSb, which is one of candidates of the Nernst elements. From this experiment, it is concluded that the Nernst coefficient is smaller than the theoretical values. On the other hand, the conductivity, the Hall coefficient ant the thermoelectric power has the values expected by the theory.

cond-mat.mtrl-sci↗
arXiv1997

Geometric contribution to the measurement of thermoelectric power and Nernst coefficient in a strong magnetic field

On the measurement of thermoelectric power and Nernst coefficient, we used two kinds of shapes for a sample. One is "Bridge shape" and the other is, we call,"Fat-Bridge shape". The latter has 5 times wider main body than the former. We used pure n-InSb in this experiment, whose carrier condensation measured at 77K was $6.6 \times 10^{-14} cm^{-3}.$ The length of sample is 17mm and temperature difference induced between the edges in that direction were about $10^{\circ}$C or $100^{\circ}$C near a room temperature range of 0 to $100^{\circ}$C. Magnetic induction applied in the perpendicular direction to temperature gradient was in the range of 0 up to 4 Tesla. In the case of "Fat-Bridge shape", we detected about 10% smaller Nernst coefficient and 1 to 10% smaller thermoelectric power comparing to the "Bridge shape". We suppose this phenomena is due to the geometric contribution on the different shape of samples.

cond-mat.mtrl-sci↗