arXiv subjects

Kikuo Harigaya

Publications and source records attributed to Kikuo Harigaya.

At least 19 recordsLinked to original sources

arXiv2013

Edge states of zigzag graphene nanoribbons with B and N doping at edge atoms

Using a tight binding model, we theoretically study the electronic properties of zigzag boron-carbon-nitride (BCN) nanoribbons where the outermost C atoms of zigzag graphene nanoribbons are replaced with B and N atoms. We show that the flat bands and edge states appear at the Fermi level when the number of B and N atoms are equal except the case of six times supercell. To investigate the origin of the edge states in BCN nanoribbons, we consider the semi-infinite graphene sheet with zigzag edge where outermost C atoms are replaced with B and N atoms. We analytically demonstrate the presence of edge states at the zigzag edges and flat bands using a transfer matrix method.

cond-mat.mtrl-sci↗

arXiv2012

Dependence of atomic arrangement on length of flat bands in zigzag BC2N nanoribbons

We theoretically study the electronic properties of BC2N nanoribbons with zigzag edges using a tight binding model. We show that the zigzag BC2N nanoribbons have the flat bands and edge states when atoms are arranged as B-C-N-C along the zigzag lines. The length of the flat bands in the wavevector space depends on the atomic arrangement. This property can be explained by the deviation of the linear dispersion of the BC2N sheet from K point of the honeycomb lattice. The charge distributions in the edge states depend on the atomic arrangement. We also show that the charge distribution of the edge states in zigzag BC2N nanoribbons where the outermost sites are occupied with B and N atoms is different from those in conventional graphene zigzag edge. Such charge distribution causes different magnetic structures. We investigate the magnetic structure of BC2N nanoribbons with zigzag edges using the Hubbard model within a mean field approximation. At the zigzag edge where the outermost sites are occupied with B and N atoms, ferromagnetic structure appears when the site energies are larger than the on-site Coulomb interaction.

cond-mat.mtrl-sci↗

arXiv2012

Appearance of Flat Bands and Edge States in Boron-Carbon-Nitride Nanoribbons

Presence of flat bands and edge states at the Fermi level in graphene nanoribbons with zigzag edges is one of the most interesting and attracting properties of nanocarbon materials but it is believed that they are quite fragile states and disappear when B and N atoms are doped at around the edges. In this paper, we theoretically investigate electronic and magnetic properties of boron-carbon-nitride (BCN) nanoribbons with zigzag edges where the outermost C atoms on the edges are alternately replaced with B and N atoms using the first principles calculations. We show that BCN nanoribbons have the flat bands and edge states at the Fermi level in both H_2 rich and poor environments. The flat bands are similar to those at graphene nanoribbons with zigzag edges, but the distributions of charge and spin densities are different between them. A tight binding model and the Hubbard model analysis show that the difference in the distribution of charge and spin densities is caused by the different site energies of B and N atoms compared with C atoms.

cond-mat.mes-hall↗

arXiv2012

Possible magnetic states in buckybowl molecules

Possible magnetic properties are studied in the buckybowl molecules: the sumanene and a part of C_60. The Hubbard model is applied to the systems. We find that the molecular structure determines the magnetism in the sumanene. On the other hand, the edge state is found along the zigzag edge of a part of C_60. Therefore, the novel property, transition from molecular magnetism to the magnetism like in nanographene, is found.

cond-mat.mtrl-sci↗

arXiv2011

Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe_4N, and Half-Metallic Ferromagnet: A Systematic Analysis

We theoretically analyze the anisotropic magnetoresistance (AMR) effects of bcc Fe (+), fcc Co (+), fcc Ni (+), Fe$_4$N (-), and a half-metallic ferromagnet (-). The sign in each ( ) represents the sign of the AMR ratio observed experimentally. We here use the two-current model for a system consisting of a spin-polarized conduction state and localized d states with spin--orbit interaction. From the model, we first derive a general expression of the AMR ratio. The expression consists of a resistivity of the conduction state of the $σ$ spin ($σ=\uparrow$ or $\downarrow$), $ρ_{s σ}$, and resistivities due to s--d scattering processes from the conduction state to the localized d states. On the basis of this expression, we next find a relation between the sign of the AMR ratio and the s--d scattering process. In addition, we obtain expressions of the AMR ratios appropriate to the respective materials. Using the expressions, we evaluate their AMR ratios, where the expressions take into account the values of $ρ_{s \downarrow}/ρ_{s \uparrow}$ of the respective materials. The evaluated AMR ratios correspond well to the experimental results.

cond-mat.mtrl-sci↗

arXiv2011

Electron density distribution of bilayer nanographene and band structures of boron-carbon-nitride systems

Bilayer graphene nanoribbon with zigzag edge is investigated with the tight binding model. Two stacking structures, alpha and beta, are considered. The band splitting is seen in the alpha structure, while the splitting in the wave number direction is found in the beta structure. The local density of states in the beta structure tend to avoid sites where interlayer hopping interactions are present. The calculation is extended to the boron-carbon-nitride systems. The qualitative properties persist when zigzag edge atoms are replaced with borons and nitrogens.

cond-mat.mes-hall↗

arXiv2011

Edge States and Stacking Effects in Nanographene Systems

cond-mat.mes-hall↗

arXiv2010

Spin-Atomic Vibration Interaction and Spin-Flip Hamiltonian of a Single Atomic Spin in a Crystal Field

We derive the spin-atomic vibration interaction $V_{\rm SA}$ and the spin-flip Hamiltonian $V_{\rm SF}$ of a single atomic spin in a crystal field. We here apply the perturbation theory to a model with the spin-orbit interaction and the kinetic and potential energies of electrons. The model also takes into account the difference in vibration displacement between an effective nucleus and electrons, $Δ{\boldmath $r$}$. Examining the coefficients of $V_{\rm SA}$ and $V_{\rm SF}$, we first show that $V_{\rm SA}$ appears for $Δ{\boldmath $r$}$$\ne$0, while $V_{\rm SF}$ is present independently of $Δ{\boldmath $r$}$. As an application, we next obtain $V_{\rm SA}$ and $V_{\rm SF}$ of an Fe ion in a crystal field of tetragonal symmetry. It is found that the magnitudes of the coefficients of $V_{\rm SA}$ can be larger than those of the conventional spin-phonon interaction depending on vibration frequency. In addition, transition probabilities per unit time due to $V_{\rm SA}$ and $V_{\rm SF}$ are investigated for the Fe ion with an anisotropy energy of $-|D|S_Z^2$, where $D$ is an anisotropy constant and $S_Z$ is the $Z$ component of a spin operator.

cond-mat.mes-hall↗

arXiv2010

Theoretical Calculation of Shrinking and Stretching in Bond Structure of Monolayer Graphite Flake via Hole Doping Treatment

This paper deals with the physics of monolayer graphite, with a particular focus on the electronics and structural properties. In contrast to the previous electronic band structure of doped single-walled carbon nanotube calculation, where just a ballistic graphite plate is considered, here for the carbon bonds length alteration is calculated in terms of hole doping. It is found that doped holes play crucial roles on the bond structure compared to that obtained no doping configurations and it changes as hole doping increasing.

cond-mat.mes-hall↗

arXiv2009

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes

Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.

cond-mat.mtrl-sci↗

arXiv2009

Persistence of Edge-State in Stacked Graphene and Nano-Graphene Materials

Nano-carbon materials are investigated intensively. In this paper, the edge-state in nanographene materials with zigzag edges is studied theoretically. In particular, while the inter-layer interactions are considered, we prove that edge states exist at the energy of the Dirac point in the doubly stacked nanographene, and in the case of the infinitely-wide lower layer case. This property applies both for the A-B and A-C stackings.

cond-mat.mtrl-sci↗

arXiv2009

What is the Shape Effect on the (Hyper)polarizabilities? A Comparison Study on the Moebius, Cyclic, and Linear Nitrogen-Substituted Polyacenes

How does the framework shape influence the static polarizability (alpha_0) and the first hyperpolarizability (beta_0)? This work, for the first time, presents a comparison study at the MP2/6-31+G(d) level, by using the nitrogen-substituted polyacenes as models: the Moebius strip with a knot, the cyclic strip without knot, and the linear strip. Opening the knot of the Moebius strip to form the cyclic strip, it leads to the increase of the alpha_0 from 268 au to 323 au and the beta_0 value increases about three times from 393 (Moebius) to 1049 au (cyclic). Further, opening the cyclic strip to form the linear strip, the alpha_0 value increases from 323 au to 476 au. While the beta_0 value as well increases about three times from 1049 (cyclic) to 2814 au (linear). The changes in the static (hyper)polarizabilities are well explained by the geometrical differences among the Moebius, cyclic, and linear nitrogen-substituted polyacenes.

physics.chem-ph↗

arXiv2009

Knot-isomers of Moebius Cyclacene: How Does the Number of Knots Influence the Structure and First Hyperpolarizability?

Four large ring molecules composed by 15 nitrogen-substituted benzene rings, named as "knot-isomers of Moebius cyclacene", i.e. non-Moebius cyclacenes without a knot (0), Moebius cyclacenes with a knot (1), non-Moebius cyclacenes with two knots (2), and Moebius cyclacenes with three knots (3), are systematically studied for their structures and nonlinear optical properties. The first hyperpolarizability (beta_0) values of these four knot-isomers structures are 4693 (0) < 10484 (2) < 25419 (3) < 60846 au (1). The beta_0 values (60846 for 1, 10484 for 2 and 25419 au for 3) of the knot-isomers with knot(s) are larger than that (4693 au for 0) of the knot-isomer without a knot. It shows that the beta_0 value can be dramatically increases (13 times) by introducing the knot(s) to the cyclacenes structures. It is found that introducing knots to cyclacenes is a new means to enhance the first hyperpolarizability.

cond-mat.mtrl-sci↗

arXiv2007

Theoretical study of a localized quantum spin reversal by the sequential injection of spins in a spin quantum dot

This is a theoretical study of the reversal of a localized quantum spin induced by sequential injection of spins for a spin quantum dot that has a quantum spin. The system consists of ``electrode/quantum well(QW)/dot/QW/electrode" junctions, in which the left QW has an energy level of conduction electrons with only up-spin. We consider a situation in which up-spin electrons are sequentially injected from the left electrode into the dot through the QW and an exchange interaction acts between the electrons and the localized spin. To describe the sequentially injected electrons, we propose a simple method based on approximate solutions from the time-dependent Schr$\ddot{\rm o}$dinger equation. Using this method, it is shown that the spin reversal occurs when the right QW has energy levels of conduction electrons with only down-spin. In particular, the expression of the reversal time of a localized spin is derived and the upper and lower limits of the time are clearly expressed. This expression is expected to be useful for a rough estimation of the minimum relaxation time of the localized spin to achieve the reversal. We also obtain analytic expressions for the expectation value of the localized spin and the electrical current as a function of time. In addition, we found that a system with the non-magnetic right QW exhibits spin reversal or non-reversal depending on the exchange interaction.

cond-mat.mes-hall↗

arXiv2006

A theoretical analysis on highly spin-polarized transport of iron nitride Fe_4N

In order to propose a ferromagnet exhibiting highly spin-polarized transport, we theoretically analyzed the spin polarization ratio of the conductivity of the bulk Fe$_4$N with a perovskite type structure, in which N is located at the body center position of fcc-Fe. The spin polarization ratio is defined by $P = (σ_\uparrow - σ_\downarrow) / (σ_\uparrow + σ_\downarrow )$, with $σ_{\uparrow(\downarrow)}$ being the conductivity at zero temperature of the up spin (down spin). The conductivity is obtained by using the Kubo formula and the Slater-Koster tight binding model, where parameters are determined from the least-square fitting of the dispersion curves by the tight binding model to those by the first principles calculation. In the vicinity of the Fermi energy, $|P|$ takes almost 1.0, indicating perfectly spin-polarized transport. In addition, by comparing Fe$_4$N to fcc-Fe (Fe$_4$N$_0$) in the ferromagnetic state with the equilibrium lattice constant of Fe$_4$N, it is shown that the non-magnetic atom N plays an important role in increasing $|P|$.

cond-mat.mtrl-sci↗

arXiv2005

Calculations of Electric Capacitance in Carbon and BN Nanotubes, and Zigzag Nanographite (BN, BCN) Ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The electronic states with the opposite electric charges separated along both edges are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano-functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons, the electronic structure is always like of semiconductors. The calculated capacitance is inversely proportional to the distance between the positive and negative electrodes.

quant-ph↗

arXiv2005

Electric capacitance as nanocondensers in zigzag nanographite ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The nearest Coulomb interactions stabilize electronic states with the opposite electric charges separated and localized along both edges. Such states are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations.

cond-mat.mtrl-sci↗

arXiv2005

Possible charge ordered states in BN and BCN nanotubes, and nanoribbons

Electronic states in boron-nitride and boron-carbon-nitride nanoribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The charge and spin polarized states are considered, and the phase diagram between two states is obtained. Next, the electric capacitance is calculated in order to examine the nano-functionalities of the system. Due to the presence of the strong site energies, the charge polarized state overcomes the spin polarized states, giving the large difference of the phase diagram in comparison with that of graphite system. The electronic structures are always like of semiconductors. The capacitance calculated for the charge polarized state with the realistic values of the Coulomb interactions is inversely proportional to the ribbon width, owing to the presence of the charge excitation energy gap.

cond-mat.str-el↗