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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... expand abstract 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. collapse abstract
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Edge States and Stacking Effects in Nanographene 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 ... expand abstractis found in the beta structure. The local density of states in the beta structure tend to avoid sites where inter-layer hopping interactions are present. collapse abstract
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J. Phys. Soc. Jpn. 79 (2010) 114721-1 - 114721-24
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... expand abstract potential energies of electrons. The model also takes into account the difference in vibration displacement between an effective nucleus and electrons, $\Delta {{\boldmath $r$}}$. Examining the coefficients of $V_{\rm SA}$ and $V_{\rm SF}$, we first show that $V_{\rm SA}$ appears for $\Delta {{\boldmath $r$}}$$\ne$0, while $V_{\rm SF}$ is present independently of $\Delta {{\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. collapse abstract
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physica status solidi (c) 2010 Oct; 7(11‐12)
Anisotropy energy, spin‐atomic vibration interaction, and spin‐flip Hamiltonian of a single atomic spin system: application to iron ion
No abstract is available for this article.
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Journal of Computational and Theoretical Nanoscience, Volume 8, 1-7, (2011)
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 ba... expand abstractllistic 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. collapse abstract
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European Physical Journal B 69, 523-528 (2009)
Effects of initial compression stress on wave propagation in carbon nanotubes
An analytical method to investigate wave propagation in single- and double- walled carbon nanotubes under initial compression stress is presented. The nanotube structures are treated within the multilayer thin shell approximation with the elastic p... expand abstractroperties taken to be those of the graphene sheet. The governing equations are derived based on Flugge equations of motion. Frequency equations of wave propagation in single and double wall carbon nanotubes are described through the effects of initial compression stress and van der Waals force. To show the effects of Initial compression stress on the wave propagation in nanotubes, the symmetrical mode can be analyzed based on the present elastic continuum model. It is shown that the wave speed are sensitive to the compression stress especially for the lower frequencies. collapse abstract
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Int. J. Mod. Phys. B 13 (1999) 2531-2544
Optical excitations in diphenylacetylene based dendrimers studied by a coupled exciton model with off-diagonal disorder
A phenomenological coupled exciton model is proposed in order to characterize optical excitations in extended dendrimers. An onsite exciton state is assigned at each phenyl rings and a nearest neighbor hopping integral which obeys the Gaussian dist... expand abstractribution is considered between the exciton states. The decreasing optical excitation energy with respect to the dendrimer size indicates the presence of exciton funnels along the backbone of the dendrimers. Therefore, the extended dendrimers can work as artificial fractal antenna systems which capture energy of light. Dynamics of an exciton is also investigated by solving time development of a wavefunction of optical excitations. It is shown that a damping term with a certain magnitude is necessary in order that optical excitations captured at the outer edge of the supermolecule move to the central areas of the molecule. collapse abstract
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AIP Conference Proceedings Vol 633(1) pp. 401-404 (2002)
Optical excitations in hexagonal nanonetwork materials
Optical excitations in hexagonal nanonetwork materials, for example, Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. The bonding of BN systems is positively polarized at the B site, and is negatively polarized at the N site... expand abstract. There is a permanent electric dipole moment along the BN bond, whose direction is from the B site to the N site. When the exciton hopping integral is restricted to the nearest neighbors, the flat band of the exciton appears at the lowest energy. The higher optical excitations have excitation bands similar to the electronic bands of graphene planes and carbon nanotubes. The symmetry of the flat exciton band is optically forbidden, indicating that the excitons related to this band will show quite long lifetime which will cause strong luminescence properties. collapse abstract
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Jpn. J. Appl. Phys. 33, L1093 (1994)
Optical absorption spectra of A6C60: Reduction of effective Coulombinteractions
Optical absorption spectra of C60^6- are theoretically investigated in order to analyze the optical properties of alkali metal doped fullerites A6C60. We use a tight binding model with long ranged Coulomb interactions and bond disorder. Optical spe... expand abstractctra are obtained by the Hartree-Fock approximation and the CI method. The Coulomb interaction parameters which are relevant to the optical spectra of A6C60 are almost the half of those of the neutral C60. The reduction of the effective Coulomb interactions is concluded for the heavily doped case of C60. collapse abstract
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physica status solidi (c) 2009 Sep; 6(10)
Switching of a single atomic spin induced by spin injection: Effect of spin relaxation
Toward the manipulation of a single atomic spin, we theoretically study the switching of a localized quantum spin induced by spin injection. The system consists of electrode/spin quantum dot/electode junctions, in which the left electrode (right elec... expand abstracttrode) has conduction electrons with up spin (with down spin), and the dot has the localized quantum spin S = (Sx; Sy; Sz). This S has a uniaxial anisotropic energy, –|D|Sz2, which shows the bistable potential between Sz = –S and S, with D being an anisotropy constant. Furthermore, S interacts with the atomic vibration. For the initial state of Sz = –S, we consider a situation in which up spin electrons exhibit the spin-flip tunneling from the left electrode to the right one through an exchange interaction between the electron spin and S. We obtain the time dependence of the current, the expectation value of Sz, and that of the vibration quantum number using the master equation approach, in which a parameter related to the spin relaxation time, τ, is taken into account. On the basis of recent experimental results about an Fe atom on CuN surface [C. F. Hirjibehedin et al., Science 317, 1197 (2007)], we apply this theory to systems of S =2. As results, systems exhibit the switching or nonswitching depending on τ (≠ ∞) and the vibration energy within a period of 5 ns, while systems of τ = ∞ bring about the switching. The switching times are evaluated to be in the range of 0.44 ns – 3.8 ns (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) collapse abstract
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Japanese Journal of Applied Physics, Vol. 44, No. 7A, 2005, pp. 5068-5072
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 sep... expand abstractarated 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. collapse abstract
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Japanese Journal of Applied Physics, Vol. 45, No. 9A, 2006, pp. 7237-7239
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 di... expand abstractagram 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. collapse abstract
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Journal of Physical Chemistry C, 113 (34), 15380 (2009)
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)... expand abstract, 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. collapse abstract
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The Journal of Physical Chemistry C 2009 Aug; 113(34)
Knot-Isomers of Möbius Cyclacene: How Does the Number of Knots Influence the Structure and First Hyperpolarizability?
Four knot-isomers of Möbius cyclacene are composed of 15 nitrogen-substituted benzo rings. They are non-Möbius cyclacenes without a knot (0), Möbius cyclacenes with a knot (1), non-Möbius cyclacenes with two knots (2), and Mo&... expand abstract#776;bius cyclacenes with three knots (3). Their structures and nonlinear optical properties are systematically studied. The order of first hyperpolarizability (β0) is 4693 (0) < 10 484 (2) < 25 419 (3) < 60 846 au (1). The β0 values of the knot-isomers with knot(s) are larger than that of the knot-isomer without a knot. It shows that the β0 value can be dramatically increased (13 times) by twisting the knot(s) to the cyclacene. Two noticeable relationships between the number of knots and the first hyperpolarizability have been observed: (i) the β0 values of one surface Möbius cyclacene (1 and 3) with an odd number of knots are larger than that of two-surface non-Möbius cyclacenes (0 and 2) with an even number of knots. (ii) For the one-surface Möbius cyclacenes, the β0 value for 1... collapse abstract
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Jpn. J. Appl. Phys. 48 (2009) 065007
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 n... expand abstractitrogen 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. collapse abstract
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Japanese Journal of Applied Physics, Vol. 48, 065007 (2009)
Exciton effects in optical absorption spectra of boron-nitride (BN) nanotubes
Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the ons... expand abstractite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV 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 nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiralities. collapse abstract
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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 exis... expand abstractt 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. collapse abstract
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Phys. Rev. B 76 (2007), 054451-1 - 054451-11
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 whi... expand abstractch 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. collapse abstract
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Phys. Rev. B 73, 172410 (2006)
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 p... expand abstractosition of fcc-Fe. The spin polarization ratio is defined by $P = (\sigma_\uparrow - \sigma_\downarrow) / (\sigma_\uparrow + \sigma_\downarrow )$, with $\sigma_{\uparrow(\downarrow)}$ 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 $|P|$. collapse abstract
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CHEMICAL PHYSICS LETTERS vol. 454, 323-326 (2008)
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 polyacen... expand abstractes 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. collapse abstract
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Prog.Theor.Phys.Suppl.113:229-238,1993
Polaron excitations in fullerenes: Theory as pi-conjugated systems
We review the recent theoretical treatment of fullerenes as pi-conjugated systems. Polaronic properties due to the Jahn-Teller type effects are mainly discussed. (1) A Su-Schrieffer-Heeger type electron-phonon model is applied to fullerenes: C_60 a... expand abstractnd C_70, and is solved with the adiabatic approximation to phonons. When the system (C_60 or C_70) is doped with one or two electrons (or holes), the additional charges accumulate along almost an equatorial line of the molecule. The dimerization becomes the weakest along the same line. Two energy levels, the occupied state and the empty state, intrude largely in the gap. The intrusion is larger in C_70 than in C_60. These are ``polarons'' in doped fullerenes. It is also found that C_60 and C_70 are related mutually with respect to electronical structures as well as lattice geometries. (2) We apply the model to the fullerene epoxide C_60O. It has the polaron-type lattice distortion around the oxygen, and also shows the energy level intrusion in the gap. (3) Optical properties of C_60 are calculated and discussed. In the absorption of the doped molecule, a new peak structure is present owing to the polaronic distortion. In the luminescence of the neutral C_60, the spacing between H_g(8)-phonon side-band peaks and the relative intensities agree well with experiments. In the dispersion of the third harmonic generation, the magnitudes of |chi^(3)| agree with those of experiments at the resonance of the lowest allowed transition as well as in the region away from the resonance. collapse abstract
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2008 Mar; 454(4-6)
What is the shape effect on the (hyper)polarizabilities? A comparison study on the Möbius, normal cyclacene, and linear nitrogen-substituted strip polyacenes
How does the framework shape influence the static polarizability ( α 0 ) and the first hyperpolarizability ( β 0 )? This work, for the first time, presents a comparison study by three models: Möbius strip cyclacene with a knot, normal cyclacene witho... expand abstractut knot, and linear strip. Opening the knot of the Möbius to form the normal cyclacene, it leads to the increase of α 0 from 268 to 323 a.u. and β 0 increases about three times from 393 to 1049 a.u. Further, opening the normal cyclacene to form the linear strip, α 0 increases from 323 to 476 a.u. While β 0 as well increases about three times from 1049 to 2814 a.u. collapse abstract
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in "22nd International Conference on The Physics of Semiconductors", (World Scientific, 1995), pp. 2101-2104
Optical response of C60 and C70 fullerenes: Exciton and lattice fluctuation effects
Molecular exciton effects in the neutral and maximally doped C60 (C70) are considered using a tight binding model with long-range Coulomb interactions and bond disorder. By comparing calculated and observed optical spectra, we conclude that relevan... expand abstractt Coulomb parameters for the doped cases are about half of those of the neutral systems. The broadening of absorption peaks is well simulated by the bond disorder model. collapse abstract
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Phys. Rev. B 52 (1995) 7968-7971
Metal-Insulator Transition in C60-Polymers
Variations in the band structures of C60-polymers are studied, when pi-conjugation conditions are changed. We look at band structures in order to discuss a metal-insulator transition, using a semi-empirical model with the Su-Schrieffer-Heeger type ... expand abstractelectron-phonon interactions. We find that electronic structures change among direct-gap insulators and the metal, depending on the degree of pi-conjugations. High pressure experiments could observe such pressure-induced metal-insulator transitions. collapse abstract
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in "Fullerene Polymers and Fullerene Polymer Composites", edited by P. C. Eklund and A. M. Rao, (Springer-Verlag, 1999), Chapter 9
Charge transfer excitons in C60-dimers and polymers
Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. ... expand abstractWe use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers. collapse abstract
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