Mechanism of magnetism in stacked nanographite with open shell electrons

Antiferromagnetism in stacked nanographite is investigated with using the Hubbard-type models. The A-B stacking or the stacking near to that of A-B type is favorable for the hexagonal nanographite with zigzag edges, in order that magnetism appears. Next, we find that the open shell electronic structure can be an origin of the decreasing magnetic moment with the decrease of the inter-graphene distance, as experiments on adsorption of molecules suggest.Comment: 13 pages; Harigaya's Web site: http://staff.aist.go.jp/k.harigaya/welcome_E.html; Nanotech. Inst. of AIST: http://unit.aist.go.jp/nanotech

Propagation of Cooper pairs in carbon nanotubes with superconducting correlations

Propagation of Cooper pairs in carbon nanotubes in the presence of superconducting correlations is studied theoretically. We find that negative and positive currents induced by impurity scatterings between electrons and holes cancel each other, and the nonmagnetic impurity does not hinder the supercurrent in the regions where the superconducting proximity effects occur. The carbon nanotube is a good conductor for Cooper pairs.Comment: 5 page

Doping Effects and Electronic States in C60-Polymers

Band structures of C60-polymers are studied changing conjugation conditions and the electron number. We use a Su-Schrieffer-Heeger type semiempirical model. In the neutral C60-polymer, electronic structures change among direct-gap insulator and the metal, depending on the degree of conjugations. The C60-polymer doped with one electron per one molecule is always a metal. The energy difference between the highest-occupied state and the lowest-unoccupied state of the neutral system becomes smaller upon doping owing to the polaron effects. The C60-polymer doped with two electrons per one C60 changes from an indirect-gap insulator to the direct-gap insulator, as the conjugations become stronger.Comment: to be published in Chem. Phys. Let

Optical Absorption in C_70 and C_76: Effects of Symmetry Reduction

Optical absorption spectra of C_70 and C_76 are calculated by a tight binding model with a long ranged Coulomb interaction. The spectrum of C_70 is in overall agreement with the experiment of molecules in a solution. The variations of the spectral shape are discussed relating with the symmetry reduction from C_70 to C_76.Comment: LaTeX, 8 pages, 3 figures; Note: Please request figures to one of the Authors. E-mail: [email protected], [email protected]

Theory of an elastic anomaly in the heavy fermion system CeTe

The elastic anomaly observed in the coherent Kondo state of Ce heavy fermion compounds is analyzed by using the Anderson lattice model simulating the energy level scheme of CeTe. The $\Gamma_7$ doublets and $\Gamma_8$ quartets of the 4f states are considered in the model. We solve the mean field equations to derive the temperature dependences of elastic constants, using the random phase approximation like expression for the interaction between the elastic strain and the crystalline field splitting. We compare the calculation with the $(c_{11} - c_{12})/2$ and $c_{44}$ constants of CeTe. The presence of the downward dip and the observed overall temperature variations of the two constants are well described by the present theory. The origin of the dip is the coupling between the elastic strain and the splitting of the $\Gamma_8$ quartets.Comment: submission to Phys. Rev. B, 16 pages, 3 figures (sent by the snail mail

Metallic and insulating states in two-dimensional C60-polymers

Variations in the band structures of the two-dimensional C60-polymers are studied, when pi-conjugation conditions are changed. We investigate the rectangular and triangular polymers, in order to discuss metal-insulator transitions, using a semiempirical model with the Su-Schrieffer-Heeger type electron-phonon interactions. We find that electronic structures change among direct-gap insulators and the metal, depending on the degree of pi-conjugations in the rectangular polymer. The triangular polymer changes from the indirect gap insulator to the metal as the pi-conjugations increase. High pressure experiments could observe such pressure-induced metal-insulator transitions.Comment: Figures should be requested to the autho

Charge transfer excitons in optical absorption spectra of 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. We 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.Comment: Figures should be requested to the autho

Excitons in hexagonal nanonetwork materials

Optical excitations in hexagonal nanonetwork materials, for example, Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. A permanent electric dipole moment, whose direction is from the B site to the N site, is considered along the BN bond. When the exciton hopping integral is restricted to the nearest neighbors, the flat band of the exciton appears at the lowest energy. The symmetry of this exciton band is optically forbidden, indicating that the excitons relaxed to this band will show quite long lifetime which will cause luminescence properties

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.Comment: 11 pages; 7 figures; accepted for publication in Japanese Journal of Applied Physic