Noriaki Hamada

Energetics of carbon nano-tubes

The cohesive energies of carbon nano-tubes are calculated using Tersoffs potential for carbon. It is shown that very thin tubules, having several carbon-atom hexagons in circumference, are energetically more favorable than graphite sheets of width equal to the circumference. We predict the existence of extremely thin tubules, say 4 A in diameter.

Electronic structures of C60 fullerides and related materials

Abstract We report microscopic total-energy electronic-structure calculations which provide the cohesion mechanism and the energy-band structures of new forms of solid carbon, C60 and related materials. We find that C60, clusters are condensed via van der Waals-like forces and the resulting face centered cubic C60, is a semiconductor with an energy gap of ≈ 1.5 eV. Both the valence-band top and the conduction-band bottom are at the zone boundary (X point), but rotation of the C60 clusters modifies the energy-band dispersion. We also find that alkali-atom doping transforms solid C60, into a strongly bonded ionic metal in which both Madelung and band energies contribute to its cohesion. The overall structure of the calculated density of states is in excellent agreement with the photoemission, inverse photoemission and X-ray emission data for both pristine and alkali-doped fullendes. A unique linear relation between the superconducting transition temperature and the Fermi-level density-of-states over a wide range of temperature is found for potassium-doped and rubidium-doped C60, under pressure. We predict that the high electronegativity of C60 hinders the hole injection with halogen-atom doping. Finally, systematic calculations for carbon micro-tubules have been performed using a realistic tight binding model. We find that micro-tubules exhibit a striking variation in electronic conduction, from a metal to narrow-gap and wide-gap semiconductors, depending on the diameter and the degree of helical arrangement of the tubule.

Electronic band structure of LaNiO3

Abstract The metallic nature of LaNiO 3 is studied by the local-density full-potential linearized augmented-plane-wave (FLAPW) band structure calculation. For a hypothetical cubic perovskite structure, we have found a large Fermi surface with a hole inside, which is similar to the Fermi surface seen in oxide high-T c superconductors. Applying a rhombohedral distortion observed experimentally, the large Fermi surface collapses into small surfaces with electrons and holes. If we stabilize a cubic structure, we may expect the superconductivity in this material on the analogy of the cubic Ba 1− x Ka x BiO 3

Electronic and geometric structures of fullerenes

We report the electronic and geometric structures of C 76 and C 84 , as well as the electronic structure of C 60 and C 70 . In the case of C 84 , we have examined all the possible geometries which are consistent with the results of the recent nuclear magnetic resonance experiment on C 84 extracted from soot. Comparison of the results with the experimental photoabsorption and photoemission spectra suggests that D 2 C 84 , proposed according to the ring-stacking growth model for fullerenes, may be the major isomer of C 84 . After the separation of isomers, the present results will be useful in determining the actual geometries of C 84

Electronic band structure of carbon nanotubes: toward the three-dimensional system

Abstract Nanotubes can be easily modified by introducing pentagons and heptagons into the hexagonal network. We construct a modified tube and higher-dimensional systems, for which we calculate the electronic band structure using a simple tight-binding model. We elucidate new features of the energy band resulting from such constructions.

Electronic structure of fullerides

Abstract We present ab-initio electronic-structure calculations of C60 fullerite and its solid-state derivatives. The obtained characteristics and the comparison with experimental data validate a superatom-concept: a C60 cluster is regarded as a constituent atom of a variety of fullerides. Roles of internal degrees-of-freedom, such as the intra-cluster vibration or the rotation of the cluster, in condensed-matter properties of the fullerides are also discussed.

Temperature dependence of Hall coefficient in high-Tc oxides

Abstract Large Fermi surfaces obtained by the local-density band-structure calculation are consistent with signs of the Hall coefficient obtained by experiment for almost all high-Tc oxide superconductors. However, a simple band-electron picture cannot explain the large temperature dependence of the Hall coefficient. We propose a phenomenology for the temperature dependence of the Hall coefficient, and discuss the origin of temperature dependences of transport coefficients.

Electronic structure of fullerenes and fullerides: artificial atoms and their solids

The authors calculated the electronic structure of fullerenes, C60, C70, C76 and C84 by using the density-functional theory (DFT) or the realistic tight-binding model. C60K and C60Cl where K and Cl are trapped inside C60, are also studied. Results are discussed from the viewpoint of the fullerenes as artificial atoms, a spherical quantum system which one can modify and make use of to construct new materials. They also report the electronic structure of solid fullerenes: pristine FCC C60, FCC C60K, and FCC BC59 (one of the sixty C atoms in each C60 is replaced by B), and fullerides: KNC60(N=1-3) and C60Br (K and Br atoms are on the interstitial sites of the C60 lattice), obtained by the DFT.