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We report total-energy electronic structure calculations that provide energetics of encapsulation of C60 in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the (10,10) nanotube, whereas the processes are endothermic for the (8,8) and (9,9) nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 A. We also find that the C(60)@(10,10) is a metal with multicarriers each of which distributes either along the nanotube or on the C60 chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.

Okada, Saito, and Oshiyama Reply: As we mentioned [1], there are only three polymerized C 60 phases which have been synthesized under pressure and whose atomic structure has been identified to date; one-dimensional orthorhombic, two-dimensional tetragonal, and twodimensional rhombohedral phases [2]. Although there have been a lot of experimental studies to explore new phases of carbon using pressure synthesis from solid C 60 [3][4][5][6] as Brazhkin and Lyapin commented [7], in most cases it is not clear what type of atomic structure the synthesized material has: Sometimes amorphous, sometimes crystal, sometimes a mixture of diamond and graphite, and sometimes totally unidentified at all.Some materials obtained via pressure synthesis from solid C 60 were reported to be superhard or even "ultrahard" (harder than diamond) and were inferred to be three-dimensional (3D) C 60 polymers from their broad x-ray diffraction profiles [8,9]. For these "superhard 3D C 60 polymers," atomic-scale network topologies had not been reported so far. Only quite recently, candidates for the atomic coordinates have been proposed [10] for the first time.In our Letter [1] we tried to provide a firm theoretical framework to consider synthesis and properties of 3D C 60 polymers. Starting from the tetragonal phase of 2D C 60 polymer, we found an ordered 3D C 60 polymer which had not been identified before and exhibited fascinating properties as described in the Letter. From the point of synthesis, for instance, the radial distribution function which experimentalists sometimes rely on to determine the structure is found to not be a simple reflection of the microscopic structures. Also the system is expected to be a candidate for a new elemental superconductor consisting entirely of carbon. At the same time, we found that our system was not a superhard or ultrahard material. Its bulk modulus is found to be 1 order of magnitude smaller than diamond [1].The present theoretical treatment (density-functional pseudopotential procedure) is expected to have enough accuracy to discuss relative hardness of various carbon based materials [11,12]. Hence there is no doubt that the system we found does not correspond directly to so-called superhard 3D C 60 polymers. On the other hand, generally it is useful to compare theoretical and experimental results carefully to innovate new materials. It is especially important in the field of nanostructure materials consisting of carbon and/or other covalent-bond elements. Their physical properties are known to depend strongly on the network topology of covalent bonds as has been clearly demonstrated in the case of carbon nanotubes [13,14].Sometimes the target new materials, with novel properties to be synthesized, can be given from the theoretical study. In this respect, a comparison between theory and experiment done by Brazhkin and Lyapin [7] is worth further consideration. The possibility of the presence of various different phases in pressure-polymerized 3D C 60 is an interesting issue to be studied theoreti...

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We report first-principles total-energy electronic-structure calculations in the density-functional theory performed for hexagonally bonded honeycomb sheets consisting of B, N, and C atoms. We find that the ground state of BNC sheets with particular stoichiometry is ferromagnetic. Detailed analyses of energy bands and spin densities unequivocally reveal the nature of the ferromagnetic ordering, leading to an argument that the BNC sheet is a manifestation of the flat-band ferromagnetism.

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The population of valence-band electronic states of fullerene peapods (C 60 @SWCNT and C 70 @SWCNT) was tuned electrochemically in 0.2 M LiClO 4 + acetonitrile. Electrochemistry of peapods is dominated by their capacitive charging without distinct faradaic processes. In situ vis-NIR spectra of C 60 /C 70 peapods are similar to those of empty nanotubes. Electrochemical charging causes reversible bleaching of the transitions between Van Hove singularities. This bleaching is mirrored by quenching of resonance Raman spectra in the regions of tube-related modes. The Raman modes of intratubular C 60 exhibit considerable intensity increase upon anodic doping of peapods, but these Raman modes are not enhanced at cathodic charging. In contrast to that, C 70 @SWCNT does not show any enhancement of Raman intensities at both anodic and cathodic potentials. All the relevant Raman modes of intratubular C 70 show symmetric charge-transfer bleaching as the tube-related modes. A suggested interpretation follows from model calculations of electronic structure of C 70 @SWCNT (17,0) and C 60 @SWCNT (17,0).