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Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tightbinding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters V n ͑n Յ 36͒, where n is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.

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Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Re-based double perovskites (DPs) have garnered substantial attention due to their high Curie temperatures (TC ) and display of complex interplay of structural and metal-insulator transitions (MIT). Here we systematically study the ground state electronic and structural properties for a family of Re-based DPs A2BReO6 (A=Sr, Ca and B=Cr, Fe), which are related by a common low energy Hamiltonian, using density functional theory + U calculations. We show that the on-site interaction U of Re induces orbital ordering (denoted C-OO), with each Re site having an occupied dxy orbital and a C-type alternation among dxz/dyz, resulting in an insulating state consistent with experimentally determined insulators Sr2CrReO6, Ca2CrReO6, and Ca2FeReO6. The threshold value of URe for orbital ordering is reduced by inducing Eg octahedral distortions of the same C-type wavelength (denoted C-OD), which serves as a structural signature of the orbital ordering; octahedral tilting also reduces the threshold. The C-OO, and the concomitant C-OD, are a spontaneously broken symmetry for the Sr based materials (i.e. a 0 a 0 c − tilt pattern), while not for the Ca based systems (i.e. a − a − b + tilt pattern). Spin-orbit coupling does not qualitatively change the physics of the C-OO/C-OD, but can induce relevant quantitative changes. We prove that a single set of UCr, UFe, URe capture the experimentally observed metallic state in Sr2FeReO6 and insulating states in other three systems. We predict that the C-OO is the origin of the insulating state in Sr2CrReO6, and that the concomitant C-OD may be experimentally observed at sufficiently low temperatures (i.e. space group P 42/m) in pure samples. Additionally, given our prescribed values of U , we show that the C-OO induced insulating state in Ca2CrReO6 will survive even if the C-OD amplitude is suppressed (e.g. due to thermal fluctuations). The role of the C-OO/C-OD in the discontinuous, temperature driven MIT in Ca2FeReO6 is discussed.