917 research outputs found
Defective transport properties of three-terminal carbon nanotube junctions
We investigate the transport properties of three terminal carbon based
nanojunctions within the scattering matrix approach. The stability of such
junctions is subordinated to the presence of nonhexagonal arrangements in the
molecular network. Such "defective" arrangements do influence the resulting
quantum transport observables, as a consequence of the possibility of acting as
pinning centers of the correspondent wavefunction. By investigating a fairly
wide class of junctions we have found regular mutual dependencies between such
localized states at the carbon network and a strikingly behavior of the
conductance. In particular, we have shown that Fano resonances emerge as a
natural result of the interference between defective states and the extended
continuum background. As a consequence, the currents through the junctions
hitting these resonant states might experience variations on a relevant scale
with current modulations of up to 75%.Comment: 8 pages, 8 figure
Doping effects on the electronic and structural properties of CoO2: An LSDA+U study
A systematic LSDA+U study of doping effects on the electronic and structural
properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer
insulator within LSDA+U and a metal with a high density of states (DOS) at the
Fermi level within LSDA. (CoO2), on the other hand, is a band
insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no
charge orderings are present. Due to the strong interaction between the doped
electron and other correlated Co d electrons, the calculated electronic
structure of (CoO2) depends sensitively on the doping level x. Zone
center optical phonon energies are calculated under the frozen phonon
approximation and are in good agreement with measured values. Softening of the
phonon at doping x ~0.25 seems to indicate a strong electron-phonon
coupling in this system. Possible intemediate spin states of Co ions, Na
ordering, as well as magnetic and charge orderings in this system are also
discussed.Comment: 11 pages, 12 figure
Discrete transverse superconducting modes in nano-cylinders
Spatial variation in the superconducting order parameter becomes significant
when the system is confined at dimensions well below the typical
superconducting coherence length. Motivated by recent experimental success in
growing single-crystal metallic nanorods, we study quantum confinement effects
on superconductivity in a cylindrical nanowire in the clean limit. For large
diameters, where the transverse level spacing is smaller than superconducting
order parameter, the usual approximations of Ginzburg-Landau theory are
recovered. However, under external magnetic field the order parameter develops
a spatial variation much stronger than that predicted by Ginzburg-Landau
theory, and gapless superconductivity is obtained above a certain field
strength. At small diameters, the discrete nature of the transverse modes
produces significant spatial variations in the order parameter with increased
average magnitude and multiple shoulders in the magnetic response.Comment: 10 pages, 8 figure
Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy
Carbon nanothreads are a new one-dimensional sp^3-bonded nanomaterial of CH stoichiometry synthesized from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume crystalline packing hundreds of micrometers across. We show high-resolution electron microscopy (HREM) images of hexagonal arrays of well-aligned thread columns that traverse the 80–100 nm thickness of the prepared sample. Diffuse scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy confirms them to be primarily sp^3-hybridized, with less than 27% sp^2 carbon, most likely associated with partially saturated “degree-4” threads
Demagnetization Protocols for Frustrated Interacting Nanomagnet Arrays
We report a study of demagnetization protocols for frustrated arrays of
interacting single domain permalloy nanomagnets by rotating the arrays in a
changing magnetic field. The most effective demagnetization is achieved by not
only stepping the field strength down while the sample is rotating, but by
combining each field step with an alternation in the field direction. By
contrast, linearly decreasing the field strength or stepping the field down
without alternating the field direction leaves the arrays with a larger
remanent magnetic moment. These results suggest that non-monotonic variations
in field magnitude around and below the coercive field are important for the
demagnetization process.Comment: 12 pages, 4 figure
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
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