234 research outputs found
Quasi one dimensional He inside carbon nanotubes
We report results of diffusion Monte Carlo calculations for both He
absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly
one dimensional He. Inside the tube, the binding energy of liquid He is
approximately three times larger than on planar graphite. At low linear
densities, He in a nanotube is an experimental realization of a
one-dimensional quantum fluid. However, when the density increases the
structural and energetic properties of both systems differ. At high density, a
quasi-continuous liquid-solid phase transition is observed in both cases.Comment: 11 pages, 3ps figures, to appear in Phys. Rev. B (RC
Electronic response of aligned multishell carbon nanotubes
We report calculations of the effective electronic response of aligned
multishell carbon nanotubes. A local graphite-like dielectric tensor is
assigned to every point of the multishell tubules, and the effective transverse
dielectric function of the composite is computed by solving Maxwell's
equations. Calculations of both real and imaginary parts of the effective
dielectric function are presented, for various values of the filling fraction
and the ratio of the internal and external radii of hollow tubules. Our full
calculations indicate that the experimentally measured macroscopic dielectric
function of carbon nanotube materials is the result of a strong electromagnetic
coupling between the tubes, which cannot be accounted for with the use of
simplified effective medium theories. The presence of surface plasmons is
investigated, and both optical absorption cross sections and energy-loss
spectra of aligned tubules are calculated.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Electronic and Magnetic Properties of Partially-Open Carbon Nanotubes
On the basis of the spin-polarized density functional theory calculations, we
demonstrate that partially-open carbon nanotubes (CNTs) observed in recent
experiments have rich electronic and magnetic properties which depend on the
degree of the opening. A partially-open armchair CNT is converted from a metal
to a semiconductor, and then to a spin-polarized semiconductor by increasing
the length of the opening on the wall. Spin-polarized states become
increasingly more stable than nonmagnetic states as the length of the opening
is further increased. In addition, external electric fields or chemical
modifications are usable to control the electronic and magnetic properties of
the system. We show that half-metallicity may be achieved and the spin current
may be controlled by external electric fields or by asymmetric
functionalization of the edges of the opening. Our findings suggest that
partially-open CNTs may offer unique opportunities for the future development
of nanoscale electronics and spintronics.Comment: 6 figures, to appear in J. Am. Chem. So
The evaluation of shear deformation for contact analysis with large displacement
A common problem encountered in the study of contact problem is the failure to obtain stable and accurate convergence result when the contact node is close to the element edge, which is referred as "critical area". In previous studies, the modification of the element force equation to apply it to a node-element contact problem using the Euler-Bernoulli beam theory [1]. A simple single-element consists two edges and a contact point was used to simulate contact phenomenon of a plane frame. The modification was proven to be effective by the convergeability of the unbalanced force at the tip of element edge, which enabled the contact node to "pass-through", resulting in precise results. However, in another recent study, we discover that, if shear deformation based on Timoshenko beam theory is taken into consideration, a basic simply supported beam coordinate afforded a much simpler and more efficient technique for avoiding the divergence of the unbalanced force in the "critical area". Using our unique and robust Tangent Stiffness Method, the improved equation can be used to overcome any geometrically nonlinear analyses, including those involving extremely large displacements
Atomic Scale Sliding and Rolling of Carbon Nanotubes
A carbon nanotube is an ideal object for understanding the atomic scale
aspects of interface interaction and friction. Using molecular statics and
dynamics methods different types of motion of nanotubes on a graphite surface
are investigated. We found that each nanotube has unique equilibrium
orientations with sharp potential energy minima. This leads to atomic scale
locking of the nanotube.
The effective contact area and the total interaction energy scale with the
square root of the radius. Sliding and rolling of nanotubes have different
characters. The potential energy barriers for sliding nanotubes are higher than
that for perfect rolling. When the nanotube is pushed, we observe a combination
of atomic scale spinning and sliding motion. The result is rolling with the
friction force comparable to sliding.Comment: 4 pages (two column) 6 figures - one ep
Electron-phonon interaction in ultrasmall-radius carbon nanotubes
We perform analysis of the band structure, phonon dispersion, and
electron-phonon interactions in three types of small-radius carbon nanotubes.
We find that the (5,5) can be described well by the zone-folding method and the
electron-phonon interaction is too small to support either a charge-density
wave or superconductivity at realistic temperatures. For ultra-small (5,0) and
(6,0) nanotubes we find that the large curvature makes these tubes metallic
with a large density of states at the Fermi energy and leads to unusual
electron-phonon interactions, with the dominant coupling coming from the
out-of-plane phonon modes. By combining the frozen-phonon approximation with
the RPA analysis of the giant Kohn anomaly in 1d we find parameters of the
effective Fr\"{o}lich Hamiltonian for the conduction electrons. Neglecting
Coulomb interactions, we find that the (5,5) CNT remains stable to
instabilities of the Fermi surface down to very low temperatures while for the
(5,0) and (6,0) CNTs a CDW instability will occur. When we include a realistic
model of Coulomb interaction we find that the charge-density wave remains
dominant in the (6,0) CNT with around 5 K while the
charge-density wave instability is suppressed to very low temperatures in the
(5,0) CNT, making superconductivity dominant with transition temperature around
one Kelvin.Comment: 20 pages. Updated 7/23/0
Chirality effects in carbon nanotubes
We consider chirality related effects in optical, photogalvanic and
electron-transport properties of carbon nanotubes. We show that these
properties of chiral nanotubes are determined by terms in the electron
effective Hamiltonian describing the coupling between the electron wavevector
along the tube principal axis and the orbital momentum around the tube
circumference. We develop a theory of photogalvanic effects and a theory of
d.c. electric current, which is linear in the magnetic field and quadratic in
the bias voltage. Moreover, we present analytic estimations for the natural
circular dichroism and magneto-spatial effect in the light absorption.Comment: 23 pages, 3 figure
Realistic description of electron-energy loss spectroscopy for One-Dimensional SrCuO
We investigate the electron-energy loss spectrum of one-dimensional undoped
CuO chains within an extended multi-band Hubbard model and an extended
one-band Hubbard model, using the standard Lanczos algorithm. Short-range
intersite Coulomb interactions are explicitly included in these models, and
long-range interactions are treated in random-phase approximation. The results
for the multi-band model with standard parameter values agree very well with
experimental spectra of SrCuO. In particular, the width of the main
structure is correctly reproduced for all values of momentum transfer. It is
shown for both models that intersite Coulomb interactions mainly lead to an
energy shift of the spectra. We find no evidence for enhanced intersite
interactions in SrCuO.Comment: 4 pages, 4 figure
Preliminary solid waste management (SWM) data survey and assessment of town, treatment site and disposal site conditions; case study southern province of Sri Lanka
In case of southern province of Sri Lanka, Solid Waste Management (SWM) can be an aggravated problem in future with respect to the present development activities in the area. To find a sustainable solution for this problem is very important to aware about present situation of SWM in the province. But there is no SWM data survey has been carried out in the province recently. Therefore under this research work a preliminary SWM data survey has been conducted in all 49 Local Authorities (LA) in Southern province. Research work was basically carried out at LA level since LA is the responsible administration body for solid waste management within its territory. Further town condition, waste treatment and disposal site conditions of each local authority were visited and assessed according to an accepted assessment criteria. Total daily waste collection in the province is 231.65 tons and it is only a 20% of total daily waste generation in the province. Composting is the most adopted waste treatment method in the province as 19 LAs out of 49 have already established composting facilities. Waste disposal is the major problem for most of the LAs as it contributes to create many socio-environmental issues. When consider waste disposal methods in southern province, 29 LAs have adopted open dumping while other LAs are burying. According to the assessment it was found that SWM in 96 percent of LAs is not up the acceptable level. The findings and results of the study have been presented in this research paper in detail
Effective electronic response of a system of metallic cylinders
The electronic response of a composite consisting of aligned metallic
cylinders in vacuum is investigated, on the basis of photonic band structure
calculations. The effective long-wavelength dielectric response function is
computed, as a function of the filling fraction. A spectral representation of
the effective response is considered, and the surface mode strengths and
positions are analyzed. The range of validity of a Maxwell-Garnett-like
approach is discussed, and the impact of our results on absorption spectra and
electron energy-loss phenomena is addressed.Comment: 15 pages, 6 figures, to appear in Phys. Rev.
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