161 research outputs found
Quasi-ballistic electron transport in double-wall carbon nanotubes
Room-temperature quasi-ballistic electron transport in double-wall carbon
nanotubes (DWNT) is demonstrated. Conductance dependence on the length was
measured by submerging DWNTs into liquid mercury. The conductance plots show
plateaus, indicating weak dependence of the electrode-tube-electrode electrical
resistance on the length of the connecting nanotube. We infer a mean free path
between 0.6 and 10 micron meters for around 80% of the DWNTs, which is in good
agreement with calculations based on the electron scattering by acoustic
phonons and by disorder
Theoretical polarization dependence of the two-phonon double-resonant Raman spectra of graphene
The experimental Raman spectra of graphene exhibit a few intense two-phonon
bands, which are enhanced through double-resonant scattering processes. Though
there are many theoretical papers on this topic, none of them predicts the
spectra within a single model. Here, we present results for the two-phonon
Raman spectra of graphene calculated by means of the quantum perturbation
theory. The electron and phonon dispersions, electronic lifetime,
electron-photon and electron-phonon matrix elements, are all obtained within a
density-functional-theory-based non-orthogonal tight-binding model. We study
systematically the overtone and combination two-phonon Raman bands, and, in
particular, the energy and polarization dependence of their Raman shift and
intensity. We find that the ratio of the integrated intensities for parallel
and cross polarized light for all two-phonon bands is between 0.33 and 0.42.
Our results are in good agreement with the available experimental data
Electronic conduction in multi-walled carbon nanotubes: Role of intershell coupling and incommensurability
Geometry incommensurability between weakly coupled shells in multi-walled
carbon nanotubes is shown to be the origin of unconventional electronic
conduction mechanism, power-law scaling of the conductance, and remarkable
magnetotransport and low temperature dependent conductivity when the dephasing
mechanism is dominated by weak electron-electron coupling
Luttinger liquid behavior in multi-wall carbon nanotubes
The low-energy theory for multi-wall carbon nanotubes including the
long-ranged Coulomb interactions, internal screening effects, and
single-electron hopping between graphite shells is derived and analyzed by
bosonization methods. Characteristic Luttinger liquid power laws are found for
the tunneling density of states, with exponents approaching their Fermi liquid
value only very slowly as the number of conducting shells increases. With minor
modifications, the same conclusions apply to transport in ropes of single-wall
nanotubes.Comment: 4 pages Revte
Electronic Structure of Carbon Nanotube Ropes
We present a tight binding theory to analyze the motion of electrons between
carbon nanotubes bundled into a carbon nanotube rope. The theory is developed
starting from a description of the propagating Bloch waves on ideal tubes, and
the effects of intertube motion are treated perturbatively in this basis.
Expressions for the interwall tunneling amplitudes between states on
neighboring tubes are derived which show the dependence on chiral angles and
intratube crystal momenta. We find that conservation of crystal momentum along
the tube direction suppresses interwall coherence in a carbon nanorope
containing tubes with random chiralities. Numerical calculations are presented
which indicate that electronic states in a rope are localized in the transverse
direction with a coherence length corresponding to a tube diameter.Comment: 15 pages, 10 eps figure
Giant magnetoresistance of multiwall carbon nanotubes: modeling the tube/ferromagnetic-electrode burying contact
We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes
with ultra small diameters. In particular, we consider the effect of the
inter-wall interactions and the lead/nanotube coupling. Comparative studies
have been performed to show that in the case when all walls are well coupled to
the electrodes, the so-called inverse GMR can appear. The tendency towards a
negative GMR depends on the inter-wall interaction and on the nanotube le ngth.
If, however, the inner nanotubes are out of contact with one of the electrodes,
the GMR remains positive even for relatively strong inter-wall interactions
regardless of the outer nanotube length. These results shed additional light on
recently reported experimental data, where an inverse GMR was found in some
multiwall carbon nanotube samples.Comment: 5 pages, 5 figure
GLOWORM-PARA: a flexible framework to simulate the population dynamics of the parasitic phase of gastrointestinal nematodes infecting grazing livestock
Gastrointestinal (GI) nematodes are a significant threat to the economic and environmental sustainability of keeping livestock, as adequate control becomes increasingly difficult due to the development of anthelmintic resistance (AR) in some systems and climate-driven changes to infection dynamics. To mitigate any negative impacts of climate on GI nematode epidemiology and slow AR development, there is a need to develop effective, targeted control strategies that minimise the unnecessary use of anthelmintic drugs and incorporate alternative strategies such as vaccination and evasive grazing. However, the impacts climate and GI nematode epidemiology may have on the optimal control strategy are generally not considered, due to lack of available evidence to drive recommendations. Parasite transmission models can support control strategy evaluation to target field trials, thus reducing the resources and lead-time required to develop evidence-based control recommendations incorporating climate stochasticity. GI nematode population dynamics arising from natural infections have been difficult to replicate and model applications have often focussed on the free-living stages. A flexible framework is presented for the parasitic phase of GI nematodes, GLOWORM-PARA, which complements an existing model of the free-living stages, GLOWORM-FL. Longitudinal parasitological data for two species that are of major economic importance in cattle, Ostertagia ostertagi and Cooperia oncophora, were obtained from seven cattle farms in Belgium for model validation. The framework replicated the observed seasonal dynamics of infection in cattle on these farms and overall, there was no evidence of systematic under- or over-prediction of faecal egg counts (FECs). However, the model under-predicted the FECs observed on one farm with very young calves, highlighting potential areas of uncertainty that may need further investigation if the model is to be applied to young livestock. The model could be used to drive further research into alternative parasite control strategies such as vaccine development and novel treatment approaches, and to understand GI nematode epidemiology under changing climate and host management
Band structures of periodic carbon nanotube junctions and their symmetries analyzed by the effective mass approximation
The band structures of the periodic nanotube junctions are investigated by
the effective mass theory and the tight binding model.
The periodic junctions are constructed by introducing pairs of a pentagonal
defect and a heptagonal defect periodically in the carbon nanotube.
We treat the periodic junctions whose unit cell is composed by two kinds of
metallic nanotubes with almost same radii, the ratio of which is between 0.7
and 1 .
The discussed energy region is near the undoped Fermi level where the channel
number is kept to two, so there are two bands.
The energy bands are expressed with closed analytical forms by the effective
mass theory with some assumptions, and they coincide well with the numerical
results by the tight binding model. Differences between the two methods are
also discussed. Origin of correspondence between the band structures and the
phason pattern discussed in Phys. Rev. B {\bf 53}, 2114, is clarified. The
width of the gap and the band are in inverse proportion to the length of the
unit cell, which is the sum of the lengths measured along the tube axis in each
tube part and along 'radial' direction in the junction part. The degeneracy and
repulsion between the two bands are determined only from symmetries.Comment: RevTeX, gif fil
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