4,253 research outputs found
Continuum theory for the piezoelectric response of chiral nanotubes under uniaxial and torsional stresses
We develop and solve a continuum theory for the piezoelectric response of
nanotubes under applied uniaxial and torsional stresses. We find that the
piezoelectric response is controlled by the chiral angle, the aspect ratio, and
two dimensionless parameters specifying the ratio of the strengths of the
electrostatic and elastic energies. The model is solved in two limiting cases
and the solutions are discussed. These systems are found to have several
unexpected physical effects not seen in conventional bulk systems, including a
strong stretch-twist coupling and the development of a significant bound charge
density in addition to a surface charge density. The model is applied to
estimate the piezoelectric response of a boron-nitride nanotube under uniform
tensile stress.Comment: 8 pages, 4 figures. Submitted to Physical Review
Switchable valley filter based on a graphene - junction in a magnetic field
Low-energy excitations in graphene exhibit relativistic properties due to the
linear dispersion relation close to the Dirac points in the first Brillouin
zone. Two of the Dirac points located at opposite corners of the first
Brillouin zone can be chosen as inequivalent, representing a new valley degree
of freedom, in addition to the charge and spin of an electron. Using the valley
degree of freedom to encode information has attracted significant interest,
both theoretically and experimentally, and gave rise to the field of
valleytronics. We study a graphene - junction in a uniform out-of-plane
magnetic field as a platform to generate and controllably manipulate the valley
polarization of electrons. We show that by tuning the external potential giving
rise to the - junction we can switch the current from one valley
polarization to the other. We also consider the effect of different types of
edge terminations and present a setup, where we can partition an incoming
valley-unpolarized current into two branches of valley-polarized currents. The
branching ratio can be chosen by changing the location of the - junction
using a gate.Comment: 8 pages, 7 figure
Continuum Theory for Piezoelectricity in Nanotubes and Nanowires
We develop and solve a continuum theory for the piezoelectric response of one
dimensional nanotubes and nanowires, and apply the theory to study
electromechanical effects in BN nanotubes. We find that the polarization of a
nanotube depends on its aspect ratio, and a dimensionless constant specifying
the ratio of the strengths of the elastic and electrostatic interactions. The
solutions of the model as these two parameters are varied are discussed. The
theory is applied to estimate the electric potential induced along the length
of a BN nanotube in response to a uniaxial stress.Comment: 4 pages in RevTex4, 2 epsf figure
Carbon Nanotubes in Helically Modulated Potentials
We calculate effects of an applied helically symmetric potential on the low
energy electronic spectrum of a carbon nanotube in the continuum approximation.
The spectrum depends on the strength of this potential and on a dimensionless
geometrical parameter, P, which is the ratio of the circumference of the
nanotube to the pitch of the helix. We find that the minimum band gap of a
semiconducting nanotube is reduced by an arbitrarily weak helical potential,
and for a given field strength there is an optimal P which produces the biggest
change in the band gap. For metallic nanotubes the Fermi velocity is reduced by
this potential and for strong fields two small gaps appear at the Fermi surface
in addition to the gapless Dirac point. A simple model is developed to estimate
the magnitude of the field strength and its effect on DNA-CNT complexes in an
aqueous solution. We find that under typical experimental conditions the
predicted effects of a helical potential are likely to be small and we discuss
several methods for increasing the size of these effects.Comment: 12 pages, 10 figures. Accepted for publication in Physical Review B.
Image quality reduced to comply with arxiv size limitation
Organizational analysis of the seed sector of rice in Guinea: stakeholders, perception and institutional linkages
This paper analyses the organization of the rice seed sector in Guinea with the overall objectives to assess how organizational settings affect seed supply to small-scale farmers and to suggest institutional changes that would favour seed service and uptake of varieties. Data were collected in Guinea, West Africa, using focus group discussions with extension workers, farmers, representatives of farmers’ associations, agro-input dealers, researchers and non-governmental organization (NGO) staff, and surveys of 91 rice farming households and 41 local seed dealers. Findings suggest that the current institutional settings and perceptions of stakeholders from the formal seed sector inhibit smallholder farmers’ access to seed. Seed interventions in the past two decades have mainly relied on the national extension system, the research institute, NGOs, farmers’ associations and contract seed producers to ensure seed delivery. Although local seed dealers play a central role in providing seed to farmers, governmental organizations operating in a linear model of formal seed sector development have so far ignored their role. We discuss the need to find common ground and alternative models of seed sector development. In particular we suggest the involvement of local seed dealers in seed development activities to better link the formal and the informal seed systems and improve smallholder farmers’ access to seed from the formal sector
Chirality Dependence of the -Momentum Dark Excitons in Carbon Nanotubes
Using a collection of twelve semiconducting carbon nanotube samples, each
highly enriched in a single chirality, we study the chirality dependence of the
-momentum dark singlet exciton using phonon sideband optical spectroscopy.
Measurements of bright absorptive and emissive sidebands of this finite
momentum exciton identify its energy as 20 - 38 meV above the bright singlet
exciton, a separation that exhibits systematic dependencies on tube diameter,
family, and semiconducting type. We present calculations that explain
how chiral angle dependence in this energy separation relates to the Coulomb
exchange interaction, and elaborate the dominance of the phonon
sidebands over the zone-center phonon sidebands over a wide range of
chiralities. The Kataura plot arising from these data is qualitatively well
described by theory, but the energy separation between the sidebands shows a
larger chiral dependence than predicted. This latter observation may indicate a
larger dispersion for the associated phonon near the point than expected
from finite distance force modeling.Comment: 24 pages, 12 figures, 1 table; slight title change, Figures 1 and 11
added, reference added, presentation improved throughout documen
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