26,085 research outputs found
Coupled ion - nanomechanical systems
We study ions in a nanotrap, where the electrodes are nanomechanical
resonantors. The ions play the role of a quantum optical system which acts as a
probe and control, and allows entanglement with or between nanomechanical
resonators.Comment: 4 pages, 2 figures, submitted for publicatio
Local Semiconducting Transition in Armchair Carbon Nanotubes: The Effect of Periodic Bi-site Perturbation on Electronic and Transport Properties of Carbon Nanotubes
In carbon nanotubes, the most abundant defects, caused for example by
irradiation or chemisorption treatments, are small perturbing clusters, i.e.
bi-site defects, extending over both A and B sites. The relative positions of
these perturbing clusters play a crucial role in determining the electronic
properties of carbon nanotubes. Using bandstructure and electronic transport
calculations, we find out that in the case of armchair metallic nanotubes a
band gap opens up when the clusters fulfill a certain periodicity condition.
This phenomenon might be used in future nanoelectronic devices in which certain
regions of single metallic nanotubes could be turned to semiconducting ones.
Although in this work we study specifically the effect of hydrogen adatom
clusters, the phenomenon is general for different types of defects. Moreover,
we study the influence of the length and randomness of the defected region on
the electron transport through it.Comment: 5 Pages, 5 Figure
Deconfinement in the Quark Meson Coupling Model
The Quark Meson Coupling Model which describes nuclear matter as a collection
of non-overlapping MIT bags interacting by the self-consistent exchange of
scalar and vector mesons is used to study nuclear matter at finite temperature.
In its modified version, the density dependence of the bag constant is
introduced by a direct coupling between the bag constant and the scalar mean
field. In the present work, the coupling of the scalar mean field with the
constituent quarks is considered exactly through the solution of the Dirac
equation. Our results show that a phase transition takes place at a critical
temperature around 200 MeV in which the scalar mean field takes a nonzero value
at zero baryon density. Furthermore it is found that the bag constant decreases
significantly when the temperature increases above this critical temperature
indicating the onset of quark deconfinement.Comment: LaTeX/TeX 15 pages (zk2.tex)+ 6 figures in TeX forma
Inter-valley plasmons in graphene
The spectrum of two-dimensional (2D) plasma waves in graphene has been
recently studied in the Dirac fermion model. We take into account the whole
dispersion relation for graphene electrons in the tight binding approximation
and the local field effects in the electrodynamic response. Near the
wavevectors close to the corners of the hexagon-shaped Brillouin zone we found
new low-frequency 2D plasmon modes with a linear spectrum. These "inter-valley"
plasmon modes are related to the transitions between the two nearest Dirac
cones.Comment: 4 pages, 2 figures; submitted in PR
Properties of solar polar coronal plumes constrained by Ultraviolet Coronagraph Spectrometer data
We investigate the plasma dynamics (outflow speed and turbulence) inside
polar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by the
UVCS instrument on SOHO at the minimum of solar cycle 22-23 with model
calculations. We consider Maxwellian velocity distributions with different
widths in plume and inter-plume regions. Electron densities are assumed to be
enhanced in plumes and to approach inter-plume values with increasing height.
Different combinations of the outflow and turbulence velocity in the plume
regions are considered. We compute line profiles and total intensities of the
\ion{H}{1} Ly and the \ion{O}{6} doublets. The observed profile shapes
and intensities are reproduced best by a small solar wind speed at low
altitudes in plumes that increases with height to reach ambient inter-plume
values above roughly 3-4 R_\sun combined with a similar variation of the
width of the velocity distribution of the scattering atoms/ions. We also find
that plumes very close to the pole give narrow profiles at heights above 2.5
R_\sun, which are not observed. This suggests a tendency for plumes to be
located away from the pole. We find that the inclusion of plumes in the model
computations provides an improved correspondence with the observations and
confirms previous results showing that published UVCS observations in polar
coronal holes can be roughly reproduced without the need for large temperature
anisotropy. The latitude distributions of plumes and magnetic flux
distributions are studied by analyzing data from different instruments on SOHO
and with SOLIS.Comment: 11 figure
Rotational dynamics and friction in double-walled carbon nanotubes
We report a study of the rotational dynamics in double-walled nanotubes using
molecular dynamics simulations and a simple analytical model reproducing very
well the observations. We show that the dynamic friction is linear in the
angular velocity for a wide range of values. The molecular dynamics simulations
show that for large enough systems the relaxation time takes a constant value
depending only on the interlayer spacing and temperature. Moreover, the
friction force increases linearly with contact area, and the relaxation time
decreases with the temperature with a power law of exponent .Comment: submitted to PR
Devil's staircase of incompressible electron states in a nanotube
It is shown that a periodic potential applied to a nanotube can lock
electrons into incompressible states. Depending on whether electrons are weakly
or tightly bound to the potential, excitation gaps open up either due to the
Bragg diffraction enhanced by the Tomonaga - Luttinger correlations, or via
pinning of the Wigner crystal. Incompressible states can be detected in a
Thouless pump setup, in which a slowly moving periodic potential induces
quantized current, with a possibility to pump on average a fraction of an
electron per cycle as a result of interactions.Comment: 4 pages, 1 figure, published versio
Calculation of the Self-energy of Open Quantum Systems
We propose an easy method of calculating the self-energy of semi-infinite
leads attached to a mesoscopic system.Comment: 6 pages, 2 figures, published in J. Phys. Soc. Jp
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
- …