26,224 research outputs found
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
Heat conductivity in small quantum systems: Kubo formula in Liouville space
We consider chains consisting of several identical subsystems weakly coupled
by various types of next neighbor interactions. At both ends the chain is
coupled to a respective heat bath with different temperature modeled by a
Lindblad formalism. The temperature gradient introduced by this environment is
then treated as an external perturbation. We propose a method to evaluate the
heat current and the local temperature profile of the resulting stationary
state as well as the heat conductivity in such systems. This method is similar
to Kubo techniques used e.g. for electrical transport but extended here to the
Liouville space.Comment: 6 pages, 1 figur
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
Finite-dimensional analogs of string s <-> t duality and pentagon equation
We put forward one of the forms of functional pentagon equation (FPE), known
from the theory of integrable models, as an algebraic explanation to the
phenomenon known in physics as st duality. We present two simple geometrical
examples of FPE solutions, one of them yielding in a particular case the
well-known Veneziano expression for 4-particle amplitude. Finally, we interpret
our solutions of FPE in terms of relations in Lie groups.Comment: LaTeX, 12 pages, 6 eps figure
Composite nucleons in scalar and vector mean-fields
We emphasize that the composite structure of the nucleon may play quite an
important role in nuclear physics. It is shown that the momentum-dependent
repulsive force of second order in the scalar field, which plays an important
role in Dirac phenomenology, can be found in the quark-meson coupling (QMC)
model, and that the properties of nuclear matter are well described through the
quark-scalar density in a nucleon and a self-consistency condition for the
scalar field. The difference between theories of point-like nucleons and
composite ones may be seen in the change of the -meson mass in nuclear
matter if the composite nature of the nucleon suppresses contributions from
nucleon-antinucleon pair creation.Comment: 10 page
Split Instability of a Vortex in an Attractive Bose-Einstein Condensate
An attractive Bose-Einstein condensate with a vortex splits into two pieces
via the quadrupole dynamical instability, which arises at a weaker strength of
interaction than the monopole and the dipole instabilities. The split pieces
subsequently unite to restore the original vortex or collapse.Comment: 4 pages, 4 figures, added figures and references, revised tex
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
From Quantum Dynamics to the Canonical Distribution: General Picture and a Rigorous Example
Derivation of the canonical (or Boltzmann) distribution based only on quantum
dynamics is discussed. Consider a closed system which consists of mutually
interacting subsystem and heat bath, and assume that the whole system is
initially in a pure state (which can be far from equilibrium) with small energy
fluctuation. Under the "hypothesis of equal weights for eigenstates", we derive
the canonical distribution in the sense that, at sufficiently large and typical
time, the (instantaneous) quantum mechanical expectation value of an arbitrary
operator of the subsystem is almost equal to the desired canonical expectation
value. We present a class of examples in which the above derivation can be
rigorously established without any unproven hypotheses.Comment: LaTeX, 8 pages, no figures. The title, abstract and some discussions
are modified to stress physical motivation of the work. References are added
to [2]. This version will appear in Phys. Rev. Lett. There is an accompanying
unpublished note (cond-mat/9707255
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
- …