154,426 research outputs found
Electronic transport in metallic carbon nanotubes with mixed defects within the strong localization regime
We study the electron transport in metallic carbon nanotubes (CNTs) with
realistic defects of different types. We focus on large CNTs with many defects
in the mesoscopic range. In a recent paper we demonstrated that the electronic
transport in those defective CNTs is in the regime of strong localization. We
verify by quantum transport simulations that the localization length of CNTs
with defects of mixed types can be related to the localization lengths of CNTs
with identical defects by taking the weighted harmonic average. Secondly, we
show how to use this result to estimate the conductance of arbitrary defective
CNTs, avoiding time consuming transport calculations
Polymers in anisotropic environment with extended defects
The conformational properties of flexible polymers in d dimensions in
environments with extended defects are analyzed both analytically and
numerically. We consider the case, when structural defects are correlated in
\varepsilon_d dimensions and randomly distributed in the remaining
d-\varepsilon_d. Within the lattice model of self-avoiding random walks (SAW),
we apply the pruned enriched Rosenbluth method (PERM) and find the estimates
for scaling exponents and universal shape parameters of polymers in environment
with parallel rod-like defects (\varepsilon_d=1). An analytical description of
the model is developed within the des Cloizeaux direct polymer renormalization
scheme
Condensing Nielsen-Olesen strings and the vortex-boson duality in 3+1 and higher dimensions
The vortex-boson (or Abelian-Higgs, XY) duality in 2+1 dimensions
demonstrates that the quantum disordered superfluid is equivalent to an ordered
superconductor and the other way around. Such a duality structure should be
ubiquitous but in 3+1 (and higher) dimensions a precise formulation of the
duality is lacking. The problem is that the topological defects become extended
objects, strings in 3+1D. We argue how the condensate of such vortex strings
must behave from the known physics of the disordered superfluid, namely the
Bose-Mott insulator. A flaw in earlier proposals is repaired, and a more direct
viewpoint, avoiding gauge fields, in terms of the physical supercurrent is laid
out, that also easily generalizes to higher-dimensional and more complicated
systems. Furthermore topological defects are readily identified; we demonstrate
that the Bose-Mott insulator supports line defects, which may be seen in cold
atom experiments.Comment: LaTeX, 25 pages, 5 figures; several revisions and addition
Universality of subleading corrections for self-avoiding walks in presence of one dimensional defects
We study three-dimensional self-avoiding walks in presence of a
one-dimensional excluded region. We show the appearance of a universal
sub-leading exponent which is independent of the particular shape and
symmetries of the excluded region. A classical argument provides the estimate:
. The numerical simulation gives .Comment: 29 pages, latex2
Topological defects in lattice models and affine Temperley-Lieb algebra
This paper is the first in a series where we attempt to define defects in
critical lattice models that give rise to conformal field theory topological
defects in the continuum limit. We focus mostly on models based on the
Temperley-Lieb algebra, with future applications to restricted solid-on-solid
(also called anyonic chains) models, as well as non-unitary models like
percolation or self-avoiding walks. Our approach is essentially algebraic and
focusses on the defects from two points of view: the "crossed channel" where
the defect is seen as an operator acting on the Hilbert space of the models,
and the "direct channel" where it corresponds to a modification of the basic
Hamiltonian with some sort of impurity. Algebraic characterizations and
constructions are proposed in both points of view. In the crossed channel, this
leads us to new results about the center of the affine Temperley-Lieb algebra;
in particular we find there a special subalgebra with non-negative integer
structure constants that are interpreted as fusion rules of defects. In the
direct channel, meanwhile, this leads to the introduction of fusion products
and fusion quotients, with interesting mathematical properties that allow to
describe representations content of the lattice model with a defect, and to
describe its spectrum.Comment: 41
Universality of subleading exponents induced by one dimensional defects the case of self-avoiding walks
In this paper we offer some simple and quite general arguments which suggest
that the first subleading exponent does not depend on the set of
broken symmetries, but only on the dimensionality of the excluded region. An
explicit value for this exponent is conjectured. We reserve analytical and
numerical details to a forthcoming paper.Comment: 5 pages, presented at the International School of Physics "Enrico
Fermi", Varenna Course CXXXIV: The Physics of Complex System
Renormalization Group results for lattice surface models
We study the phase diagram of statistical systems of closed and open
interfaces built on a cubic lattice. Interacting closed interfaces can be
written as Ising models, while open surfaces as Z(2) gauge systems. When the
open surfaces reduce to closed interfaces with few defects, also the gauge
model can be written as an Ising spin model. We apply the lower bound
renormalization group (LBRG) transformation introduced by Kadanoff (Phys. Rev.
Lett. 34, 1005 (1975)) to study the Ising models describing closed and open
surfaces with few defects. In particular, we have studied the Ising-like
transition of self-avoiding surfaces between the random-isotropic phase and the
phase with broken global symmetry at varying values of the mean curvature. Our
results are compared with previous numerical work. The limits of the LBRG
transformation in describing regions of the phase diagram where not
ferromagnetic ground-states are relevant are also discussed.Comment: 24 pages, latex, 5 figures (available upon request to
[email protected]
Avoiding power broadening in optically detected magnetic resonance of single NV defects for enhanced DC-magnetic field sensitivity
We report a systematic study of the magnetic field sensitivity of a magnetic
sensor based on a single Nitrogen-Vacancy (NV) defect in diamond, by using
continuous optically detected electron spin resonance (ESR) spectroscopy. We
first investigate the behavior of the ESR contrast and linewidth as a function
of the microwave and optical pumping power. The experimental results are in
good agreement with a simplified model of the NV defect spin dynamics, yielding
to an optimized sensitivity around 2 \mu T/\sqrt{\rm Hz}. We then demonstrate
an enhancement of the magnetic sensitivity by one order of magnitude by using a
simple pulsed-ESR scheme. This technique is based on repetitive excitation of
the NV defect with a resonant microwave \pi-pulse followed by an optimized
read-out laser pulse, allowing to fully eliminate power broadening of the ESR
linewidth. The achieved sensitivity is similar to the one obtained by using
Ramsey-type sequences, which is the optimal magnetic field sensitivity for the
detection of DC magnetic fields
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