1,970 research outputs found
Transport through quasi-ballistic quantum wires: the role of contacts
We model one-dimensional transport through each open channel of a quantum
wire by a Luttinger liquid with three different interaction parameters for the
leads, the contact regions and the wire, and with two barriers at the contacts.
We show that this model explains several features of recent experiments, such
as the flat conductance plateaux observed even at finite temperatures and for
different lengths, and universal conductance corrections in different channels.
We discuss the possibility of seeing resonance-like structures of a fully open
channel at very low temperatures.Comment: revtex, 5 pages, 1 eps figure; clarifications added in light of new
experiment
Resonance in a Tomonaga-Luttinger liquid
We study a homogeneous Tomonaga-Luttinger liquid with backscattering
potential. A perturbative computation of the conductance at and near resonance
is given. We find that the backscattering of one electron dominates that of two
electrons for an interaction parameter and that the resonance point
depends on temperature. Our results may be relevant for recent experiments on
shot-noise in FQHE, where the charge 1/3 and not is measured on
resonance.Comment: 15 pages, three Figures. v2: Definite version, Citations added,
presentation improved. To appear in Phys. Rev. B, Rapid Co
Junctions of one-dimensional quantum wires - correlation effects in transport
We investigate transport of spinless fermions through a single site dot
junction of M one-dimensional quantum wires. The semi-infinite wires are
described by a tight-binding model. Each wire consists of two parts: the
non-interacting leads and a region of finite extent in which the fermions
interact via a nearest-neighbor interaction. The functional renormalization
group method is used to determine the flow of the linear conductance as a
function of a low-energy cutoff for a wide range of parameters. Several fixed
points are identified and their stability is analyzed. We determine the scaling
exponents governing the low-energy physics close to the fixed points. Some of
our results can already be derived using the non-self-consistent Hartree-Fock
approximation.Comment: version accepted for publication in Phys. Rev. B, 14 pages, 7 figures
include
SIMULATION AND MODELING THE DEFERRED BEHAVIOR INDUCED BY THE CREEP OF A REINFORCED CONCRETE SECTION
In the reinforced concrete structures, the differed deformations due to creep increase with time. They become dominating compared to the instantaneous strains, corresponding to a given loading. In order to ensure itself of the good behavior of an element or a concrete structure reinforced in service and with the absolute limit, it is necessary to correctly evaluate the instantaneous and differed deformations and displacements. Within the framework of this study one is interested in modeling and simulation of the behaviors instantaneous and differed from a zone bent out of reinforced concrete. For the simulation of the instantaneous behavior, in composed inflection, an existing program is used and of the complements of validations is realized. For the modeling of the differed behavior, a formulation based on the method of the adjusted effective module is presented. Another data-processing program is developed. It makes it possible to simulate the deformations of a zone bent under loading supported in time. The differed effects are taken into account by using the coefficient of creep recommended by the CEB 90. The simulations carried out on experimental tests allow the validation of the approach used. The results show that the best functioning of the approach used by the program and the comparison test-calculation is satisfactory
Fluctuation-Dissipation Relations of a Tunnel Junction Driven by a Quantum Circuit
We derive fluctuation-dissipation relations for a tunnel junction driven by a
high impedance microwave resonator, displaying strong quantum fluctuations. We
find that the fluctuation-dissipation relations derived for classical forces
hold, provided the effect of the circuit's quantum fluctuations is incorporated
into a modified non-linear curve. We also demonstrate that all
quantities measured under a coherent time dependent bias can be reconstructed
from their dc counterpart with a photo-assisted tunneling relation. We confirm
these predictions by implementing the circuit and measuring the dc current
through the junction, its high frequency admittance and its current noise at
the frequency of the resonator.Comment: Publisehd as Physical Review Letters, 114, 12680
Interactions between sub-10 nm iron and cerium oxide nanoparticles and 3T3 fibroblasts : the role of the coating and aggregation state
Recent nanotoxicity studies revealed that the physico-chemical
characteristics of engineered nanomaterials play an important role in the
interactions with living cells. Here, we report on the toxicity and uptake of
the cerium and iron oxide sub-10 nm nanoparticles by NIH/3T3 mouse fibroblasts.
Coating strategies include low-molecular weight ligands (citric acid) and
polymers (poly(acrylic acid), MW = 2000 g mol-1). Electrostatically adsorbed on
the surfaces, the organic moieties provide a negatively charged coating in
physiological conditions. We find that most particles were biocompatible, as
exposed cells remained 100% viable relative to controls. Only the bare and the
citrate-coated nanoceria exhibit a slight decrease of the mitochondrial
activity for cerium concentrations above 5 mM (equivalent to 0.8 g L-1). We
also observe that the citrate-coated particles are internalized by the cells in
large amounts, typically 250 pg per cell after a 24 h incubation for iron
oxide. In contrast, the polymer-coated particles are taken up at much lower
rates (< 30 pg per cell). The strong uptake shown by the citrate-coated
particles is related to the destabilization of the dispersions in the cell
culture medium and their sedimentation down to the cell membranes. In
conclusion, we show that the uptake of nanomaterials by living cells depends on
the coating of the particles and on its ability to preserve the colloidal
nature of the dispersions.Comment: 9 figures, 2 table
Junction of several weakly interacting quantum wires: a renormalization group study
We study the conductance of three or more semi-infinite wires which meet at a
junction. The electrons in the wires are taken to interact weakly with each
other through a short-range density-density interaction, and they encounter a
general scattering matrix at the junction. We derive the renormalization group
equations satisfied by the S-matrix, and we identify its fixed points and their
stabilities. The conductance between any pair of wires is then studied as a
function of physical parameters such as temperature. We discuss the possibility
of observing the effects of junctions in present day experiments, such as the
four-terminal conductance of a quantum wire and crossed quantum wires.Comment: RevTeX, 13 pages, including 4 eps figure
Transport theory of carbon nanotube Y junctions
We describe a generalization of Landauer-B\"uttiker theory for networks of
interacting metallic carbon nanotubes. We start with symmetric starlike
junctions and then extend our approach to asymmetric systems. While the
symmetric case is solved in closed form, the asymmetric situation is treated by
a mix of perturbative and non-perturbative methods. For N>2 repulsively
interacting nanotubes, the only stable fixed point of the symmetric system
corresponds to an isolated node. Detailed results for both symmetric and
asymmetric systems are shown for N=3, corresponding to carbon nanotube Y
junctions.Comment: submitted to New Journal of Physics, Focus Issue on Carbon Nanotubes,
15 pages, 3 figure
Discovery of a New Transient Magnetar Candidate: XTE J1810-197
We report the discovery of a new X-ray pulsar, XTE J1810-197. The source was
serendipitously discovered on 2003 July 15 by the Rossi X-ray Timing Explorer
(RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a
5.54 s spin-period and a soft spectrum (photon index ~ 4). We detect the source
in earlier RXTE observations back to 2003 January. These show that a transient
outburst began between 2002 November 17 and 2003 January 23 and that the pulsar
has been spinning down since then, with a high rate Pdot ~ 10^-11 s/s showing
significant timing noise, but no evidence for Doppler shifts due to a binary
companion. The rapid spin-down rate and slow spin-period imply a super-critical
magnetic field B=3x10^14 G and a young characteristic age < 7600 yr. These
properties are strikingly similar to those of anomalous X-ray pulsars and soft
gamma repeaters, making the source a likely new magnetar. A follow-up Chandra
observation provided a 2".5 radius error circle within which the 1.5 m
Russian-Turkish Optical Telescope RTT150 found a limiting magnitude of
R_c=21.5, in accord with other recently reported limits. The source is present
in archival ASCA and ROSAT data as well, at a level 100 times fainter than the
\~ 3 mCrab seen in 2003. This suggests that other X-ray sources that are
currently in a state similar to the inactive phase of XTE J1810-197 may also be
unidentified magnetars awaiting detection via a similar activity.Comment: Submitted to ApJL; 4 pages; 4 figure
The Deepest Radio Study of the Pulsar Wind Nebula G21.5-0.9: Still No Evidence for the Supernova Shell
We report on sensitive new 1.4-GHz VLA radio observations of the pulsar wind
nebula G21.5-0.9, powered by PSR J1833-1034, and its environs. Our observations
were targeted at searching for the radio counterpart of the shell-like
structure seen surrounding the pulsar wind nebula in X-rays. Some such radio
emission might be expected as the ejecta from the <~ 1000 yr old supernova
expand and interact with the surrounding medium. We find, however, no radio
emission from the shell, and can place a conservative 3-sigma upper limit on
its 1-GHz surface brightness of 7 x 10^-22 W/m^2/Hz/sr, comparable to the
lowest limits obtained for radio emission from shells around other pulsar-wind
nebulae. Our widefield radio image also shows the presence of two extended
objects of low-surface brightness. We re-examine previous 327-MHz images, on
which both the new objects are visible. We identify the first, G21.64-0.84, as
a new shell-type supernova remnant, with a diameter of ~13' and an unusual
double-shell structure. The second, G21.45-0.59, ~1' in diameter, is likely an
HII region.Comment: 8 Pages, submitted to MNRA
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