7,589 research outputs found
Anderson Localization in Disordered Vibrating Rods
We study, both experimentally and numerically, the Anderson localization
phenomenon in torsional waves of a disordered elastic rod, which consists of a
cylinder with randomly spaced notches. We find that the normal-mode wave
amplitudes are exponentially localized as occurs in disordered solids. The
localization length is measured using these wave amplitudes and it is shown to
decrease as a function of frequency. The normal-mode spectrum is also measured
as well as computed, so its level statistics can be analyzed. Fitting the
nearest-neighbor spacing distribution a level repulsion parameter is defined
that also varies with frequency. The localization length can then be expressed
as a function of the repulsion parameter. There exists a range in which the
localization length is a linear function of the repulsion parameter, which is
consistent with Random Matrix Theory. However, at low values of the repulsion
parameter the linear dependence does not hold.Comment: 10 pages, 6 figure
Stability of conductance oscillations in monatomic sodium wires
We study the stability of conductance oscillations in monatomic sodium wires
with respect to structural variations. The geometry, the electronic structure
and the electronic potential of sodium wires suspended between two sodium
electrodes are obtained from self-consistent density functional theory
calculations. The conductance is calculated within the framework of the
Landauer-B\"utttiker formalism, using the mode-matching technique as formulated
recently in a real-space finite-difference scheme [Phys. Rev. B \textbf{70},
195402 (2004)]. We find a regular even-odd conductance oscillation as a
function of the wire length, where wires comprising an odd number of atoms have
a conductance close to the quantum unit , and even-numbered
wires have a lower conductance. The conductance of odd-numbered wires is stable
with respect to geometry changes in the wire or in the contacts between the
wire and the electrodes; the conductance of even-numbered wires is more
sensitive. Geometry changes affect the spacing and widths of the wire
resonances. In the case of odd-numbered wires the transmission is on-resonance,
and hardly affected by the resonance shapes, whereas for even-numbered wires
the transmission is off-resonance and sensitive to the resonance shapes.
Predicting the amplitude of the conductance oscillation requires a
first-principles calculation based upon a realistic structure of the wire and
the leads. A simple tight-binding model is introduced to clarify these results.Comment: 16 pages, 20 figure
Structural fluctuations and quantum transport through DNA molecular wires: a combined molecular dynamics and model Hamiltonian approach
Charge transport through a short DNA oligomer (Dickerson dodecamer) in
presence of structural fluctuations is investigated using a hybrid
computational methodology based on a combination of quantum mechanical
electronic structure calculations and classical molecular dynamics simulations
with a model Hamiltonian approach. Based on a fragment orbital description, the
DNA electronic structure can be coarse-grained in a very efficient way. The
influence of dynamical fluctuations arising either from the solvent
fluctuations or from base-pair vibrational modes can be taken into account in a
straightforward way through time series of the effective DNA electronic
parameters, evaluated at snapshots along the MD trajectory. We show that charge
transport can be promoted through the coupling to solvent fluctuations, which
gate the onsite energies along the DNA wire
Tratamiento de la demodicosis canina generalizada con dosis reducidas de ivermectina oral
La administración diaria de ivermectina a dosis altas (0,6 mg/kg) es la mejor alternativa disponible para tratar aquellos casos de sarna demodécica resistente al almiraz
Spectral signatures of symmetry-breaking dynamical phase transitions
Large deviation theory provides the framework to study the probability of
rare fluctuations of time-averaged observables, opening new avenues of research
in nonequilibrium physics. One of the most appealing results within this
context are dynamical phase transitions (DPTs), which might occur at the level
of trajectories in order to maximize the probability of sustaining a rare
event. While the Macroscopic Fluctuation Theory has underpinned much recent
progress on the understanding of symmetry-breaking DPTs in driven diffusive
systems, their microscopic characterization is still challenging. In this work
we shed light on the general spectral mechanism giving rise to continuous DPTs
not only for driven diffusive systems, but for any jump process in which a
discrete symmetry is broken. By means of a symmetry-aided
spectral analysis of the Doob-transformed dynamics, we provide the conditions
whereby symmetry-breaking DPTs might emerge and how the different dynamical
phases arise from the specific structure of the degenerate eigenvectors. We
show explicitly how all symmetry-breaking features are encoded in the
subleading eigenvectors of the degenerate manifold. Moreover, by partitioning
configuration space into equivalence classes according to a proper order
parameter, we achieve a substantial dimensional reduction which allows for the
quantitative characterization of the spectral fingerprints of DPTs. We
illustrate our predictions in three paradigmatic many-body systems: (i) the 1D
boundary-driven weakly asymmetric exclusion process (WASEP), which exhibits a
particle-hole symmetry-breaking DPT for current fluctuations, (ii) the and
-state Potts model, which displays discrete rotational symmetry-breaking DPT
for energy fluctuations, and (iii) the closed WASEP which presents a continuous
symmetry-breaking DPT to a time-crystal phase characterized by a rotating
condensate
New Cosmological Structures on Medium Angular Scales Detected with the Tenerife Experiments
We present observations at 10 and 15 GHz taken with the Tenerife experiments
in a band of the sky at Dec.=+35 degrees. These experiments are sensitive to
multipoles in the range l=10-30. The sensitivity per beam is 56 and 20 microK
for the 10 and the 15 GHz data, respectively. After subtraction of the
prediction of known radio-sources, the analysis of the data at 15 GHz at high
Galactic latitude shows the presence of a signal with amplitude Delta Trms ~ 32
microK. In the case of a Harrison-Zeldovich spectrum for the primordial
fluctuations, a likelihood analysis shows that this signal corresponds to a
quadrupole amplitude Q_rms-ps=20.1+7.1-5.4 microK, in agreement with our
previous results at Dec.+=40 degrees and with the results of the COBE DMR.
There is clear evidence for the presence of individual features in the RA range
190 degrees to 250 degrees with a peak to peak amplitude of ~110 microK. A
preliminary comparison between our results and COBE DMR predictions for the
Tenerife experiments clearly indicates the presence of individual features
common to both. The constancy in amplitude over such a large range in frequency
(10-90 GHz) is strongly indicative of an intrinsic cosmological origin for
these structures.Comment: ApJ Letters accepted, 13 pages Latex (uses AASTEX) and 4 encapsulated
postscript figures
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