308 research outputs found
Heavy atom quantum diffraction by scattering from surfaces
Typically one expects that when a heavy particle collides with a surface, the
scattered angular distribution will follow classical mechanics. The heavy mass
assures that the de Broglie wavelength of the incident particle in the
direction of the propagation of the particle (the parallel direction) will be
much shorter than the characteristic lattice length of the surface, thus
leading to a classical description. Recent work on molecular interferometry has
shown that by increasing the perpendicular coherence length, one may observe
interference of very heavy species passing through a grating. Here we show,
using quantum mechanical simulations, that the same effect will lead to quantum
diffraction of heavy particles colliding with a surface. We find that the
effect is robust with respect to the incident energy, the angle of incidence
and the mass of the particle. It may also be used to verify the quantum nature
of the surface and its fluctuations at very low temperatures.Comment: 9 pages, 3 figure
Dissipating the Langevin equation in the presence of an external stochastic potential
In the Langevin formalism, the delicate balance maintained between the
fluctuations in the system and their corresponding dissipation may be upset by
the presence of a secondary, space-dependent stochastic force, particularly in
the low friction regime. In prior work, the latter was dissipated
self-consistently through an additional uniform (mean-field) friction [Shepherd
and Hernandez, J. Chem. Phys., 115, 2430-2438 (2001).] An alternative approach
to ensure that equipartition is satisfied relies on the use of a
space-dependent friction while ignoring nonlocal correlations. The approach is
evaluated with respect to its ability to maintain constant temperature for two
simple one-dimensional, stochastic potentials of mean force wherein the
friction can be evaluated explicitly when there is no memory in the barriers.
The use of a space-dependent friction is capable of providing qualitatively
similar results to those obtained previously, but in extreme cases, deviations
from equipartition may be observed due to the neglect of the memory effects
present in the stochastic potentials.Comment: 9 pages, 5 figures, to appear in J. Chem. Phy
Coherent quantum transport in disordered systems I: The influence of dephasing on the transport properties and absorption spectra on one-dimensional systems
Excitonic transport in static disordered one dimensional systems is studied
in the presence of thermal fluctuations that are described by the
Haken-Strobl-Reineker model. For short times, non-diffusive behavior is
observed that can be characterized as the free-particle dynamics in the
Anderson localized system. Over longer time scales, the environment-induced
dephasing is sufficient to overcome the Anderson localization caused by the
disorder and allow for transport to occur which is always seen to be diffusive.
In the limiting regimes of weak and strong dephasing quantum master equations
are developed, and their respective scaling relations imply the existence of a
maximum in the diffusion constant as a function of the dephasing rate that is
confirmed numerically. In the weak dephasing regime, it is demonstrated that
the diffusion constant is proportional to the square of the localization length
which leads to a significant enhancement of the transport rate over the
classical prediction. Finally, the influence of noise and disorder on the
absorption spectrum is presented and its relationship to the transport
properties is discussed.Comment: 23 pages, 7 figure
A hybrid stochastic hierarchy equations of motion approach to treat the low temperature dynamics of non-Markovian open quantum systems
The hierarchical equations of motion technique has found widespread success
as a tool to generate the numerically exact dynamics of non-Markovian open
quantum systems. However, its application to low temperature environments
remains a serious challenge due to the need for a deep hierarchy that arises
from the Matsubara expansion of the bath correlation function. Here we present
a hybrid stochastic hierarchical equation of motion (sHEOM) approach that
alleviates this bottleneck and leads to a numerical cost that is nearly
independent of temperature. Additionally, the sHEOM method generally converges
with fewer hierarchy tiers allowing for the treatment of larger systems.
Benchmark calculations are presented on the dynamics of two level systems at
both high and low temperatures to demonstrate the efficacy of the approach.
Then the hybrid method is used to generate the exact dynamics of systems that
are nearly impossible to treat by the standard hierarchy. First, exact energy
transfer rates are calculated across a broad range of temperatures revealing
the deviations from the Forster rates. This is followed by computations of the
entanglement dynamics in a system of two qubits at low temperature spanning the
weak to strong system-bath coupling regimes.Comment: 20 pages, 6 figure
Damage identification in composite panels using guided waves
A methodology for the identification of barely visible impact damage using guided waves on a typical aircraft composite structure is implemented. Delaminations and debondings have been introduced in two stiffened panels by means of impact loads
Quantum Diffusion on Molecular Tubes: Universal Scaling of the 1D to 2D Transition
The transport properties of disordered systems are known to depend critically
on dimensionality. We study the diffusion coefficient of a quantum particle
confined to a lattice on the surface of a tube, where it scales between the 1D
and 2D limits. It is found that the scaling relation is universal and
independent of the disorder and noise parameters, and the essential order
parameter is the ratio between the localization length in 2D and the
circumference of the tube. Phenomenological and quantitative expressions for
transport properties as functions of disorder and noise are obtained and
applied to real systems: In the natural chlorosomes found in light-harvesting
bacteria the exciton transfer dynamics is predicted to be in the 2D limit,
whereas a family of synthetic molecular aggregates is found to be in the
homogeneous limit and is independent of dimensionality.Comment: 10 pages, 6 figure
An exact equilibrium reduced density matrix formulation I: The influence of noise, disorder, and temperature on localization in excitonic systems
An exact method to compute the entire equilibrium reduced density matrix for
systems characterized by a system-bath Hamiltonian is presented. The approach
is based upon a stochastic unraveling of the influence functional that appears
in the imaginary time path integral formalism of quantum statistical mechanics.
This method is then applied to study the effects of thermal noise, static
disorder, and temperature on the coherence length in excitonic systems. As
representative examples of biased and unbiased systems, attention is focused on
the well-characterized light harvesting complexes of FMO and LH2, respectively.
Due to the bias, FMO is completely localized in the site basis at low
temperatures, whereas LH2 is completely delocalized. In the latter, the
presence of static disorder leads to a plateau in the coherence length at low
temperature that becomes increasingly pronounced with increasing strength of
the disorder. The introduction of noise, however, precludes this effect. In
biased systems, it is shown that the environment may increase the coherence
length, but only decrease that of unbiased systems. Finally it is emphasized
that for typical values of the environmental parameters in light harvesting
systems, the system and bath are entangled at equilibrium in the single
excitation manifold. That is, the density matrix cannot be described as a
product state as is often assumed, even at room temperature. The reduced
density matrix of LH2 is shown to be in precise agreement with the steady state
limit of previous exact quantum dynamics calculations.Comment: 37 pages, 12 figures. To appear in Phys. Rev.
Efectividad de un audiovisual como preparación para la cirugía en pacientes pediátricos
Las investigaciones sobre los pacientes pediátricos quirúrgicos, ponen en evidencia la urgente necesidad de preparlos psicológicamente para la hospitalización y la intervención. A este respecto, estudios realizados fuera de nuestro país han demostrado que la visualización de un audiovisual acerca de la hospitalización y el proceso quirúrgico es una técnica eficaz para ayudar a los niños y a sus progenitores a afrontar dicha situación. El objetivo del presente trabajo ha consistido en comprobar la eficacia de esta técnica en nuestro contexto hospitalario. Los resultados indican que la proyección de un audiovisual antes de la hospitalización es muy recomendable, dado que: 1) aclara dudas; 2) da una imagen más familiar del hospital; 3) estimula a los padres a hablar con sus hijos y 4) aumenta la satisfacción de los padres respecto a la información. Uno de los datos obtenidos que mayor refleja la eficacia del audiovisual es que el 98.2% de los padres recomendaría la visualización del audiovisual a otras familias.Research on paediatric surgical patients points to the urgent need to prepare them psychologically for hospitalisation and surgery. In this respect, studies conducted outside Spain have shown that seeing a video on hospitalisation and procedures for surgery is an effective technique for helping children and their parents to deal with this situation. The aim of the present work was to verify the effectiveness of this technique within the context of our hospitals. The results indicate that it is highly advisable to show such a video to patients and their parents before admission to hospital, since this: 1) clears up any doubts they might have; 2) gives a friendlier image of the hospitil; 3) encourages parents to talk to their children and 4) increases satisfaction among parents concerning the information they receive. One of the dadta which most clearly reflects the effectiveness of using the type of the audiovisual described is that 98.2% of parents would recommend other families to see it
Identification of barely visible impact damages on a stiffened composite panel with a probability-based approach
A probability-based damage detection algorithm has been implemented in order to identify barely visible impact damages in two composite stiffened panels by means of Acousto Ultrasonics (AU). A modification of RAPID (reconstruction algorithm for probabilistic inspection of defects) has been implemented to adapt the algorithm to the current structures and transducer networks. An improvement of 40% in the localization accuracy is obtained with the new algorith
SIG para la gestión eficiente de la información urbanística local
La dificultad de hacer el seguimiento del grado de cumplimiento del planeamiento
urbanístico con la realidad construida y cambiante en la ciudad, demanda el uso de
herramientas de Sistemas de Información Geográfica (SIG) en la gestión local para
desarrollar y mantener un conocimiento preciso de las características físicas, urbanas y económicas del parque inmobiliario, así como de las determinaciones de planeamiento que
afectan a los valores de su potencial edificable y urbano. En este trabajo se propone implementar un nuevo proceso innovador para el estudio y la gestión de la ciudad con SIG, dónde se establecen las pautas para obtener el potencial urbanístico de un ámbito, un barrio o una ciudad, a partir de la comparación de la realidad existente con el planeamiento urbanístico, detectando de manera automática tanto las parcelas que no agotan su potencial
como las que están sobreedificadas. Para ello se ha realizado un estudio piloto en el núcleo antiguo del barrio de Sant Andreu de Barcelona, el cual está en un proceso de modificación del planeamiento general vigente, afrontando el reto de desarrollar nuevas herramientas que permitan emular las consecuencias de las modificaciones y acometer una gestión eficiente de la información urbanística local.Peer Reviewe
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