3,312 research outputs found
Elastic interactions of active cells with soft materials
Anchorage-dependent cells collect information on the mechanical properties of
the environment through their contractile machineries and use this information
to position and orient themselves. Since the probing process is anisotropic,
cellular force patterns during active mechanosensing can be modelled as
anisotropic force contraction dipoles. Their build-up depends on the mechanical
properties of the environment, including elastic rigidity and prestrain. In a
finite sized sample, it also depends on sample geometry and boundary conditions
through image strain fields. We discuss the interactions of active cells with
an elastic environment and compare it to the case of physical force dipoles.
Despite marked differences, both cases can be described in the same theoretical
framework. We exactly solve the elastic equations for anisotropic force
contraction dipoles in different geometries (full space, halfspace and sphere)
and with different boundary conditions. These results are then used to predict
optimal position and orientation of mechanosensing cells in soft material.Comment: Revtex, 38 pages, 8 Postscript files included; revised version,
accepted for publication in Phys. Rev.
Information Extraction and Modeling from Remote Sensing Images: Application to the Enhancement of Digital Elevation Models
To deal with high complexity data such as remote sensing images presenting metric resolution over large areas, an innovative, fast and robust image processing system is presented.
The modeling of increasing level of information is used to extract, represent and link image features to semantic content.
The potential of the proposed techniques is demonstrated with an application to enhance and regularize digital elevation models based on information collected from RS images
Development and Construction of a new Photoelectron Imaging Spectrometer for Studying the Spectroscopy and Ultrafast Dynamics of Molecular Anions
We present a detailed account of the development, construction, and commissioning of a new experiment for studying the spectroscopy and ultrafast dynamics of molecular anions in the gas phase. The new instrument incorporates: an electrospray ionisation source, which is capable of generating a vast class of molecular anions; a Wiley-McLaren time-of-flight mass spectrometer; and a compact photoelectron imaging arrangement for anions, which negates the use of pulsed high voltages. We use this
instrument in conjunction with a femtosecond laser system to perform the first ultrafast time-resolved photoelectron imaging experiments on molecular anions generated through electrospray ionisation.
A method for reconstructing three dimensional charged particle distributions from their associated two dimensional projections on an imaging detector plane is described. This new method utilises: (1) onion-peeling in polar co-ordinates (POP) to perform the reconstruction; and (2) basis set concepts to significantly enhance the algorithms computational speed. We compare this new POP algorithm with other reconstruction algorithms, which shows that the method is as good as the benchmark pBASEX method
in terms of accuracy. Importantly, we show that it is also computationally fast, allowing images to be reconstructed as they are acquired in a typical imaging experiment.
Original work is presented which investigates the spectroscopy and ultrafast excited dynamics of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion. The photoelectron spectrum of TCNQ– is measured at 3.1 eV, which is used to gain insight into the electronic structure and geometries of both the anion and neutral states. Time-resolved photoelectron imaging experiments explore the relaxation dynamics of its first excited 1 2B3u state, which we show undergoes internal conversion back to the 2B2g
ground state on a timescale of 650 fs. Results also provide evidence of a wave packet motion on the excited state, which exhibits a characteristic frequency of 30 cm–1.
Finally, we describe, for the first time, a formulism which allows ultrafast relaxation timescales to be extracted from the photoelectron angular distributions of
isoenergetic photoelectron features. As an example, we use the time-resolved photoelectron angular distributions of a nearly isoenergetic feature in the photoelectron images of TCNQ–. From this model we extract a relaxation time for the 1 2B3u state, which quantitatively agrees with those extracted from fits to the features in the photoelectron spectra derived from the images
Selected aspects of complex, hypercomplex and fuzzy neural networks
This short report reviews the current state of the research and methodology
on theoretical and practical aspects of Artificial Neural Networks (ANN). It
was prepared to gather state-of-the-art knowledge needed to construct complex,
hypercomplex and fuzzy neural networks.
The report reflects the individual interests of the authors and, by now
means, cannot be treated as a comprehensive review of the ANN discipline.
Considering the fast development of this field, it is currently impossible to
do a detailed review of a considerable number of pages.
The report is an outcome of the Project 'The Strategic Research Partnership
for the mathematical aspects of complex, hypercomplex and fuzzy neural
networks' meeting at the University of Warmia and Mazury in Olsztyn, Poland,
organized in September 2022.Comment: 46 page
Hot Electrons Regain Coherence in Semiconducting Nanowires
The higher the energy of a particle is above equilibrium the faster it
relaxes due to the growing phase-space of available electronic states it can
interact with. In the relaxation process phase coherence is lost, thus limiting
high energy quantum control and manipulation. In one-dimensional systems high
relaxation rates are expected to destabilize electronic quasiparticles. We show
here that the decoherence induced by relaxation of hot electrons in
one-dimensional semiconducting nanowires evolves non-monotonically with energy
such that above a certain threshold hot-electrons regain stability with
increasing energy. We directly observe this phenomenon by visualizing for the
first time the interference patterns of the quasi-one-dimensional electrons
using scanning tunneling microscopy. We visualize both the phase coherence
length of the one-dimensional electrons, as well as their phase coherence time,
captured by crystallographic Fabry-Perot resonators. A remarkable agreement
with a theoretical model reveals that the non-monotonic behavior is driven by
the unique manner in which one dimensional hot-electrons interact with the cold
electrons occupying the Fermi-sea. This newly discovered relaxation profile
suggests a high-energy regime for operating quantum applications that
necessitate extended coherence or long thermalization times, and may stabilize
electronic quasiparticles in one dimension
Increased Dimensionality of Raman Cooling in a Slightly Nonorthogonal Optical Lattice
We experimentally study the effect of a slight nonorthogonality in a
two-dimensional optical lattice onto resolved-sideband Raman cooling. We find
that when the trap frequencies of the two lattice directions are equal, the
trap frequencies of the combined potential exhibit an avoided crossing and the
corresponding eigenmodes are rotated by 45 degrees relative to the lattice
beams. Hence, tuning the trap frequencies makes it possible to rotate the
eigenmodes such that both eigenmodes have a large projection onto any desired
direction in the lattice plane, in particular, onto the direction along which
Raman cooling works. Using this, we achieve two-dimensional Raman ground-state
cooling in a geometry where this would be impossible, if the eigenmodes were
not rotated. Our experiment is performed with a single atom inside an optical
resonator but this is inessential and the scheme is expected to work equally
well in other situations
Analysis of Sea Surface Features by Using X-Band Radar Data Sets
En este trabajo se recoge el estudio de algunos de los fenómenos que ocurren en el océano debido al oleaje mediante técnicas de teledetección en el rango de las microondas.
Estos fenómenos están relacionados con los diferentes mecanismos de formación de la imagen radar en banda X y en condiciones de incidencia tangente.
Dichos mecanismos permiten detectar fenómenos en dichas imágenes radar (conocidas como “clutter” marino para propósitos de navegación), como son la relación de dispersión del oleaje, sus armónicos superiores y la contribución espectral conocida en la literatura científica como “group line”. Para el estudio de estos fenómenos se emplean los espectros de las imágenes proporcionadas por diferentes estaciones que utilizan tecnología basadas en radar de navegación en banda X. Los sistemas radar proporcionan una secuencia de imágenes en el dominio del tiempo que, gracias a la descomposición tridimensional de Fourier, permite obtener dichos espectros correspondientes de la secuencia de imágenes radar para su posterior análisis.
Así, el espectro de la secuencia de imágenes de radar marino proporciona información sobre la distribución de la energía del oleaje, haciendo visible todos los fenómenos relacionados con el oleaje, el viento local, etc.
El estudio del “clutter”, o del ruido de fondo del espectro, también es importante ya que permite la estimación de la altura significativa de las olas.
En este trabajo se recoge un estudio detallado de la detección del “group line” y de la relación de dispersión del oleaje en función de la dirección de los diferentes ángulos de azimut que barren la imagen del radar, así como para diferentes alcances a partir de la ubicación del radar, además, de un estudio de la relación señal ruido considerando los fenómenos anteriores, así como de la máscara de iluminación de la superficie del mar, debida al efecto de ensombrecimiento de la antena radar, que también contiene las principales contribuciones del espectro de la imagen.
A partir del análisis de las diferentes contribuciones del espectro de la imagen radar, y utilizando diversas técnicas de inteligencia artificial, se desarrollan algoritmos que mejoran la estima de parámetros oceanográficos, como la altura significativa del oleaje y las corrientes superficiales
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