To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
We study the propagation of water waves over a ridge structured at the subwavelength scale using homogenization techniques able to account for its finite extent. The calculations are conducted in the time domain considering the full three-dimensional problem to capture the effects of the evanescent field in the water channel over the structured ridge and at its boundaries. This provides an effective two-dimensional wave equation which is a classical result but also non-intuitive transmission conditions between the region of the ridge and the surrounding regions of constant immersion depth. Numerical results provide evidence that the scattering properties of a structured ridge can be strongly influenced by the evanescent fields, a fact which is accurately captured by the homogenized model.
We consider the propagation of water waves in a waveguide with a surface-piercing circular cylinder. A plane wave interacting with the cylinder leads to a Fano resonance resulting in strong scattering with a large reflection coefficient. Using a smoothly varying bathymetry whose shape is optimized, we show both numerically and experimentally that broadband and robust backscattering reduction can be obtained below the first cutoff frequency.
In this paper we study both theoretically and experimentally the inverse problem of indirectly measuring the shape of a localized bottom deformation with a non-instantaneous time evolution, from either an instantaneous global state (space-based inversion) or a local time-history record (time-based inversion) of the free-surface evolution. Firstly, the mathematical inversion problem is explicitly defined and uniqueness of its solution is established. We then show that this problem is ill-posed in the sense of Hadamard, rendering its solution unstable. In order to overcome this difficulty, we introduce a regularization scheme as well as a strategy for choosing the optimal value of the associated regularization parameter. We then conduct a series of laboratory experiments in which an axisymmetric three-dimensional bottom deformation of controlled shape and time evolution is imposed on a layer of water of constant depth, initially at rest. The detailed evolution of the air–liquid interface is measured by means of a free-surface profilometry technique providing space- and time-resolved data. Based on these experimental data and employing our regularization scheme, we are able to show that it is indeed possible to reconstruct the seabed profile responsible for the linear free-surface dynamics either by space- or time-based inversions. Furthermore, we discuss the different relative advantages of each type of reconstruction, their associated errors and the limitations of the inverse determination.
Molecular biology techniques have enabled us to prepare and select RNA aptamers that can bind specifically to small targets. RNA oligonucleotides can also be used as fluorescent probes. We have combined the two approaches to obtain Aptamer Beacons, in which molecular recognition is linked to the emission of an optical signal. These RNA biosensors could be used to detect directly the signatures of life in samples of mineral and extra-terrestrial material.
The main goals of the French Mars exploration program are the development of a netlander for geophysical studies. This project is conducted in the framework of a consortium of European partners (Finland, Germany and Belgium), and an orbiter vehicle of the future Mars sample return mission (MSR) is being developed in cooperation with NASA. Also, we discuss the preparatory program for volatile and rare gas analysis, sample pollution, site selection and Astrobiology.
We consider the role of the dwarf planet Ceres on the secular dynamics of the asteroid main belt. Specifically, we examine the post impact evolution of asteroid families due to the interaction of their members with the linear nodal secular resonance with Ceres. First, we find the location of this resonance and identify which asteroid families are crossed by its path. Next, we summarize our results for three asteroid families, namely (1726) Hoffmeister, (1128) Astrid and (1521) Seinajoki which have irregular distributions of their members in the proper elements space, indicative of the effect of the resonance. We confirm this by performing a set of numerical simulations, showcasing that the perturbing action of Ceres through its linear nodal secular resonance is essential to reproduce the actual shape of the families.
The study of the evolution of organic matter subjected to space conditions, and more specifically to Solar photons in the vacuum ultraviolet range (120–200 nm) has been undertaken in low-Earth orbit since the 1990s, and implemented on various space platforms. This paper describes a photochemistry experiment called AMINO, conducted during 22 months between 2009 and 2011 on the EXPOSE-R ESA facility, outside the International Space Station. Samples with relevance to astrobiology (connected to comets, carbonaceous meteorites and micrometeorites, the atmosphere of Titan and RNA world hypothesis) have been selected and exposed to space environment. They have been analysed after return to the Earth. This paper is not discussing the results of the experiment, but rather gives a general overview of the project, the details of the hardware used, its configuration and recent developments to enable long-duration exposure of gaseous samples in tight closed cells enabling for the first time to derive quantitative results from gaseous phase samples exposed in space.
Careful examination of the present metabolism and in vitro selection of various catalytic RNAs strongly support the RNA world hypothesis as a crucial step of the origins and early life evolution. Small functional RNAs were exposed from 10 March 2009 to 21 January 2011 to space conditions on board the International Space Station in the EXPOSE-R mission. The aim of this study was to investigate the preservation or modification properties such as integrity of RNAs after space exposition. The exposition to the solar radiation has a strong degradation effect on the size distribution of RNA. Moreover, the comparison between the in-flight samples, exposed to the Sun and not exposed, indicates that the solar radiation degrades RNA bases.
Intangible assets can play a strategic role in the implementation of differentiated strategies in foreign markets. The literature has addressed the impact of intangible assets on both exports and financial performance and the effects of exports on company financial performance (profit and risk). This article aims to analyze the effect of exports on the relationship between intangibles and company performance in the wine industry. Empirical studies show that intangibles have a positive but diminishing impact on exports. The effect of exports on financial performance differs depending on whether we consider corporations or cooperatives. While intangible expenses reduce company risk in both samples whatever the level of export intensity, the effects are different with profit. In corporations, intangible expenses have a positive impact on profit only when there is a high level of expenses and a high level of export intensity. (JEL Classifications: G32, L25, Q12, Q13)
The centrosome is the principal microtubule organization center in cells, giving rise to microtubule-based organelles (e.g., cilia, flagella). The aim was to study the osteocyte centrosome morphology at an ultrastructural level in relation to its mechanosensitive function. Osteocyte centrosomes and cilia in tibial cortical bone were explored by acetylated alpha-tubulin (AαTub) immunostaining under confocal microscopy. For the first time, fine ultrastructure and spatial orientation of the osteocyte centrosome were explored by transmission electron microscopy on serial ultrathin sections. AαTub-positive staining was observed in 94% of the osteocytes examined (222/236). The mother centriole formed a short primary cilium and was longer than the daughter centriole due to an intermediate zone between centriole and cilium. The proximal end of the mother centriole was connected with the surface of daughter centriole by striated rootlets. The mother centriole exhibited distal appendages that interacted with the cell membrane and formed a particular structure called “cilium membrane prolongation.” The primary cilium was mainly oriented perpendicular to the long axis of bone. Mother and daughter centrioles change their original mutual orientation during the osteocyte differentiation process. The short primary cilium is hypothesized as a novel type of fluid-sensing organelle in osteocytes.
We present an experimental study of drop impact on a solid surface in the spreading regime with no splashing. Using the space–time-resolved Fourier transform profilometry technique, we can follow the evolution of the drop shape during the impact. We show that a self-similar dynamical regime drives the drop spreading until the growth of a viscous boundary layer from the substrate selects a residual minimal film thickness. Finally, we discuss the interplay between capillary and viscous effects in the spreading dynamics, which suggests a pertinent impact parameter.
It is often believed that without instruments, endogenous sample selection models are identified only if a covariate with a large support is available (see, e.g., Chamberlain, 1986, Journal of Econometrics 32, 189–218; Lewbel, 2007, Journal of Econometrics141, 777–806) . We propose a new identification strategy mainly based on the condition that the selection variable becomes independent of the covariates for large values of the outcome. No large support on the covariates is required. Moreover, we prove that this condition is testable. We finally show that our strategy can be applied to the identification of generalized Roy models.
The theory of the interaction of elastic waves with dislocations is reviewed, as is the extent to which it has been tested by experiment. There are two essential ingredients to the wave-dislocation interaction: one is that, when a wave hits a dislocation, the latter will respond by moving in some fashion. The other is that, when a dislocation moves, it generates (“radiates”) elastic waves. For a linearly elastic solid continuum, both phenomena can be described by equations that are linear outside the dislocation core. One is a linear elastic wave equation with a right-hand-side term that is localized at the dislocation position. The other is a linear equation for the vibrations of a string (that is coincident with the dislocation), with an external loading provided by the wave. This provides the basic mechanism for the scattering of elastic waves by dislocations, and it can be worked out in considerable detail for pinned dislocation segments and prismatic dislocation loops in infinite media, as well as for the scattering of surface (Rayleigh) elastic waves by subsurface dislocation segments.
The results for the scattering by a single dislocation can be used as input in a multiple scattering formalism to study the properties of a coherent wave propagating in a solid with many dislocations present. Expressions for the effective velocity of propagation, and for the disorder-induced (as distinct from the internal losses) attenuation can be found. They test successfully with Resonant Ultrasound Spectroscopy (RUS) experimental measurements.
Open problems, possible further applications and current efforts are discussed.
High quality GaAs and InP have been grown on silicon substrates, using low pressure metalorganic chemical vapor deposition technique. The growth temperature is 550ºC andthe growth rate 100 A/min.
Photoluminescence, X-ray diffraction and electrochemical profiling verified the high quality of these layers. The use of superlattices as buffer layers, (GaAs/GaInP) in the case of GaAs/Si and (GalnAsP/InP) in the case of InP/Si, decreased the amount of misfit dislocations in the epitaxial layer. Carrier concentrations as low as 5.1015 cm-3 have been measured by electrochemical profiling.
GaInAs/GaAs/GaInP multiquantum well laser structures have been grown by chemical beam epitaxy (CBE) using conventional sources (hydrides as group V element sources). Large area lasers were photolitographically defined and mounted for continuous wave (CW) measurements. CW output power levels of 600 mW at 25°C are reported from 100 μm wide, 300 μm long laser diodes without any facet treatment. At these levels, the delivered current is 2A, with an associated voltage of less than 1.7 V. The characteristic temperature of the structure is 95 K.
The same structures were then grown using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP). The large area laser diodes were characterized under pulsed conditions. For a 300 μm long cavity, threshold current density of 390 A/cm2 and external quantum efficiency of 0.6 W/A (2 facets) were obtained, demonstrating the suitability of TBP and TBAs as substitutes of arsine and phosphine in chemical beam epitaxy for laser fabrication.
High quality GaAs and InP have been grown on silicon substrates, using low pressure metalorganic chemical vapor deposition technique. The growth temperature is 550°C and the growth rate 100 A/min.
Photoluminescence, X-ray diffraction and electrochemical profiling verified the high quality of these layers. The use of superlattices as buffer layers, (GaAs/GaInP) in the case of GaAs/Si and (GaInAsP/InP) in the case of InP/Si, decreased the amount of misfit dislocations in the epitaxial layer. Carrier concentrations as low as 5.1015 cm−3 have been measured by electrochemical profiling.
The low temperature growth procedure used in the case of GaAs to introduce high concentrations of deep traps such as arsenic antisite defects has been extended to the growth of InP by gas source molecular beam epitaxy. The low temperature growth of InP induces a strong group V stoechiometric deviation (of the order of +1%). On the other hand, Secondary Ion Mass Spectrometry reveals high levels of hydrogen ranging from 3.1018 to 3.1019 cm−3 depending on growth temperature. Undoped layers are found to be resistive without any post annealing. Annealing experiments above 250°C lead to conductive layers suggesting a passivation effect of both shallow donors and acceptors by hydrogen.