Ref HAL: hal-01157624_v1
Ref Arxiv: 1412.6057
DOI: 10.1103/PhysRevD.91.064043
WoS: 000351428400006
Ref. & Cit.: NASA ADS
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16 Citations
Résumé:

We study the canonical structure of the real first order formulation of general relativity on a null foliation. We use a tetrad decomposition which allows to elegantly encode the nature of the foliation in the norm of a vector in the fibre bundle. The resulting constraint structure shows some peculiarities. In particular, the dynamical Einstein equations propagating the physical degrees of freedom appear in this formalism as second class tertiary constraints, which puts them on the same footing as the Hamiltonian constraint of the Ashtekar's connection formulation. We also provide a framework to address the issue of zero modes in gravity, in particular, to study the non-perturbative fate of the zero modes of the linearized theory. Our results give a new angle on the dynamics of general relativity and can be used to quantize null hypersurfaces in the formalism of loop quantum gravity or spin foams.

Commentaires: 35 pages, 1 figure; minor corrections, added references. Réf Journal: Phys. Rev. D 91, 064043 (2015)

Ref HAL: hal-01154792_v1
DOI: 10.1103/PhysRevB.91.205424
WoS: 000354972700012
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19 Citations
Résumé:

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density,measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons.We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μm is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the excitonemission pattern on the scale up to 100 μm. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

Ref HAL: hal-01152155_v1
PMID 25956109
DOI: 10.1063/1.4919645
WoS: 000354258200032
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15 Citations
Résumé:

We use computer simulations to study the relaxation dynamics of a model for oil-in-water microemulsion droplets linked with telechelic polymers. This system exhibits both gel and glass phases and we show that the competition between these two arrest mechanisms can result in a complex, three-step decay of the time correlation functions, controlled by two different localization lengthscales. For certain combinations of the parameters, this competition gives rise to an anomalous logarithmic decay of the correlation functions and a subdiffusive particle motion, which can be understood as a simple crossover effect between the two relaxation processes. We establish a simple criterion for this logarithmic decay to be observed. We also find a further logarithmically slow relaxation related to the relaxation of floppy clusters of particles in a crowded environment, in agreement with recent findings in other models for dense chemical gels. Finally, we characterize how the competition of gel and glass arrest mechanisms affects the dynamical heterogeneities and show that for certain combination of parameters these heterogeneities can be unusually large. By measuring the four-point dynamical susceptibility, we probe the cooperativity of the motion and find that with increasing coupling this cooperativity shows a maximum before it decreases again, indicating the change in the nature of the relaxation dynamics. Our results suggest that compressing gels to large densities produces novel arrested phases that have a new and complex dynamics.

Ref HAL: hal-01148904_v1
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Résumé:

Liquid sheets are formed by the collision of a liquid drop on a small solid target. Upon impact, the drop flattens into a radial sheet expanding in the air bounded by a thicker rim. A pure water sheet spreads out radially until it reaches a maximum diameter and then retracts due to the effect of surface tension. The destabilization mechanism is drastically modified when a dilute oil in water emulsion is used. The liquid sheet spreads out radially but now holes perforate the sheet before the retraction, as already observed for some surfactant solutions [1]. The holes do not perturb significantly the velocity field of the liquid sheet; they growth with a constant velocity given by the Culicks’s law until they merge together and form a web of ligaments, which are then destabilized into droplets. We use a simple experimental optical method we have developed recently to get time and space resolved measurements of the thickness field of the liquid sheet [2]. We show that each perforation’s event (hole) is preceded by a hole’s precursor (thinning zone of the liquid sheet) whose thickness profile and growth’s velocity have been measured . Interestingly each rupture event (transition from holes’s precursor to true hole) of the sheet is clearly evidenced by a discontinuity of the growth’s velocity of the instability. These experiments are appropriate to gain an understanding on the physical mechanisms governing the perforation of thick films of emulsion.

Ref HAL: hal-01148771_v1
PMID 25587999
DOI: 10.1021/la504579c
WoS: 000349575200007
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1 Citation
Résumé:

The influence of the surface charge distribution on the interaction between nanosized particles in water is reported. The distribution of charges at the surface of initially neutral microemulsion droplets has been modulated by additions of various oligomeric cationic surfactants. The osmotic compressibility of the doped microemulsions was measured by light and small-angle neutrons scattering and reveals that the overall effective interaction induced by the ionic groups is repulsive. However, particular charge distributions decrease the osmotic compressibility much less than others. Independent measurements of the activity of the bromide counterions with specific electrodes evidence a significant decrease in the effective charge, which, however, cannot account for the osmotic compressibility in the framework of the primitive model. The q dependence of the structure factor reveals an attractive contribution over a short distance. Numerical studies assign this attractive contribution to the overlap of hydration shells that are extended as a result of the charge localization.

Ref HAL: hal-01145505_v1
DOI: 10.1063/1.4906878
WoS: 000350546000006
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16 Citations
Résumé:

One-dimensional (1D) nanostructure devices are at the frontline of studies on future electronics, although issues like massive parallelization, doping control, surface effects, and compatibility with silicon industrial requirements are still open challenges. The recent rogresses in atomic to nanometer scale control of materials morphology, size, and composition including the growth of axial, radial and branched nanowirebased heterostructures, make the nanowire (NW) an ideal building block for implementing rectifying diodes or detectors that could be well operated into the Terahertz (THz), thanks to their typical achievable attofarad order capacitance. Here, we report on our recent progresses in the development of 1D InAs or InAs/InSb NW-based field effect transistors (FETs) exploiting novel morphologies and/or material combinations effective for addressing the goal of a semiconductor plasma-wave THz detector array technology. Through a critical review of material-related parameters (NW doping concentration, geometry and/or material choice) and antenna-related issues, here we underline the crucial aspects that can affect detection performance across the THz frequency region.

Ref HAL: hal-01145503_v1
DOI: 10.1109/TTHZ.2015.2398313
WoS: 000351080300019
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33 Citations
Résumé:

THz beam shaping via a single diffractive optical element is used to convert a divergent beam into a focal line segment perpendicular to the optical axis. The novel structure was designed for narrowband applications as a kinoform element and we successfully applied it in active, high-speed, THz linear scanners. The theoretical approach and experimental results are presented.