Dynamics of formation and decay of coherence in a polariton condensate

We study the dynamics of formation and decay of a condensate of microcavity polaritons. We investigate the relationship between the number of particles, the emission's linewidth and its degree of linear polarization which serves as the order parameter. Tracking the condensate's formation, we show that, even when interactions are negligible, coherence is not determined only by occupation of the ground state. As a result of the competition between the coherent and thermal fractions of the condensate, the highest coherence is obtained some time after the particle number has reached its maximum

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\sim 4$ meV to zero, at a field of $\sim$35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press

Interplay of the exciton and electron-hole plasma recombination on the photoluminescence dynamics in bulk GaAs

We present a systematic study of the exciton/electron-hole plasma photoluminescence dynamics in bulk GaAs for various lattice temperatures and excitation densities. The competition between the exciton and electron-hole pair recombination dominates the onset of the luminescence. We show that the metal-to-insulator transition, induced by temperature and/or excitation density, can be directly monitored by the carrier dynamics and the time-resolved spectral characteristics of the light emission. The dependence on carrier density of the photoluminescence rise time is strongly modified around a lattice temperature of 49 K, corresponding to the exciton binding energy (4.2 meV). In a similar way, the rise-time dependence on lattice temperature undergoes a relatively abrupt change at an excitation density of 120-180x10^15 cm^-3, which is about five times greater than the calculated Mott density in GaAs taking into account many body corrections.Comment: 15 pages, 7 figures, submitted to Phys. Rev.

Observation of long-lived polariton states in semiconductor microcavities across the parametric threshold

The excitation spectrum around the pump-only stationary state of a polariton optical parametric oscillator (OPO) in semiconductor microcavities is investigated by time-resolved photoluminescence. The response to a weak pulsed perturbation in the vicinity of the idler mode is directly related to the lifetime of the elementary excitations. A dramatic increase of the lifetime is observed for a pump intensity approaching and exceeding the OPO threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following onset of the soft Goldstone mode

Remote Estimation of Crop Health [ABSTRACT]

First sentence of abstract: In this paper we discuss developed techniques to remotely assess the fraction of photosynthetically active radiation absorbed by green vegetation [fAPAR-GREEN=fAPAR*(green LAI/total LAI)], fractional green vegetation cover (FGVC), green leaf area index (GLAI) green leaf biomass (GLB) and net ecosystem carbon dioxide exchange (NEE) in crops

Application of mesenchymal stem cells in bone regenerative procedures in oral implantology. A literature review

Objective: The aim of this work was to review de literature about the role of mesenchymal stem cells in bone regenerative procedures in oral implantology, specifically, in the time require to promote bone regeneration. Study Desing: A bibliographic search was carried out in PUBMED with a combination of different key words. Animal and human studies that assessed histomorphometrically the influence of mesenchymal stem cells on bone regeneration procedures in oral implantology surgeries were examined. Reults: - Alveolar regeneration: Different controlled histomorphometric animal studies showed that bone regeneration is faster using stem cells seeded in scaffolds than using scaffolds or platelet rich plasma alone. Human studies revealed that stem cells increase bone regeneration. - Maxillary sinus lift: Controlled studies in animals and in humans showed higher bone regeneration applying stem cells compared with controls. - Periimplantary bone regeneration and alveolar distraction: Studies in animals showed higher regeneration when stem cells are used. In humans, no evidence of applying mesenchymal stem cells in these regeneration procedures was found. Conclusion: Stem cells may promote bone regeneration and be useful in bone regenerative procedures in oral implantology, but no firm conclusions can be drawn from the rather limited clinical studies so far performed

Expansion-Free Cavity Evolution: Some exact Analytical Models

We consider spherically symmetric distributions of anisotropic fluids with a central vacuum cavity, evolving under the condition of vanishing expansion scalar. Some analytical solutions are found satisfying Darmois junction conditions on both delimiting boundary surfaces, while some others require the presence of thin shells on either (or both) boundary surfaces. The solutions here obtained model the evolution of the vacuum cavity and the surrounding fluid distribution, emerging after a central explosion. This study complements a previously published work where modeling of the evolution of such kind of systems was achieved through a different kinematical condition.Comment: 9 pages, Revtex. Typos corrected. Published in Int. J. Mod. Phys.

Modelling and simulation of the sea-landing of aerial vehicles using the Particle Finite Element Method

In this paper the Particle Finite Element Method (PFEM) is applied to the simulation of the sea-landing of an unmanned aerial vehicle (UAV). The problem of interest consists in modelling the impact of the vehicle against the water surface, analyzing the main kinematic and dynamic quantities (such as loads exerted upon the capsule at the moment of the impact). The PFEM, a methodology well-suited for free-surface flow simulation is used for modelling the water while a rigid body model is chosen for the vehicle. The vehicle under consideration is characterized by low weight. This leads to difficulties in modelling the fluid–structure interaction using standard Dirichlet–Neumann coupling. We apply a modified partitioned strategy introducing the interface Laplacian into the pressure Poisson's equation for obtaining a convergent FSI solution. The paper concludes with an industrial example of a vehicle sea-landing modelled using PFEM

Self-consistent model for ambipolar tunneling in quantum-well systems

We present a self-consistent approach to describe ambipolar tunneling in asymmetrical double quantum wells under steady-state excitation and extend the results to the case of tunneling from a near-surface quantum well to surface states. The results of the model compare very well with the behavior observed in photoluminescence experiments in $InGaAs/InP$ asymmetric double quantum wells and in near-surface $AlGaAs/GaAs$ single quantum wells.Comment: 10 pages, REVTeX 3.