Modelling the water mass exchange through navigational channels connecting adjacent coastal basins - application to the Channel of Potidea (North Aegean Sea)

The research objective is the detection of the mechanism of the water mass exchange through a navigational channel connecting two adjacent coastal basins. The research involves the application of a mathematical model in parallel to in-situ measurements. The hydrodynamic circulation in the greater area of the NW Aegean Sea is modeled by means of a barotropic circulation model. Wind, Coriolis and Tide are the main forcings taken into account. The flow through the channel is resolved at a subgrid scale by means of a local open channel flow model. The comparison between field measurements, recorded during a limited period, and the model results supports the model verification. The study is integrated by an operational application of the model under various realistic forcings. The results help to gain a better understanding of the mechanisms regulating the water mass exchange and the consequent interaction between two adjacent connected coastal basins. From the case study of the Potidea channel it is revealed that the water mass exchange under mean wind forcing is of the same order as the one induced by the tidal forcing

Modelling the cohesive sediment transport in the marine environment: the case of Thermaikos Gulf

The transport of fine-grained sediments in the marine environment entails risks of pollutant intrusions from substances absorbed onto the cohesive flocks&apos; surface, gradually released to the aquatic field. These substances include nutrients such as nitrate, phosphate and silicate compounds from drainage from fertilization of adjacent cultivated areas that enter the coastal areas through rivers and streams, or trace metals as remainders from urban and industrial activities. As a consequence, knowledge on the motion and distribution of sediment particles coming from a given pollutant source is expected to provide the &apos;bulk&apos; information on pollutant distribution, necessary for determining the region of influence of the source and to estimate probable trophic levels of the seawater and potential environmental risks. In that aim a numerical model has been developed to predict the fate of the sediments introduced to the marine environment from different pollution sources, such as river outflows, erosion of the seabed, aeolian transported material and drainage systems. <br><br> The proposed three-dimensional mathematical model is based on the particle tracking method, according to which matter concentration is expressed by particles, each representing a particular amount of sedimentary mass, passively advected and dispersed by the currents. The processes affecting characteristics and propagation of sedimentary material in the marine environment, incorporated in the parameterization, apart from advection and dispersion, include cohesive sediment and near-bed processes. The movement of the particles along with variations in sedimentary characteristics and state, carried by each particle as personal information, are traced with time. Specifically, concerning transport processes, the local seawater velocity and the particle&apos;s settling control advection, whereas the random Brownian motion due to turbulence simulates turbulent diffusion. The vertical stratification of the water-column is taken into consideration by appropriate damping of the vertical diffusion term. Variations in cohesive sediment properties during the abidance in the aquatic environment include coagulation and flock break-up processes, quantification of the effects of ambient density to the density of the cohesive aggregate and the associated alterations to the falling speed of the particle. In the vicinity of the seabed, particles may deposit and gradually consolidate with time, the particles remain settled onto the bed, re-enter the flow at a later temporal point or may enter the water column for the first time, originating from the erosion of the bed. The occurrence of each of the aforementioned near-bed processes is defined according to the prevailing benthic shear stress conditions. <br><br> The mathematical model has been applied to the Thermaikos Gulf, an area of high environmental and socioeconomic importance but also a region of significant pollutant forcing from various anthropogenic activities taking place in the adjoining land. Various kinds of outputs can be extracted, such as trajectories of the overall movement of specific particles and related alterations of their characteristics with time, snapshots of the domain with respect to suspended or deposited matter and natural concentrations of sediments at every required temporal and spatial point. Indicative results from yearly and monthly simulations, using input baroclinic circulation data from the North Aegean Sea model and river discharges are presented and discussed, including outputs from a Typical One-Year Simulation (TOYS), the simulation of the period from 3 September 2001 to 31 August 2002 (S1A2) and the January 2003 experiment (J03). <br><br> The description of the processes that have been incorporated in the parameterization covers the most significant factors controlling transport and mixing of fine grained sediments in the marine environment, thus validating the accuracy and completeness of the model. One of the major advantages, apart from the observation of the phenomena in scales smaller than the grid size, describing the natural processes more accurately, is the flexibility in accepting various pollutant sources and the applicability to different domains with minor modifications. The model has been incorporated in the MFSTEP project, as part of the developed operational forecasting system for the Mediterranean Sea. The application can be used for the prognosis of the seawater quality for current and for future conditions, enabling employment as part of a near-real time observation system or to formulate decisions for the protection of the seawater environment

Observation of inversion, hysteresis, and collapse of spin in optically trapped polariton condensates

The spin and intensity of optically trapped polariton condensates are studied under steady-state elliptically-polarised nonresonant pumping. Three distinct effects are observed: (1) spin inversion where condensation occurs in the opposite handedness from the pump, (2) spin/intensity hysteresis as the pump power is scanned, and (3) a sharp ‘spin collapse’ transition in the condensate spin as a function of the pump ellipticity. We show these effects are strongly dependent on trap size and sample position and are linked to small counterintuitive energy differences between the condensate spin components. Our results, which fail to be fully described within the commonly used nonlinear equations for polariton condensates, show that a more accurate microscopic picture is needed to unify these phenomena in a two-dimensional condensate theory

Spontaneous spin bifurcations and ferromagnetic phase transitions in a spinor exciton-polariton condensate

We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatially trapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two elliptically polarized (up to 95% circularly-polarized) states with opposite handedness of polarization. The magnetized condensate remains stable for many seconds at 5 K, but at higher temperatures it can flip from one magnetic orientation to another. We optically address these states and demonstrate the inversion of the magnetic state by resonantly injecting 100-fold weaker pulses of opposite spin. Theoretically, these phenomena can be well described as spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations.This work was supported by Grants EPSRC No. EP/G060649/1, EU No. CLERMONT4 235114, EU No. INDEX 289968, Spanish MEC (MAT2008-01555), Greek GSRT ARISTEIA Apollo program and Fundación La Caixa, and Mexican CONACYT No. 251808. FP acknowledges financial support through an EPSRC doctoral prize fellowship at the University of Cambridge and a Schrödinger fellowship at the University of Oxford.This is the final version of the article. It first appeared from the American Physical Society via http://dx.doi.org/10.1103/PhysRevX.5.03100

Theory of neutral and charged exciton scattering with electrons in semiconductor quantum wells

Electron scattering on both neutral ($X$) and charged ($X^-$) excitons in quantum wells is studied theoretically. A microscopic model is presented, taking into account both elastic and dissociating scattering. The model is based on calculating the exciton-electron direct and exchange interaction matrix elements, from which we derive the exciton scattering rates. We find that for an electron density of $10^9 {\rm cm}^{-2}$ in a GaAs QW at $T=5K$, the $X^-$ linewidth due to electron scattering is roughly twice as large as that of the neutral exciton. This reflects both the $X^-$ larger interaction matrix elements compared with those of $X$, and their different dependence on the transferred momentum. Calculated reflection spectra can then be obtained by considering the three electronic excitations of the system, namely, the heavy-hole and light-hole 1S neutral excitons, and the heavy-hole 1S charged exciton, with the appropriate oscillator strengths.Comment: 18 pages, 12 figure

Spin Order and Phase Transitions in Chains of Polariton Condensates

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice.We acknowledge Grants No. EPSRC EP/L027151/1, No. EU INDEX 289968, No. ERC “POLAFLOW” Starting Grant, ERC LINASS 320503, Spanish MEC (MAT2008- 01555), Mexican CONACYT 251808, Leverhulme Trust Grant No. VP1-2013-011 and Fundación La Caixa. H. S. and I. S. acknowledge support by the Research Fund of the University of Iceland, The Icelandic Research Fund, Grant No. 163082-051. T. L. was supported by the MOE AcRF Tier 1 Grant No. 2016-T1-001-084. P. S. acknowledges financial support from the Stavros Niarchos Foundation, “ARCHERS” project

Vortices in polariton OPO superfluids

This chapter reviews the occurrence of quantised vortices in polariton fluids, primarily when polaritons are driven in the optical parametric oscillator (OPO) regime. We first review the OPO physics, together with both its analytical and numerical modelling, the latter being necessary for the description of finite size systems. Pattern formation is typical in systems driven away from equilibrium. Similarly, we find that uniform OPO solutions can be unstable to the spontaneous formation of quantised vortices. However, metastable vortices can only be injected externally into an otherwise stable symmetric state, and their persistence is due to the OPO superfluid properties. We discuss how the currents charactering an OPO play a crucial role in the occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure

Resonant nonstationary amplification of polychromatic laser pulses and conical emission in an optically dense ensemble of neon metastable atoms

Experimental and numerical investigation of single-beam and pump-probe interaction with a resonantly absorbing dense extended medium under strong and weak field-matter coupling is presented. Significant probe beam amplification and conical emission were observed. Under relatively weak pumping and high medium density, when the condition of strong coupling between field and resonant matter is fulfilled, the probe amplification spectrum has a form of spectral doublet. Stronger pumping leads to the appearance of a single peak of the probe beam amplification at the transition frequency. The greater probe intensity results in an asymmetrical transmission spectrum with amplification at the blue wing of the absorption line and attenuation at the red one. Under high medium density, a broad band of amplification appears. Theoretical model is based on the solution of the Maxwell-Bloch equations for a two-level system. Different types of probe transmission spectra obtained are attributed to complex dynamics of a coherent medium response to broadband polychromatic radiation of a multimode dye laser.Comment: 9 pages, 13 figures, corrected, Fig.8 was changed, to be published in Phys. Rev.

Angle-resonant stimulated polariton amplifier

We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification induced by stimulated energy relaxation

Progress on a spherical TPC for low energy neutrino detection

The new concept of the spherical TPC aims at relatively large target masses with low threshold and background, keeping an extremely simple and robust operation. Such a device would open the way to detect the neutrino-nucleus interaction, which, although a standard process, remains undetected due to the low energy of the neutrino-induced nuclear recoils. The progress in the development of the fist 1 m$^3$ prototype at Saclay is presented. Other physics goals of such a device could include supernova detection, low energy neutrino oscillations and study of non-standard properties of the neutrino, among others.Comment: 3 pages, talk given at the 9th Workshop on Topics in Astroparticle and Underground Physics, Zaragoza, September 10-1