First-order dissipative phase transition in an exciton-polariton condensate

We investigate the phase diagram of a two-dimensional driven-dissipative system of polaritons coupled to an excitonic reservoir. We find that two critical points exists. The first corresponds to the quasicondensation and the second to a first-order phase transition from the nonuniform state with spatially modulated density to a uniform state. The latter is related to the modulational instability of a homogeneous state due to the repulsive interactions with the noncondensed reservoir. The first-order character of the transition is evidenced by a discontinuity in the density and the correlation length as well as the phase coexistence and metastability. Moreover, we show that a signature of a Berezinskii-Kosterlitz-Thouless-like transition can be observed in the nonuniform phase

Angular dependence of giant Zeeman effect for semimagnetic cavity polariton

The observation of spin-related phenomena of microcavity polaritons has been limited due to the weak Zeeman effect of nonmagnetic semiconductors. We demonstrate that the incorporation of magnetic ions into quantum wells placed in a nonmagnetic microcavity results in enhanced effects of magnetic field on exciton-polaritons. We show that in such a structure the Zeeman splitting of exciton-polaritons strongly depends on the photon-exciton detuning and polariton wave vector. Our experimental data are explained by a model where the impact of magnetic field on the lower polariton state is directly inherited from the excitonic component, and the coupling strength to the cavity photon is modified by an external magnetic field

The Pull-out Capacity of Suction Caissons in Model Investigations

A small-scale model experiment on the pull-out resistance of suction caissons is described. The pull-out force and suction developed within the caisson in the extraction process were recorded during the experiment. A simple breakout model, together with an elementary static formulae, is applied to predict the results obtained experimentally. There is a reasonably good agreement between the experimental results and predictions. An extensive discussion of the approach applied is included. The analysis presented in this paper is original, as it differs from other approaches mentioned in this paper, and leads to acceptable predictions. At the end, the results are also compared with another approach for predicting the capacity of suction caissons

Estimation of Stresses in a Dry Sand Layer Tested on Shaking Table

Theoretical analysis of shaking table experiments, simulating earthquake response of a dry sand layer, is presented. The aim of such experiments is to study seismic-induced compaction of soil and resulting settlements. In order to determine the soil compaction, the cyclic stresses and strains should be calculated first. These stresses are caused by the cyclic horizontal acceleration at the base of soil layer, so it is important to determine the stress field as function of the base acceleration. It is particularly important for a proper interpretation of shaking table tests, where the base acceleration is controlled but the stresses are hard to measure, and they can only be deduced. Preliminary experiments have shown that small accelerations do not lead to essential settlements, whilst large accelerations cause some phenomena typical for limit states, including a visible appearance of slip lines. All these problems should be well understood for rational planning of experiments. The analysis of these problems is presented in this paper. First, some heuristic considerations about the dynamics of experimental system are presented. Then, the analysis of boundary conditions, expressed as resultants of respective stresses is shown. A particular form of boundary conditions has been chosen, which satisfies the macroscopic boundary conditions and the equilibrium equations. Then, some considerations are presented in order to obtain statically admissible stress field, which does not exceed the Coulomb-Mohr yield conditions. Such an approach leads to determination of the limit base accelerations, which do not cause the plastic state in soil. It was shown that larger accelerations lead to increase of the lateral stresses, and the respective method, which may replace complex plasticity analyses, is proposed. It is shown that it is the lateral stress coefficient K0 that controls the statically admissible stress field during the shaking table experiments

Complex investigations of landslides in the Brda river valley in Koronowo near Bydgoszcz

In the recentyears intensified landslidingwas observed in the surroundings and in the town of Koronowo near Bydgoszcz. (Krajeñskie Lake District). The gently undulated moraine upland, a wide valley of the Brda river, its relatively steep valleysides and incised valleys of tributaries are morphologicfeatures related to modeling by North Polish Glaciation. Quaternary tills alternated with fluvioglacial sands and gravels are underalined by Miocene clay and mud with brown coal intercalations. Due to such morphologic and geologic setting the study area isprone to slope instability. Failure apt soil massifs were assigned to 4 groups. Out of 32 identified landslides, 9 were selected for detailed examination. To register a nature and rate of failures a complex monitoring system was installed on slopes where the selected landslides resulted in severe damages to municipal infrastructure. The system was facilitated with inclinometers andpiezometers, supported by a network of GPS-RTK geodetic benchmarks as well as with a recording weather station. The performed examination revealed that the most unfavourable arrangement of the layers is, when under a non-cohesive soil (e.g. sand, gravel) lies a cohesive impermeable layer (e.g. clay, loam). The landslides are rather shallow ones (except 2 incidents) with slow rate of displacement of an order offew mm/year. The landslide triggeringfactor is water originatingfrom precipitation and snow melting. Influence of water was especially significant in early Spring 2011, due to the combined effect ofsnow melting and infiltration of thawing water originating from the former, exceptionally high rainfalls. The influence of hydrologic conditions on slope deformations is complex. There is a significant timelag between a movement initiation and unfavourable hydrometeorological conditions. That is exemplified with the landslide that was initiated in February-March 2011 in consequence of atmospheric conditions of November-December 2010. The obtained results formed the background for inventing engineering treatment measures aiming at current remedial stabilization of slopes and mitigation ofpossible landsliding in the future

Landslide susceptibility and risk assessment in a non-mountainous region : a case study of Koronowo, northern Poland

The study deals with landslide threats in a low-relief region which exemplifies an area rarely perceived as prone to such geohazards. Actually, in the gently undulated landscape in the vicinity of Koronowo at the Brda River (South Pomeranian Lake District, northern Poland) intensified landsliding was observed in the recent years. The field mapping and examination of air photos showed that endangered terrains are fairly extensive and cannot be limited to initially identified slope failure incidences (hot-spots). The devised landslide susceptibility and hazard map is an outcome of predictive modelling using empirical likelihood ratio function (LR) with respect to seven evidential layers: elevation, slope angle, slope aspect, flow accumulation, surface deposits, depth to glacial raft, land cover as well as the landslide scars. The values of potential losses were calculated based on the mapped land-use categories and current market prices of estates and services. The final map, which resulted from combining landslide hazard with potential losses, shows damage propensity in a spatial scale of the town surroundings. It is meant as a supportive tool for decision-making with regard to allocating funds for stabilization measures or planning placement of new investments. On this background, stabilization solutions for selected sites are assessed in a cost-benefit context

Switching of exciton character in double InGaN/GaN quantum wells

International audienceThe dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching