Theory of traveling filaments in bistable semiconductor structures

We present a generic nonlinear model for current filamentation in semiconductor structures with S-shaped current-voltage characteristics. The model accounts for Joule self-heating of a current density filament. It is shown that the self-heating leads to a bifurcation from static to traveling filament. Filaments start to travel when increase of the lattice temperature has negative impact on the cathode-anode transport. Since the impact ionization rate decreases with temperature, this occurs for a wide class of semiconductor systems whose bistability is due to the avalanche impact ionization. We develop an analytical theory of traveling filaments which reveals the mechanism of filament motion, find the condition for bifurcation to traveling filament, and determine the filament velocity.Comment: 13 pages, 5 figure

Monitoring the Permafrost Conditions along Pipeline Routes in Central Yakutia, Russia

Pipelines are critical infrastructure for Yakutia, transporting vital supplies to communities in this vast northern region. The pipeline routes in central Yakutia traverse areas of ice-rich permafrost that is sensitive to temperature changes. This study examined the thermal state of permafrost in undisturbed and disturbed settings along the Lena River to Mundulakh Reservoir water pipeline and the Mastakh to Yakutsk gas pipeline and considered the effects of climatic fluctuations and surface disturbances (forest removal and wildfire) over the monitoring periods of 28 and 18 years, respectively. The geocryological conditions in the study area, as well as the methods of permafrost thermal monitoring, are described. The observation results demonstrated significant increases in the mean annual ground temperature in the upper permafrost layers, as well as in the active-layer thickness following fire and post-fire clearing. At some sites, progressive deepening of the active layer caused the formation of taliks and reached the top of massive ground ice, initiating permafrost degradation. The development of extreme ground temperatures in the layer of annual temperature fluctuations was analyzed according to the combination of seasonal and annual meteorological conditions and the type of anthropogenic impact. The research results can be extrapolated to similar anthropogenic landscapes

Investigation of the influence of parameters of laser processing by continuous radiation of the visible range on the spectral characteristics of silver-containing films of titanium dioxide

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Monitoring the Permafrost Conditions along Pipeline Routes in Central Yakutia, Russia

Pipelines are critical infrastructure for Yakutia, transporting vital supplies to communities in this vast northern region. The pipeline routes in central Yakutia traverse areas of ice-rich permafrost that is sensitive to temperature changes. This study examined the thermal state of permafrost in undisturbed and disturbed settings along the Lena River to Mundulakh Reservoir water pipeline and the Mastakh to Yakutsk gas pipeline and considered the effects of climatic fluctuations and surface disturbances (forest removal and wildfire) over the monitoring periods of 28 and 18 years, respectively. The geocryological conditions in the study area, as well as the methods of permafrost thermal monitoring, are described. The observation results demonstrated significant increases in the mean annual ground temperature in the upper permafrost layers, as well as in the active-layer thickness following fire and post-fire clearing. At some sites, progressive deepening of the active layer caused the formation of taliks and reached the top of massive ground ice, initiating permafrost degradation. The development of extreme ground temperatures in the layer of annual temperature fluctuations was analyzed according to the combination of seasonal and annual meteorological conditions and the type of anthropogenic impact. The research results can be extrapolated to similar anthropogenic landscapes

Long-Term Variability in Ground Thermal State in Central Yakutia’s Tuymaada Valley

This paper presents the results of long-term temperature monitoring at the Yakutsk and Zeleny Lug stations, which are experimental sites, for the thermal state of valley permafrost landscapes under the conditions of modern climate warming. An analysis of the long-term data from meteorological stations in the region clearly showed one of the highest trends of increase in the mean annual air temperature in the north of Russia. Here, we established quantitative regularities in the long-term variability of the ground temperature at the bottom of the active layer and at zero amplitude. The dynamics of the ground temperature of the layer of zero amplitude during climate warming indicate the thermal stability of permafrost. The main regulating factor of the thermal state of grounds in permafrost landscapes is short-term fluctuations in the regime of snow accumulation. Active layer thickness is characterized by low interannual variability, weak climate warming responses, and insignificant trends. The results of studies of the thermal regime of soils can be extended to the same types of valley landscapes in the Lena River, and are a reliable basis for predicting heat transfer in natural and disturbed landscapes

Analyse en temps réel de l'accord de plasmon induit par laser dans un composite de verre nanoporeux

International audienceLaser-induced structuring in nanoporous glass composites is promising for numerous emerging applications in photonics and plasmonics. Local laser irradiation activates an interplay of photo-thermo-chemical mechanisms that are extremely difficult to control. The choice of optimum laser parameters to fabricate structures with desired properties remains extremely challenging. Another challenging issue is the investigation of the properties of laser-induced buried structures. In this paper, we propose a way to control the plasmonic structures formation inside a nanoporous glass composite with doped silver/copper ions that are induced by laser irradiation. Experimental and numerical investigations both demonstrate the capacities of the procedure proving its validity and application potential. In particular, we register the transmitted laser power to analyze and control the modification process. Spectral micro-analysis of the irradiated region shows a multilayer plasmonic structure inside the glass composite. Subsequently, the effective medium theory connects the measured spectral data to the numerically estimated size, concentration, and chemical composition of the secondary phase across the initial GC sample and the fabricated structure.La structuration induite par laser dans les composites de verre nanoporeux est prometteuse pour de nombreuses applications émergentes en photonique et plasmonique. L'irradiation laser locale active un jeu de mécanismes photo-thermochimiques extrêmement difficiles à contrôler. Le choix des paramètres laser optimaux pour fabriquer des structures avec les propriétés souhaitées reste extrêmement difficile. L'étude des propriétés des structures enterrées induites par laser constitue un autre problème difficile. Dans cet article, nous proposons un moyen de contrôler la formation des structures plasmoniques à l'intérieur d'un composite de verre nanoporeux avec des ions argent / cuivre dopés qui sont induits par irradiation laser. Les investigations expérimentales et numériques démontrent les capacités de la procédure à prouver sa validité et son potentiel d'application. En particulier, nous enregistrons la puissance laser transmise pour analyser et contrôler le processus de modification. La micro-analyse spectrale de la région irradiée montre une structure plasmonique multicouche à l'intérieur du composite de verre. Par la suite, la théorie du milieu efficace relie les données spectrales mesurées à la taille, à la concentration et à la composition chimique estimées numériquement de la phase secondaire à travers l'échantillon GC initial et la structure fabriquée

Femtosecond laser direct nanostructuring of ion doped porous dielectric and semiconductor films: mechanisms and applications

International audienceFemtosecond laser provides unique possibilities of the direct nanostructuring of both surfaces and highly localized volumes of dielectric materials. Here, we show extraordinary capacities of these lasers to create various surface and volume structures and to change their colors in mesoporous ion-doped matrixes. In addition to wide band gap dielectric materials, ion-impregnated semiconductor materials are used. Our experiments are performed by using several laser systems, configurations and beams. Firstly, two-beam interference set-up was used to produce grating-like structures. Secondly, a single scanning laser beams was used to produce periodic surface nanostructures. The roles of laser fluence, wavelength and polarization direction are evidenced. To better understand the mechanisms involved, a detailed multi-physical modeling is performed. Simulation results allow us to determine nanoparticle growth rate that relies not only on nanoparticle absorption, but also on the heat diffusion, photo-oxidation, and other reactions. The main laser parameters determine if the main mechanism is a thermal-activation or a photo-activation. Finally, the related applications in optics, photonics, catalysis and medicine are discussed

Local Annealing of Ag-TiO2 Nanocomposite Films with Plasmonic Response by CW UV Laser Scanning

International audienceSemiconductor sol-gel films containing plasmonic nanoparticles are being increasingly used in wet analytics (µ-TAS systems) as functional substrates for surface enhanced Raman spectroscopy (SERS), as optical elements, and as photovoltaic and photocatalytic devices. A local change in the structure of such materials with predictable properties of the modified region opens up new possibilities for the creation of integrated circuits and multifunctional systems. Here, we considered the mechanism of local modification of TiO2 thin films structure containing plasmon nanoparticles as a result of laser annealing. The material processing was carried out by scanning with a continuous wave (CW) semiconductor laser at a wavelength of 405 nm and at radiation intensity from 35 to 85 kW/cm 2. The modification region differed in optical characteristics and structural features from the original film. As a result of the laser processing, a heat source was formed that ensured the crystal nucleation and growth of brookite up to an intensity of 55.4 kW/cm 2. A subsequent increase in intensity led to the transformation of brookite into anatase. The crystal phase formation in the obtained track was accompanied by a change in the relief in its cross section and a decrease in the plasmon resonance peak. The density of the film in the modified region increased, which was accompanied by a decrease in its thickness by 20% from the original film thickness. The disappearance of plasmon resonance in the modified region contributed to a decrease in the absorption capacity and, as a consequence, to a sharp decrease in temperature at the central part of the heat source

Landscape Dynamics in the Caspian Lowlands Since the Last Deglaciation Reconstructed From the Pedosedimentary Sequence of Srednaya Akhtuba, Southern Russia

Surface Kastanozem of the Lower Volga area was first studied as a part of the pedocomplex, with the lower part (148⁻160 cm) formed in Early Khvalynian Chocolate clays (13⁻15 ka), the middle part (100⁻148 cm) in a mixed clay-loess sediment sand, and the upper part (0⁻100 cm) in loess. This resulted from local aeolian transport, with the source material derived from the rewinding of marine sediments. They are enriched in aggregates of Chocolate clays and glauconitic grains of a fine sand-course silt size and have similar contents of clay minerals. The high salinity of similar types evidences marine genesis for both Chocolate clays and source material for loess sediments. Clay fragments of a sand and silt size are responsible for the heavy texture and high gypsum content of loess. The study of soils with the focus on micromorphology and clay mineralogy allows the identification of the complex character of a shift from marine to sub-areal sedimentation. This shift was accompanied by short breaks in sedimentation, allowing the development of synlithogenic soil horizons of Late Khvalynian, after-Khvanynian, and Boreal time. The features of shallowly buried soil horizons confirm increased aridity after the last deglaciation. Surface Calcic Kastanozem is a full Holocene soil reflecting the present environment. However, it is deeply influenced by shallow buried soil horizons and Chocolate clays