Effect of rapid thermal annealing on barrier height and 1/f noise of Ni/GaN Schottky barrier diodes

Current-voltage (as a function of temperature), capacitance-voltage, and 1/f noise characteristics of Ni/GaN Schottky barrier diodes (SBDs) as function of rapid thermal annealing (RTA) are studied. It is found that RTA treatments of SBDs at 450 °C for 60 s resulted in a significant improvement of ideality factor and Schottky barrier height: the ideality factor decreased from 1.79 to 1.12 and the barrier height increased from 0.94 to 1.13 eV. The spectral power density of current fluctuations in the diode subjected to RTA at 450 °C is found to be two orders of magnitude lower as compared to the as-deposited diode. Improved diode characteristics and decreased 1/f noise in RTA treated (450 °C/60 s) diode are attributed to reduced level of barrier inhomogeneities at the metal-semiconductor interface and explained within the framework of the spatial inhomogeneity model

Transgenerational Effects of Stress Exposure on Offspring Phenotypes in Apomictic Dandelion

Heritable epigenetic modulation of gene expression is a candidate mechanism to explain parental environmental effects on offspring phenotypes, but current evidence for environment-induced epigenetic changes that persist in offspring generations is scarce. In apomictic dandelions, exposure to various stresses was previously shown to heritably alter DNA methylation patterns. In this study we explore whether these induced changes are accompanied by heritable effects on offspring phenotypes. We observed effects of parental jasmonic acid treatment on offspring specific leaf area and on offspring interaction with a generalist herbivore; and of parental nutrient stress on offspring root-shoot biomass ratio, tissue P-content and leaf morphology. Some of the effects appeared to enhance offspring ability to cope with the same stresses that their parents experienced. Effects differed between apomictic genotypes and were not always consistently observed between different experiments, especially in the case of parental nutrient stress. While this context-dependency of the effects remains to be further clarified, the total set of results provides evidence for the existence of transgenerational effects in apomictic dandelions. Zebularine treatment affected the within-generation response to nutrient stress, pointing at a role of DNA methylation in phenotypic plasticity to nutrient environments. This study shows that stress exposure in apomictic dandelions can cause transgenerational phenotypic effects, in addition to previously demonstrated transgenerational DNA methylation effects

Density-Independent Mortality and Increasing Plant Diversity Are Associated with Differentiation of Taraxacum officinale into r- and K-Strategists

Background: Differential selection between clones of apomictic species may result in ecological differentiation without mutation and recombination, thus offering a simple system to study adaptation and life-history evolution in plants. Methodology/Principal Findings: We caused density-independent mortality by weeding to colonizer populations of the largely apomictic Taraxacum officinale (Asteraceae) over a 5-year period in a grassland biodiversity experiment (Jena Experiment). We compared the offspring of colonizer populations with resident populations deliberately sown into similar communities. Plants raised from cuttings and seeds of colonizer and resident populations were grown under uniform conditions. Offspring from colonizer populations had higher reproductive output, which was in general agreement with predictions of r-selection theory. Offspring from resident populations had higher root and leaf biomass, fewer flower heads and higher individual seed mass as predicted under K-selection. Plants grown from cuttings and seeds differed to some degree in the strength, but not in the direction, of their response to the r- vs. K-selection regime. More diverse communities appeared to exert stronger K-selection on resident populations in plants grown from cuttings, while we did not find significant effects of increasing species richness on plants grown from seeds. Conclusions/Significance: Differentiation into r- and K-strategists suggests that clones with characteristics of r-strategists were selected in regularly weeded plots through rapid colonization, while increasing plant diversity favoured the selection of clones with characteristics of K-strategists in resident populations. Our results show that different selection pressures may result in a rapid genetic differentiation within a largely apomictic species. Even under the assumption that colonizer and resident populations, respectively, happened to be r- vs. K-selected already at the start of the experiment, our results still indicate that the association of these strategies with the corresponding selection regimes was maintained during the 5-year experimental period

Epigenetic variation in plant responses to defence hormones

BACKGROUND AND AIMS: There is currently much speculation about the role of epigenetic variation as a determinant of heritable variation in ecologically important plant traits. However, we still know very little about the phenotypic consequences of epigenetic variation, in particular with regard to more complex traits related to biotic interactions. METHODS: Here, a test was carried out to determine whether variation in DNA methylation alone can cause heritable variation in plant growth responses to jasmonic acid and salicylic acid, two key hormones involved in induction of plant defences against herbivores and pathogens. In order to be able to ascribe phenotypic differences to epigenetic variation, the hormone responses were studied of epigenetic recombinant inbred lines (epiRILs) of Arabidopsis thaliana – lines that are highly variable at the level of DNA methylation but nearly identical at the level of DNA sequence. KEY RESULTS: Significant heritable variation was found among epiRILs both in the means of phenotypic traits, including growth rate, and in the degree to which these responded to treatment with jasmonic acid and salicylic acid. Moreover, there was a positive epigenetic correlation between the responses of different epiRILs to the two hormones, suggesting that plant responses to herbivore and pathogen attack may have a similar molecular epigenetic basis. CONCLUSIONS: This study demonstrates that epigenetic variation alone can cause heritable variation in, and thus potentially microevolution of, plant responses to defence hormones. This suggests that part of the variation of plant defences observed in natural populations may be due to underlying epigenetic, rather than entirely genetic, variation

Plant functional traits in studies of vegetation changes in response to grazing and mowing: towards a use of more specific traits

This thesis develops a large eddy simulation framework for engineering applications using the finite element method. It focuses on the numerical formulation, the wall modelling approach as well as the generation of turbulent inflow conditions, with emphasis on incompressible flows. A low-dissipation formulation is introduced that uses a non-incremental fractional step method to stabilize the pressure and allow the use of finite element pairs that do not satisfy the inf-sup condition, such as equal order interpolation for velocity and pressure. This stabilization introduces an error of O(dt, h^2) (for linear elements) in the conservation of kinetic energy, while the final scheme preserves momentum and angular momentum. Explicit subgrid scale models are used for turbulent closure. Temporal discretization is performed through an explicit, energy-conserving Runge Kutta scheme, coupled with an eigenvalue-based time step estimator. The formulation is compared with a residual-based Variational Multiscale method in three common benchmark cases: the decaying isotropic turbulence, the Taylor-Green vortex and the turbulent channel flow at Ret = 395, 950 and 2003. Both formulations provide very accurate predictions, however it is observed that for the Variational Multiscale method, the best results are obtained for different values of the stabilization constants, depending on the case and the Reynolds number. On the other hand, the new formulation provides favorable results without any need for ad hoc tuning. The formulation is further evaluated in the flow over a sphere and the flow around an Ahmed body, where very good agreement with the reference DNS data is obtained. A new approach is introduced for wall modelling in a finite element context. Instead of the classical finite element method, where part of the domain is omitted and the wall model accounts for it, the mesh extends all the way to the wall, as is commonly done in finite differences and finite volumes. The new approach is tested in a turbulent channel flow at Ret = 2003, a neutrally stratified atmospheric boundary layer and the flow over a wall-mounted hump, where it is shown to offer a great improvement over the classical finite element method. The effect of time-averaging the wall model input, as well as moving the exchange interface further away from the wall is also evaluated. In addition, preliminary work is presented on a two-layer non-equilibrium wall model that uses time-averaging to filter the excess Reynolds stresses. It is tested in a turbulent channel flow at Ret = 2003 with accurate results. Significant savings on the computational cost are also achieved by using a wall-model grid that is coarser in the tangential directions, with minimal impact on the results. Furthermore a method of synthesizing turbulent inflow conditions through the diffusion process is compared with a precursor method on the flow over a three-dimensional hill, providing results of similar quality at significantly less computational cost. Finally, the complete framework is evaluated on the flow around the DrivAer model, a realistic car model developed to facilitate aerodynamic investigations of passenger vehicles, as well as the flow over the Bolund hill, a hill whose geometry represents a scaled-down model of the typical wind farm site. Despite the complexity of the flows and the coarse grids utilized, good agreement with the reference data is achieved.Esta tesis desarrolla un marco de simulación de grandes vortices para aplicaciones de ingeniería utilizando el método de elementos finitos. Se enfoca en la formulación numérica, el método de modelado de la pared y la generación de condiciones de entrada turbulenta, con énfasis en flujos incompresibles. Se introduce una formulación de baja disipación que utiliza un método de paso fraccional no incremental para estabilizar la presión y permitir el uso de pares de elementos finitos que no satisfacen la condición inf-sup, como la interpolación de igual orden para la velocidad y la presión. Esta estabilización introduce un error de O(dt, h^2) (para elementos lineales) en la conservación de la energía cinética, mientras que el esquema final conserva el momento y el momento angular. Se utilizan modelos explícitos de subescala para el cierre turbulento. La discretización temporal se realiza a través de un esquema de Runge Kutta explícito que conserva la energía, junto con un estimador de paso de tiempo basado en valores propios. La formulación se compara con un método `Variational Multiscale¿ basado en residuos en tres casos comunes de referencia: el decaimiento de la turbulencia isotrópica, el vórtice de Taylor-Green y el flujo del canal turbulento a Ret = 395, 950 y 2003. Ambas formulaciones proporcionan predicciones muy precisas. Sin embargo, se observa que para el método `Variational Multiscale¿, los mejores resultados se obtienen para diferentes valores de las constantes de estabilización, según el caso y el número de Reynolds. Por otro lado, la nueva formulación proporciona resultados favorables sin necesidad de ajustes ad hoc. La formulación se evalúa más a fondo en el flujo sobre una esfera y el flujo alrededor de un cuerpo de Ahmed, donde se obtiene un muy buen acuerdo con los datos de referencia del DNS. Se introduce un nuevo enfoque para el modelado de la pared en un contexto de elementos finitos. A diferencia del método clásico de elementos finitos, donde se omite parte del dominio que es tenido en cuenta por el modelo de pared, la malla se extiende hasta la pared, como se hace comúnmente en diferencias finitas y volúmenes finitos. El nuevo enfoque se prueba en un flujo de canal turbulento en Ret = 2003, una capa límite atmosférica estratificada neutral y el flujo sobre una joroba montada en la pared, donde se muestra que ofrece una gran mejora sobre el método clásico de elementos finitos. También se evalúa el efecto de promediar en el tiempo la entrada del modelo de pared, así como mover la interfaz de intercambio más lejos de la pared. Además, el trabajo preliminar se presenta en un modelo de pared de no equilibrio de dos capas que utiliza un promedio de tiempo para filtrar el exceso de tensiones de Reynolds. Se prueba en un flujo de canal turbulento en Ret = 2003 con resultados muy satisfactorios. También se logran ahorros significativos en el costo computacional mediante el uso de una malla de modelo de pared que es más gruesa en las direcciones tangenciales, con un impacto mínimo en los resultados. Además, se compara un método para generar condiciones de entrada turbulenta a través de un proceso de difusión con un método precursor en el flujo sobre una colina tridimensional. El primer método proporciona resultados de calidad similar con un costo computacional significativamente menor. Finalmente, el marco completo se evalúa sobre el flujo alrededor del modelo DrivAer, un modelo de automóvil realista desarrollado para facilitar las investigaciones aerodinámicas de los vehículos de pasajeros, así como el flujo sobre la colina de Bolund, una colina cuya geometría representa un modelo a escala de un parque eólico. A pesar de la complejidad de los flujos y del uso de mallas relativamente gruesas, se logra un buen acuerdo con los datos de referencia

Plant functional traits in studies of vegetation changes in response to grazing and mowing: towards a use of more specific traits

International audiencePlants' abilities to function are difficult to evaluate directly in the field. Therefore, a number of attempts have been made to determine easily measurable surrogates - plant functional traits (PFTs), In particular, the value of PFTs as tools for predicting vegetation responses to management (i.e., grazing and mowing) is the focus of a large number of studies. However, recent Studies using PFTs to predict the effect of pasture management in different regions did not give consistent predictions for the same set of PFTs. This lead to the suggestion that more specific traits better suited for a specific region be used in the future. We consider the identification of the roost adaptative traits for surviving grazing and mowing in different biomes an important goal. Using temperate grasslands in Europe as an example, we show that (i) plant height, often considered as the best predictor of species response to grassland management, is coupled with other more relevant functional traits, and that (ii) clonal traits have important, often neglected functions in the response of species to grassland management. We conclude that single traits cannot be the only basis for predicting vegetation changes under pasture management and, therefore, a functional analysis of the trade-off between key traits is needed

Plant functional traits in studies of vegetation changes in response to grazing and mowing: towards a use of more specific traits

International audiencePlants' abilities to function are difficult to evaluate directly in the field. Therefore, a number of attempts have been made to determine easily measurable surrogates - plant functional traits (PFTs), In particular, the value of PFTs as tools for predicting vegetation responses to management (i.e., grazing and mowing) is the focus of a large number of studies. However, recent Studies using PFTs to predict the effect of pasture management in different regions did not give consistent predictions for the same set of PFTs. This lead to the suggestion that more specific traits better suited for a specific region be used in the future. We consider the identification of the roost adaptative traits for surviving grazing and mowing in different biomes an important goal. Using temperate grasslands in Europe as an example, we show that (i) plant height, often considered as the best predictor of species response to grassland management, is coupled with other more relevant functional traits, and that (ii) clonal traits have important, often neglected functions in the response of species to grassland management. We conclude that single traits cannot be the only basis for predicting vegetation changes under pasture management and, therefore, a functional analysis of the trade-off between key traits is needed