Identification Of Physical Mechanisms Research Articles

Interplay of Thermal and Electronic Effects in the Mott Transition of Nanosized VO2 Phase Change Memory Devices

Volatile memory devices relying on the Mott-type insulator-to-metal transition of vanadium oxide (VO2) are widely utilized in the field of neuromorphic computing. Such devices, however, are realized in a nanoscale geometry, where the switching relies on the self-heating of an ultrasmall spot as well as the presence of extremely high electric fields in the active region. In this paper, we investigate the interplay of such nanoscale thermal and nonlinear electronic phenomena by investigating the temperature and voltage dependent conduction properties of our custom-designed VO2 devices, where a V-shaped electrode focuses the switching to an ultrasmall single-spot active region. This simplified spatial structure of the active volume facilitates the device modeling and the identification of physical mechanisms behind the phase transition. We find that purely thermal or electronic effects fail to describe the device operation, however, according to our finite element simulations, a combined electronic and thermal model provides a precise description of the device characteristics. These results facilitate the understanding as well as the thermal and electronic design of novel VO2-based neuronal devices.

Spatiotemporal modelling of ozone distribution in the State of California

This paper is concerned with the spatiotemporal mapping of monthly 8-h average ozone ( O 3) concentrations over California during a 15-years period. The basic methodology of our analysis is based on the spatiotemporal random field (S/TRF) theory. We use a S/TRF decomposition model with a dominant seasonal O 3 component that may change significantly from site to site. O 3 seasonal patterns are estimated and separated from stochastic fluctuations. By means of Bayesian Maximum Entropy (BME) analysis, physically meaningful and sufficiently detailed space–time maps of the seasonal O 3 patterns are generated across space and time. During the summer and winter months the seasonal O 3 concentration maps exhibit clear and progressively changing geographical patterns over time, suggesting the existence of relationships in accordance with the typical physiographic and climatologic features of California. BME mapping accuracy can be superior to that of other techniques commonly used by EPA; its framework can rigorously assimilate useful data sources that were previously unaccounted for; the generated maps offer valuable assessments of the spatiotemporal O 3 patterns that can be helpful in the identification of physical mechanisms and their interrelations, the design of human exposure and population health models, and in risk assessment. As they focus on the seasonal patterns, the maps are not contingent on short-time and locally prevalent weather conditions, which are of no interest in a global and non-forecasting framework. Moreover, the maps offer valuable insight about the space–time O 3 concentration patterns and are, thus, helpful for disentangling the influence of explanatory factors or even for identifying some influential ones that could have been otherwise overlooked.

GCM Simulations of the Climate in the Central United States

Abstract A diagnostic analysis of relationships between central U.S. climate characteristics and various flow and scalar fields was used to evaluate nine global coupled ocean–atmosphere general circulation models (CGCMs) participating in the Coupled Model Intercomparison Project (CMIP). To facilitate identification of physical mechanisms causing biases, data from 21 models participating in the Atmospheric Model Intercomparison Project (AMIP) were also used for certain key analyses. Most models reproduce basic features of the circulation, temperature, and precipitation patterns in the central United States, although no model exhibits small differences from the observationally based data for all characteristics in all seasons. Model ensemble means generally produce better agreement with the observationally based data than any single model. A fall precipitation deficiency, found in all AMIP and CMIP models except the third-generation Hadley Centre CGCM (HadCM3), appears to be related in part to slight biases in the flow on the western flank of the Atlantic subtropical ridge. In the model mean, the ridge at 850 hPa is displaced slightly to the north and to the west, resulting in weaker southerly flow into the central United States. The CMIP doubled-CO2 transient runs show warming (1°–5°C) for all models and seasons and variable precipitation changes over the central United States. Temperature (precipitation) changes are larger (mostly less) than the variations that are observed in the twentieth century and the model variations in the control simulations.

Physical mechanisms governing the dynamics of Los Angeles smog aerosol

The aerosol size distributions measured during the 1969 Pasadena Smog Study and those obtained later in laboratory smog-simulation experiments are analyzed. Emphasis is on the identification of physical mechanisms and parameters which are responsible for the daily aerosol concentration changes. The role of relative humidity, solar radiation intensity, coagulation, and condensation is discussed. Data from field measurements and artificial humidification experiments indicate that by changing the relative humidity, the submicron aerosol volume concentration, as well as the total light scattering, may be changed by at least a factor of two. The effect of solar radiation on the photochemical gas-particle conversion rate was investigated by inflating a plastic bag with particulate-free ambient air and exposing it to solar radiation. The observed nuclei-generation rates in Pasadena were on the order of 10 5 nuclei/sec. The solar radiation was found to be necessary for the nuclei production. Laboratory experiments performed in Minnesota suggest that the growth of photochemical nuclei after formation is governed by simultaneous coagulation and condensation. During late night hours, the aerosol was found to decay according to the laws of coagulation. The size distributions were observed to approach a universal form which could be simulated by laboratory aging experiments and by numerical (Monte Carlo) simulation. The mean coagulation coefficient for the smog decay was between 2 × 10 −9 and 10 −8 cm 3/sec. During the daytime, coagulation was found to limit the total number concentration to 2 × 10 5/cm 3 by rapid removal of small particles (0.01 μm) by the larger ones (0.1 μm). It appears that the important particle diameter range 0.1 < D p < 1.0 μm, which on the average contains about 60% of the total aerosol mass fraction, was not affected significantly by coagulation. A comparison of field data with laboratory experiments and numerical calculations suggests that the noontime accumulation of aerosol mass in the 0.1 < D p < 1.0 μm subrange is primarily due to condensation of photochemically produced supersaturated vapors on the existing particles.