6,079 research outputs found
Resonant tunneling transistors with controllable negative differential resistances
Three-terminal devices based on resonant tunneling through two quantum barriers separated by a quantum well are presented and analyzed theoretically. Each proposed device consists of a resonant tunneling double barrier heterostructure integrated with a Schottky barrier field-effect transistor configuration. The essential feature of these devices is the presence, in their output current-voltage (I_{D} - V_{D}) curves, of negative differential resistances controlled by a gate voltage. Because of the high-speed characteristics associated with tunnel structures, these devices could find applications in tunable millimeter-wave oscillators, negative resistance amplifiers, and high-speed digital circuits
The resurrection of group selection as a theory of human cooperation
Two books edited by members of the MacArthur Norms and Preferences Network (an interdisciplinary group, mainly anthropologists and economists) are reviewed here. These books in large part reflect a renewed interest in group selection
that has occurred among these researchers: they promote the theory that human cooperative behavior evolved via selective processes which favored biological and/or cultural group-level adaptations as opposed to individual-level adaptations. In support of this theory, an impressive collection of cross-cultural data are presented which suggest that participants in experimental economic games often do not behave as self-interested income maximizers; this lack of self-interest is regarded as evidence of group selection. In this review, problems with these data and with the theory are discussed. On the data side, it is argued that even if a behavior seems individually-maladaptive in a game context, there is no reason to believe that it would have been that way in ancestral contexts, since the environments of experimental games do not at all resemble those in which ancestral humans would have interacted cooperatively. And on the theory side, it is argued that it is premature to invoke group selection in order to explain human cooperation, because more parsimonious individual-level theories have not yet been exhausted. In summary, these books represent ambitious interdisciplinary contributions on an important topic, and they include unique and useful data; however, they do not make a convincing case that the evolution of human cooperation required group selection
Electrical behavior of GaAsâAlAs heterostructures
We report an experimental study of the electrical behavior of GaAsâAlAsâGaAs heterostructures grown by metalâorganic chemical vapor deposition. The structures consisted of a layer of AlAs several thousand angstroms thick sandwiched between layers of GaAs which were a few microns thick. The top layer of GaAs was doped degenerately n-type with Se, while the bottom layer was nondegenerately doped. Capacitanceâvoltage (CâV) and curentâvoltage (IâV) curves were obtained as a function of temperature, illumination, and rate of data acquisition. Deep-level transient spectroscopy (DLTS) measurements were also made. The CâV showed hysteresis near zero bias with the capacitance being larger when the voltage was swept from reverse to forward bias in the dark. The CâV displayed a light sensitive peak near zero bias. With illumination, the capacitance was greater, and no hysteresis was observed. We explain these phenomena as being due to deep levels near the AlAsâGaAs interface; DLTS has confirmed this. IâV curves taken in darkness also showed hysteresis. We take this as further evidence of deep levels. Additionally, capacitance failed to level off in reverse bias, indicating a lack of inversion in the samples
Intense physical activity is associated with cognitive performance in the elderly
Numerous studies have reported positive impacts of physical activity on cognitive function. However, the majority of these studies have utilised physical activity questionnaires or surveys, thus results may have been influenced by reporting biases. Through the objective measurement of routine levels of physical activity via actigraphy, we report a significant association between intensity, but not volume, of physical activity and cognitive functioning. A cohort of 217 participants (aged 60â89 years) wore an actigraphy unit for 7 consecutive days and underwent comprehensive neuropsychological assessment. The cohort was stratified into tertiles based on physical activity intensity. Compared with individuals in the lowest tertile of physical activity intensity, those in the highest tertile scored 9%, 9%, 6% and 21% higher on the digit span, digit symbol, Rey Complex Figure Test (RCFT) copy and Rey Figure Test 30-min recall test, respectively. Statistically, participants in the highest tertile of physical activity intensity performed significantly better on the following cognitive tasks: digit symbol, RCFT copy and verbal fluency test (all P<0.05). The results indicate that intensity rather than quantity of physical activity may be more important in the association between physical activity and cognitive function
RIO Country Report 2017: Cyprus
The R&I Observatory country report 2017 provides a brief analysis of the R&I system covering the economic context, main actors, funding trends & human resources, policies to address R&I challenges, and R&I in national and regional smart specialisation strategies. Data is from Eurostat, unless otherwise referenced and is correct as at January 2018. Data used from other international sources is also correct to that date. The report provides a state-of-play and analysis of the national level R&I system and it's challenges, to support the European Semester.JRC.B.7-Knowledge for Finance, Innovation and Growt
Intention of preserving forest remnants among landowners in the Atlantic Forest: The role of the ecological context via ecosystem services
Unravelling the psychological processes determining landowners' support towards forest conservation is crucial, particularly in rural areas of the tropics, where most forest remnants are within private lands. As humanânature connections are known to shape proâenvironmental behaviours, the intention of preserving forest remnants should ultimately be determined by the ecological context people live in. Here, we investigate the pathways through which the ecological context (forest cover), via direct contact with forests and ecosystem services and disservices, influence the psychological antecedents of conservation behaviour (beliefs, attitude and intention of preserving forest remnants). We conceptualized a model based on the Reasoned Action Approach, using the ecological context and these three forest experiences as background factors, and tested the model using Piecewise Structural Equation Modelling. Data were collected through an interviewâbased protocol applied to 106 landowners across 13 landscapes varying in forest cover in a consolidated rural region in the Brazilian Atlantic Forest. Our results indicate that: (a) ecosystem services are more important than disservices for shaping intention of preserving forests, particularly nonâprovisioning services; (b) contact with forest has an indirect effect on intention, by positively influencing the frequency of receiving ecosystem services; (c) people living in more forested ecological contexts have more contact with forests, receive ecosystem services more frequently and, ultimately, have stronger intention of preserving forests. Hence, our study suggests a dangerous positive feedback loop between deforestation, the extinction of forest experiences and impairment of humanânature connections. Local demands across the full range of ecosystem services, the balance between services and disservices and the ecological context people live in should be considered when developing conservation initiatives in tropical rural areas
Current transport mechanisms in GaAs/AlAs tunnel structures grown by metalâorganic chemical vapor deposition
Elastic and inelastic tunneling processes are investigated in GaAsâAlAsâGaAs double heterojunctions grown in the [100] direction by metalâorganic chemical vapor deposition (MOCVD). The AlAs quantum barriers in the heterostructures studied are doped p-type with Mg. Theoretical calculations of tunneling currents are performed and compared with experimental IâV data. It is found that for structures with thin AlAs barriers, the dominant current transport mechanism at low temperatures is tunneling through the AlAs band gap at both the Gamma and X points. This is consistent with inelastic processes observable in first (dI/dV) and second (d2I/dV2) derivative spectra obtained with modulation techniques. A simple model, developed for calculating impurity-assisted tunneling currents, shows that the role of barrier impurities becomes more important as the barrier is grown thicker. Implications of some of these results for resonant tunneling heterostructures consisting of two AlAs quantum barriers separated by a GaAs quantum well are discussed. Experimental second derivative spectra showing reproducible features are also presented for these double barrier structures
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