Elastic and inelastic tunneling characteristics of AIAs/GaAs heterojunctions

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Thermal tuning of organic dye lasers

Non-mechanical method for tuning liquid laser wavelengths involves electrically varying temperature of laser medium. Technique is used to investigate behavior of laser dyes, and may lead to broad, tunable, light source for spectroscopy measurements of long path absorption

Inelastic tunneling characteristics of AIAs/GaAs heterojunction barriers

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Electrical behavior of GaAs–AlAs heterostructures

We report an experimental study of the electrical behavior of GaAs–AlAs–GaAs heterostructures grown by metal–organic 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. Capacitance–voltage (C–V) and curent–voltage (I–V) curves were obtained as a function of temperature, illumination, and rate of data acquisition. Deep-level transient spectroscopy (DLTS) measurements were also made. The C–V showed hysteresis near zero bias with the capacitance being larger when the voltage was swept from reverse to forward bias in the dark. The C–V 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 AlAs–GaAs interface; DLTS has confirmed this. I–V 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

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

Experimental pig-to-pig transmission dynamics for African swine fever virus, Georgia 2007/1 strain

African swine fever virus (ASFV) continues to cause outbreaks in domestic pigs and wild boar in Eastern European countries. To gain insights into its transmission dynamics, we estimated the pig-to-pig basic reproduction number (R 0) for the Georgia 2007/1 ASFV strain using a stochastic susceptible-exposed-infectious-recovered (SEIR) model with parameters estimated from transmission experiments. Models showed that R 0 is 2·8 [95% confidence interval (CI) 1·3–4·8] within a pen and 1·4 (95% CI 0·6–2·4) between pens. The results furthermore suggest that ASFV genome detection in oronasal samples is an effective diagnostic tool for early detection of infection. This study provides quantitative information on transmission parameters for ASFV in domestic pigs, which are required to more effectively assess the potential impact of strategies for the control of between-farm epidemic spread in European countries.ISSN:0950-2688ISSN:1469-440

Method to Suppress Isobaric and Polyatomic Interferences for Measurements of Highly Siderophile Elements in Desilicified Geological Samples

Sample decomposition using inverse aqua regia at elevated temperatures and pressures (e.g., Carius tube or high‐pressure asher) is the most common method used to extract highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) from geological samples. Recently, it has been recognised that additional HF desilicification is necessary to better recover HSEs, potentially contained within silicate or oxide minerals in mafic samples, which cannot be dissolved solely by inverse aqua regia. However, the abundance of interfering elements tends to increase in the eluent when conventional ion‐exchange purification procedures are applied to desilicified samples. In this study, we developed an improved purification method to determine HSEs in desilicified samples. This method enables the reduction of the ratios of isobaric and polyatomic interferences, relative to the measured intensities of HSE isotope masses, to less than a few hundred parts per million. Furthermore, the total procedural blanks are either comparable to or lower than conventional methods. Thus, this method allows accurate and precise HSE measurements in mafic and ultramafic geological samples, without the need for interference corrections. Moreover, the problem of increased interfering elements, such as Zr for Pd and Cr for Ru, is circumvented for the desilicified samples

On designing observers for time-delay systems with nonlinear disturbances

This is the post print version of the article. The official published version can be obtained from the link below - Copyright 2002 Taylor & Francis LtdIn this paper, the observer design problem is studied for a class of time-delay nonlinear systems. The system under consideration is subject to delayed state and non-linear disturbances. The time-delay is allowed to be time-varying, and the non-linearities are assumed to satisfy global Lipschitz conditions. The problem addressed is the design of state observers such that, for the admissible time-delay as well as non-linear disturbances, the dynamics of the observation error is globally exponentially stable. An effective algebraic matrix inequality approach is developed to solve the non-linear observer design problem. Specifically, some conditions for the existence of the desired observers are derived, and an explicit expression of desired observers is given in terms of some free parameters. A simulation example is included to illustrate the practical applicability of the proposed theory.The work of Z. Wang was supported in part by the University of Kaiserslautern of Germany and the Alexander von Humboldt Foundation of Germany

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