Nonexponential relaxations in a two-dimensional electron system in silicon

The relaxations of conductivity have been studied in a strongly disordered two-dimensional (2D) electron system in Si after excitation far from equilibrium by a rapid change of carrier density ns at low temperatures T. The dramatic and precise dependence of the relaxations on ns and T strongly suggests (a) the transition to a glassy phase as T-->0, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics.     

Mixed-mode oscillations in a three time-scale model for the dopaminergic neuron

Mixed-mode dynamics is a complex type of dynamical behavior that has been observed both numerically and experimentally in numerous prototypical systems in the natural sciences. The compartmental Wilson-Callaway model for the dopaminergic neuron is an example of a system that exhibits a wide variety of mixed-mode patterns upon variation of a control parameter. One characteristic feature of this system is the presence of multiple time scales. In this article, we study the Wilson-Callaway model from a geometric point of view. We show that the observed mixed-mode dynamics is caused by a slowly varying canard structure. By appropriately transforming the model equations, we reduce them to an underlying three-dimensional canonical form that can be analyzed via a slight adaptation of the approach developed by M. Krupa, N. Popovic, and N. Kopell (unpublished).     

Universal behavior of the resistance noise across the metal-insulator transition in silicon inversion layers

Studies of low-frequency resistance noise show that the glassy freezing of the two-dimensional (2D) electron system in the vicinity of the metal-insulator transition occurs in all Si inversion layers. The size of the metallic glass phase, which separates the 2D metal and the (glassy) insulator, depends strongly on disorder, becoming extremely small in high-mobility samples. The behavior of the second spectrum, an important fourth-order noise statistic, indicates the presence of long-range correlations between fluctuators in the glassy phase, consistent with the hierarchical picture of glassy dynamics.     

Derivative spectrophotometric method for determination of acidity constants of single step acid-base equilibria

A general derivative spectrophotometric method for determination of acidity constants is developed. The method appears suitable in cases when classical spectrophotometry cannot be employed due to little differences in the absorption spectra of conjugated acid-base pairs. Based on theoretical considerations, seven variants of the method have been established and their validity was checked, determining acidity constants of lorazepam and flurazepam as model compounds.     

The inner charge defect method as an analog to the quantum defect method

A method is proposed for the calculation of one-electron wave functions for excited bound and free atomic states. For the interaction potential between the outer electron and the atomic core, we have adopted the following potential: V(r) = q₀/r for r < r₀ and V(r) = -1/r for r > r₀, where r₀ is approximately equal to the core radius, q₀(∈, 1) = Δq + 1, and Δq > 0 is the inner charge defect. It is shown, for atomic argon, that the method has about the same accuracy as those of Bates and Damgard and Brugess and Seaton.     

Analysis of Tea for Metals by Flame and Graphite Furnace Atomic Absorption Spectrometry with Multivariate Analysis

The concentrations of iron, zinc, manganese, copper, chromium, nickel, lead, arsenic, and cadmium were determined using flame and graphite furnace atomic absorption spectrometry in nine herbal tea samples. Hawthorn, yarrow, elderflower, and bearberry are herbal teas for which the metal content has been rarely determined. The concentration of cadmium in St John’s wort exceeded the maximum permissible limit of 0.3?mg/kg. The metals were also determined in aqueous extracts following 5, 10, and 20?min of boiling and using different preparations, including acidification with lemon juice. The influence of these factors on metal release was demonstrated using multivariate analyses by redundancy analysis and principal component analysis. The metal release was not considerably affected by the boiling time, while the acidity of the medium was generally positively correlated. For the infusions, arsenic was detected only in acidified mint tea. The extraction efficiencies of metals were determined and classified as highly, moderately, and poorly extractable, with chromium showing largest variations.     

Aging effects across the metal-insulator transition in two dimensions

Aging effects in the relaxations of conductivity of a two-dimensional electron system in Si have been studied as a function of carrier density. They reveal an abrupt change in the nature of the glassy phase at the metal-insulator transition (MIT): (a) while full aging is observed in the insulating regime, there are significant departures from full aging on the metallic side of the MIT, before the glassy phase disappears completely at a higher density n(g): (b) the amplitude of the relaxations peaks just below the MIT, and it is strongly suppressed in the insulating phase. Other aspects of aging, including large non-Gaussian noise and similarities to spin glasses, also have been discussed.     

On silver-thallous iodide suspensions

Summary In order to obtain general information on the colloid stability of a mixed system, silver-thallous iodide systems were preliminarily examined. In all the systems observed, the molar fractions of each metal, f_Ag and f_Tl were continuously changed by 5% so that the silver-thallous iodide systems consisted of 21 different samples. Tyndallometry was used as a simple and sufficiently accurate method for determination of colloid stability of investigated systems. For the control of the substance change in the suspensions X-ray diffraction analysis was employed. The results obtained show that at the higher percentage of silver or thallium in the systems, resulting stabilities are dependent on the stability of the main component, while in the case of equal amounts of metals two stability maxima were attained. Maxima were pointed out in discussion as the prominent points of the system.

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Zusammenfassung Um allgemeine Information über die kolloide Stabilität eines Mischsystems zu erhalten, untersuchten wir das System Silber-Thallium-Jodid. Die Molfraktionen der Metallef _Ag undf _Tl wurden kontinuierlich um 5% geändert, so daß insgesamt 21 Silber-Thallium-JodidProben untersucht wurden. Zur Bestimmung der kolloidalen Stabilität der untersuchten Systeme wurde als eine einfache und genügend genaue Methode dieTyndallometrie angewendet. Die Ag-Tl-J-Niederschläge wurden röntgenographisch untersucht. Die Ergebnisse zeigen, daß bei hohen Silberoder Thalliumgehalten die Stabilität durch die Hauptsubstanz bestimmt wird, während im Mittelbereich zwei Maxima in der Stabilität auftreten.

     

Magnetic Force Formulae for Magnets at Small Distances

In [7], the magnetic force on subregions of rigid magnetized bodies was studied as a discrete-to-continuum limit. The derived force formula includes a new term, which depends on the underlying crystalline lattice structure ℒ. It originates from contributions of magnetic dipole-dipole interactions of dipole moments close to the boundary of the considered subregion. Further studies of this new term have led to the question of how the magnetic force between two idealized magnets, which are a distance ε > 0 apart, depends on ε as ε → 0. In this article, analytical aspects of this question are discussed, cf. [5], where also numerical experiments are performed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)