Information criteria for nonlinear time series models

In this paper the performance of different information criteria for simultaneous model class and lag order selection is evaluated using simulation studies. We focus on the ability of the criteria to distinguish linear and nonlinear models. In the simulation studies, we consider three different versions of the commonly known criteria AIC, SIC and AICc. In addition, we also assess the performance of WIC and evaluate the impact of the error term variance estimator. Our results confirm the findings of different authors that AIC and AICc favor nonlinear over linear models, whereas weighted versions of WIC and all versions of SIC are able to successfully distinguish linear and nonlinear models. However, the discrimination between different nonlinear model classes is more difficult. Nevertheless, the lag order selection is reliable. In general, information criteria involving the unbiased error term variance estimator overfit less and should be preferred to using the usual ML estimator of the error term variance. © 2016 by De Gruyter

Avalanches and Self-Organized Criticality in Superconductors

We review the use of superconductors as a playground for the experimental study of front roughening and avalanches. Using the magneto-optical technique, the spatial distribution of the vortex density in the sample is monitored as a function of time. The roughness and growth exponents corresponding to the vortex landscape are determined and compared to the exponents that characterize the avalanches in the framework of Self-Organized Criticality. For those situations where a thermo-magnetic instability arises, an analytical non-linear and non-local model is discussed, which is found to be consistent to great detail with the experimental results. On anisotropic substrates, the anisotropy regularizes the avalanches

GaN/AlN Quantum Dots for Single Qubit Emitters

We study theoretically the electronic properties of $c$-plane GaN/AlN quantum dots (QDs) with focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band k.p theory we calculate the optical interband transitions of the QDs and their polarization properties. We show that an anisotropy of the QD confinement potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to a pronounced linear polarization of the ground state and excited state transitions. An externally applied uniaxial stress can be used to either induce a linear polarization of the ground-state transition for emission of single polarized photons or even to compensate the polarization induced by the structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte

Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films

We present numerical and analytical studies of coupled nonlinear Maxwell and thermal diffusion equations which describe nonisothermal dendritic flux penetration in superconducting films. We show that spontaneous branching of propagating flux filaments occurs due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heat generation. The branching is triggered by a thermomagnetic edge instability which causes stratification of the critical state. The resulting distribution of magnetic microavalanches depends on a spatial distribution of defects. Our results are in good agreement with experiments performed on Nb films.Comment: 4 pages, 3 figures, see http://mti.msd.anl.gov/aran_h1.htm for extensive collection of movies of dendritic flux and temperature pattern

How Tribo-Oxidation Alters the Tribological Properties of Copper and Its Oxides

Tribochemical reactions in many applications determine the performance and lifetime of individual parts or entire engineering systems. The underlying processes are however not yet fully understood. Here, the tribological properties of copper and its oxides are investigated under mild tribological loading and for dry sliding. The oxides represent the late stages of a copper–sapphire tribo‐contact, once the whole copper surface is covered with an oxide. For this purpose, high‐purity copper, thermally‐oxidized and sintered Cu$_{2}$O and CuO samples are tribologically loaded and eventually formed wear particles analyzed. The tribological behavior of the oxides is found to be beneficial for a reduction of the coefficient of friction (COF), mainly due to an increase in hardness. The results reveal tribochemical reactions when copper oxides are present, irrespective of whether they form during sliding or are existent from the beginning. Most strikingly, a reduction of copper oxide to metallic copper is observed in X‐ray photoelectron spectroscopy measurements. A more accurate understanding of tribo‐oxidation will allow for manufacturing well‐defined surfaces with enhanced tribological properties. This paves the way for extending the lifetime of contacts evincing tribo‐oxidation

How Tribo-Oxidation Alters the Tribological Properties of Copper and Its Oxides

Tribochemical reactions in many applications determine the performance and lifetime of individual parts or entire engineering systems. The underlying processes are however not yet fully understood. Here, the tribological properties of copper and its oxides are investigated under mild tribological loading and for dry sliding. The oxides represent the late stages of a copper–sapphire tribo‐contact, once the whole copper surface is covered with an oxide. For this purpose, high‐purity copper, thermally‐oxidized and sintered Cu$_{2}$O and CuO samples are tribologically loaded and eventually formed wear particles analyzed. The tribological behavior of the oxides is found to be beneficial for a reduction of the coefficient of friction (COF), mainly due to an increase in hardness. The results reveal tribochemical reactions when copper oxides are present, irrespective of whether they form during sliding or are existent from the beginning. Most strikingly, a reduction of copper oxide to metallic copper is observed in X‐ray photoelectron spectroscopy measurements. A more accurate understanding of tribo‐oxidation will allow for manufacturing well‐defined surfaces with enhanced tribological properties. This paves the way for extending the lifetime of contacts evincing tribo‐oxidation

Image states in metal clusters

The existence of image states in small clusters is shown, using a quantum-mechanical many-body approach. We present image state energies and wave functions for spherical jellium clusters up to 186 atoms, calculated in the GW approximation, where G is the Green's function and W is the dynamically screened Coulomb interaction, which by construction contains the dynamic long-range correlation effects that give rise to image effects. In addition, we find that image states are also subject to quantum confinement. To extrapolate our investigations to clusters in the mesoscopic size range, we propose a semiclassical model potential, which we test against our full GW results