Johannes Kepler University of Linz

About: Johannes Kepler University of Linz is a education organization based out in Linz, Oberösterreich, Austria. It is known for research contribution in the topics: Thin film & Quantum dot. The organization has 6605 authors who have published 19243 publications receiving 385667 citations.

Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N : The relationship between structural, electronic, and magnetic properties

Abstract: We report on the metalorganic chemical vapor deposition of $\mathrm{GaN}:\mathrm{Fe}$ and $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ layers on $c$-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably ${\mathrm{Fe}}_{x}\mathrm{N}$ with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content $x\ensuremath{\lesssim}0.4%$, indicating that the concentration of ${\mathrm{Fe}}^{2+}$ ions increases at the expense of Fe ions in the $3+$ charge state. This effect is witnessed by PL measurements as changes in the intensity of the ${\mathrm{Fe}}^{3+}$-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of ${\mathrm{Fe}}^{3+}$ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, ${\mathrm{Fe}}^{2+}$ does not produce any EPR response. However, the presence of Fe ions in the $2+$ charge state may account for a temperature-independent Van Vleck--type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the $3+$ charge state is linked to the gradient in the Fe concentration. In layers with iron content $x\ensuremath{\gtrsim}0.4%$ the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic---i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.

Recommender Systems Leveraging Multimedia Content

Abstract: Recommender systems have become a popular and effective means to manage the ever-increasing amount of multimedia content available today and to help users discover interesting new items. Today’s recommender systems suggest items of various media types, including audio, text, visual (images), and videos. In fact, scientific research related to the analysis of multimedia content has made possible effective content-based recommender systems capable of suggesting items based on an analysis of the features extracted from the item itself. The aim of this survey is to present a thorough review of the state-of-the-art of recommender systems that leverage multimedia content, by classifying the reviewed papers with respect to their media type, the techniques employed to extract and represent their content features, and the recommendation algorithm. Moreover, for each media type, we discuss various domains in which multimedia content plays a key role in human decision-making and is therefore considered in the recommendation process. Examples of the identified domains include fashion, tourism, food, media streaming, and e-commerce.

A heuristic solution method for node routing based solid waste collection problems

Abstract: This paper considers a real world waste collection problem in which glass, metal, plastics, or paper is brought to certain waste collection points by the citizens of a certain region. The collection of this waste from the collection points is therefore a node routing problem. The waste is delivered to special sites, so called intermediate facilities (IF), that are typically not identical with the vehicle depot. Since most waste collection points need not be visited every day, a planning period of several days has to be considered. In this context three related planning problems are considered. First, the periodic vehicle routing problem with intermediate facilities (PVRP-IF) is considered and an exact problem formulation is proposed. A set of benchmark instances is developed and an efficient hybrid solution method based on variable neighborhood search and dynamic programming is presented. Second, in a real world application the PVRP-IF is modified by permitting the return of partly loaded vehicles to the depots and by considering capacity limits at the IF. An average improvement of 25% in the routing cost is obtained compared to the current solution. Finally, a different but related problem, the so called multi-depot vehicle routing problem with inter-depot routes (MDVRPI) is considered. In this problem class just a single day is considered and the depots can act as an intermediate facility only at the end of a tour. For this problem several instances and benchmark solutions are available. It is shown that the algorithm outperforms all previously published metaheuristics for this problem class and finds the best solutions for all available benchmark instances.

Parallel Resolved Open Source CFD-DEM: Method, Validation and Application

Abstract: In the following paper the authors present a fully parallelized Open Source method for calculating the interaction of immersed bodies and surrounding fluid. Acombination of computational fluid dynamics (CFD) and a discrete element method (DEM) accounts for the physics of both the fluid and the particles. The objects considered are relatively big compared to the cells of the fluid mesh, i.e. they cover several cells each. Thus this fictitious domain method (FDM) is called resolved. The implementation is realized within the Open Source framework CFDEMcoupling (www.cfdem.com), which provides an interface between OpenFOAM® based CFD-solvers and the DEM software LIGGGHTS (www.liggghts.com). While both LIGGGHTS and OpenFOAM® were already parallelized, only a recent improvement of the algorithm permits the fully parallel computation of resolved problems. Alongside with a detailed description of the method, its implementation and recent improvements, a number of application and validation examples is presented in...