The role of Mg complexes in the degradation of InGaN-based LEDs

The electrical and optical properties of commercial blue InGaN-based LEDs after room temperature aging tests at DC-forward currents from 50 to 100 mA are studied by Current–Voltage, Capacitance–Voltage, Electroluminescence and Cathodoluminescence techniques. An increase of the reverse and low-bias forward currents, of the parasitic series resistance and a substantial optical intensity reduction are observed in all the devices investigated with or without heat sink. In addition, only in devices aged at 100 mA without heat sink, the onset of a broad optical band peaked at about 3.1 eV and an apparent doping decrease of about four times, as obtained by Capacitance–Voltage measurements, are found. Temperature-dependent luminescence analyses show the quenching of the 3.1 eV band above 200 K, suggesting its donor–acceptor-pair nature. The band onset is interpreted as a result of the Mg dopant instability in the p-type layers, correlated to the device self-heating inducing junction temperature above 300 C. The band is attributed to Mg-related metastable complexes, such as Mg–H₂, acting as shallow acceptors. Due to their unstable nature, the behaviour of the 3.1 eV emission is studied under controlled electron-beam irradiation in the SEM. Its time evolution during 60 min of irradiation reveals an almost complete quenching in the Cathodoluminescence spectra, which is attributed to the dissociation of the Mg–H₂ complexes.     

Informational analysis of seismic sequences by applying the Fisher Information Measure and the Shannon entropy: An application to the 2004–2010 seismicity of Aswan area (Egypt)

The interevent-time (IET) and interevent-distance (IED) series of seismic events occurred at Aswan area (Egypt) from 2004 to 2010 were investigated by means of the Fisher Information Measure and the Shannon entropy. The analysis was performed varying the depth and the magnitude thresholds. The results point out to an increase of level of organization and order with the decrease of magnitude threshold and the increase of depth threshold for the IET series, while the IED series are characterized by a level of uncertainty approximately constant with the threshold magnitude. The complexity measure, calculated as the product of the Fisher Information Measure and the Shannon entropy power, presents very similar pattern for both the types of seismic series, indicating an increasing complexity with the decrease of the threshold magnitude and the increase of the threshold depth.     

Multi-scale properties of large eddy simulations: correlations between resolved-scale velocity-field increments and subgrid-scale quantities

We provide analytical and numerical results concerning multi-scale correlations between the resolved velocity field and the subgrid-scale (SGS) stress-tensor in large eddy simulations (LES). Following previous studies for Navier–Stokes equations, we derive the exact hierarchy of LES equations governing the spatio-temporal evolution of velocity structure functions of any order. The aim is to assess the influence of the subgrid model on the inertial range intermittency. We provide a series of predictions, within the multifractal theory, for the scaling of correlation involving the SGS stress and we compare them against numerical results from high-resolution Smagorinsky LES and from a-priori filtered data generated from direct numerical simulations (DNS). We find that LES data generally agree very well with filtered DNS results and with the multifractal prediction for all leading terms in the balance equations. Discrepancies are measured for some of the sub-leading terms involving cross-correlation between resolved velocity increments and the SGS tensor or the SGS energy transfer, suggesting that there must be room to improve the SGS modelisation to further extend the inertial range properties for any fixed LES resolution.     

Tight-binding models for ultracold atoms in optical lattices: general formulation and applications

Tight-binding models for ultracold atoms in optical lattices can be properly defined by using the concept of maximally localized Wannier functions for composite bands. The basic principles of this approach are reviewed here, along with different applications to lattice potentials with two minima per unit cell, in one and two spatial dimensions. Two independent methods for computing the tight-binding coefficients—one ab initio, based on the maximally localized Wannier functions, the other through analytic expressions in terms of the energy spectrum—are considered. In the one dimensional case, where the tight-binding coefficients can be obtained by designing a specific gauge transformation, we consider both the case of quasi resonance between the two lowest bands, and that between s and p orbitals. In the latter case, the role of the Wannier functions in the derivation of an effective Dirac equation is also reviewed. Then, we consider the case of a two dimensional honeycomb potential, with particular emphasis on the Haldane model, its phase diagram, and the breakdown of the Peierls substitution. Tunable honeycomb lattices, characterized by movable Dirac points, are also considered. Finally, general considerations for dealing with the interaction terms are presented.     

Electro-oxidation of ethanol using PtSnRh/C electrocatalysts prepared by an alcohol-reduction process

PtRh/C (90:10), PtRh/C (50:50), PtSn/C (50:50), and PtSnRh/C (50:40:10) electrocatalysts were prepared by an alcohol-reduction process using ethylene glycol as solvent and reduction agent and Vulcan Carbon XC72 as supports. The electrocatalysts were characterized by energy-dispersive X-ray analysis, X-ray diffraction, and transmission electron microscopy. The electro-oxidation of ethanol was studied by cyclic voltammetry chronoamperometry at room temperature and on a single cell of a direct ethanol fuel cell at 100 °C. Cyclic voltammetry and chronoamperometry experiments showed that PtSnRh/C and PtSn/C electrocatalysts have similar performance for ethanol oxidation at room temperature, while the activity of PtRh/C electrocatalysts was very low. At 100 °C on a single cell, PtSnRh/C showed superior performance compared to PtSn/C and PtRh/C electrocatalysts.     

A Small and Non-simple Geometric Transition

Following notation introduced in the recent paper (Rossi Int. J. Geom. Methods Mod. Phys. 12(5), 2015), this paper is aimed to present in detail an example of a small geometric transition which is not a simple one i.e. a deformation of a conifold transition. This is realized by means of a detailed analysis of the Kuranishi space of a Namikawa cuspidal fiber product, which in particular improves the conclusion of Y. Namikawa in Remark 2.8 and Example 1.11 of Namikawa (Topology 41(6), 1219–1237, 2002). The physical interest of this example is presenting a geometric transition which can’t be immediately explained as a massive black hole condensation to a massless one, as described by Strominger (Nucl. Phys. B451, 97–109, 1995).     

Mean Field Limit of Interacting Filaments and Vector Valued Non-linear PDEs

Families of N interacting curves are considered, with long range, mean field type, interaction. They generalize models based on classical interacting point particles to models based on curves. In this new set-up, a mean field result is proven, as \(N\rightarrow \infty \). The limit PDE is vector valued and, in the limit, each curve interacts with a mean field solution of the PDE. This target is reached by a careful formulation of curves and weak solutions of the PDE which makes use of 1-currents and their topologies. The main results are based on the analysis of a nonlinear Lagrangian-type flow equation. Most of the results are deterministic; as a by-product, when the initial conditions are given by families of independent random curves, we prove a propagation of chaos result. The results are local in time for general interaction kernel, global in time under some additional restriction. Our main motivation is the approximation of 3D-inviscid flow dynamics by the interacting dynamics of a large number of vortex filaments, as observed in certain turbulent fluids; in this respect, the present paper is restricted to smoothed interaction kernels, instead of the true Biot–Savart kernel.     

Microwave-assisted acylation of 7-amino-5-aryl-6-cyanopyrido[2,3-d]pyrimidines

A microwave-assisted method is described for monoacylating 7-amino-5-aryl-6-cyanopyrido[2,3-d]pyrimidines using excess acid chlorides in pyridine. A diacylated intermediate is effectively deacylated to the product amide by a macroporous-Tris resin. A small library of 17 amides was prepared to validate the method. The integration of commercial microwave technology into the ArQule chemistry platform is also discussed.