Enhanced Luminescence of InGaN / GaN Vertical Light Emitting Diodes with an InGaN Protection Layer

We have investigated the effectiveness of a thin n-In0.2Ga0.8N layer inserted in the bottom n-GaN layer of InGaN/GaN blue light emitting diodes (LEDs) for the protection of multiple quantum wells during the laser lift-off process for vertical LED fabrication. The photoluminescence properties of the InGaN/GaN lateral LEDs are nearly identical irrespective of the existence of the n-In0.2Ga0.8N insertion layer in the bottom n-GaN layer. However, such an insertion is found to effectively increase the photoluminescence intensity of the multiple quantum well and the carrier lifetime of the vertical LEDs. These improvements are attributed to the reduced defect generations in the vertical LEDs during the laser lift-off process due to the presence of the protection layer. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3521

Metabolomics for the rapid dereplication of bioactive compounds from natural sources

Plant science

Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy

Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78 cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.Comment: Invited review in a special issue "Superconductivity in MgB_2: Physics and Applications

MgB2 and Mg1-xAlxB2 single crystals: high-pressure growth and physical properties

Single crystals of MgB2 have been grown with a high-pressure cubic anvil technique. They grow via the peritectic decomposition of the MgNB9 ternary nitride. The crystals are of a size up to 2x1x0.1mm3 with a weight up to 230 micrograms. Typically they have transition temperatures between 38 and 38.6 K with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that the MgB2 phase is stable at least up to 2190C at high hydrostatic pressure in the presence of Mg vapor under high pressure. Substitution of aluminum for magnesium in single crystals leads to stepwise decrease of Tc. This indicates a possible appearance of superstructures or phases with different Tc's. The upper critical field decreases with Al doping.Comment: 6 pages, 2 figures. submitted to Physica C (M2S-Rio proceedings). Text and Fig.2 revise

Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties

We demonstrate the preparation of large, free standing iron pnictide single crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties are investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent growth. We show direct evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.Comment: 22 pages, 9 figure

Density functional method for nonequilibrium electron transport

We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density functional theory (DFT) as implemented in the well tested Siesta approach (which uses non-local norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states). We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias (including selfconsistency and the solution of the electrostatic problem) is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: (1) single atom carbon wires connected to aluminum electrodes with extended or finite cross section, (2) single atom gold wires, and finally (3) large carbon nanotube systems with point defects.Comment: 18 pages, 23 figure

Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations

The approach of the elastic continuum limit in small amorphous bodies formed by weakly polydisperse Lennard-Jones beads is investigated in a systematic finite-size study. We show that classical continuum elasticity breaks down when the wavelength of the sollicitation is smaller than a characteristic length of approximately 30 molecular sizes. Due to this surprisingly large effect ensembles containing up to N=40,000 particles have been required in two dimensions to yield a convincing match with the classical continuum predictions for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk systems. The existence of an effective length scale \xi is confirmed by the analysis of the (non-gaussian) noisy part of the low frequency vibrational eigenmodes. Moreover, we relate it to the {\em non-affine} part of the displacement fields under imposed elongation and shear. Similar correlations (vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table

Exponential Metric Fields

The Laser Interferometer Space Antenna (LISA) mission will use advanced technologies to achieve its science goals: the direct detection of gravitational waves, the observation of signals from compact (small and dense) stars as they spiral into black holes, the study of the role of massive black holes in galaxy evolution, the search for gravitational wave emission from the early Universe. The gravitational red-shift, the advance of the perihelion of Mercury, deflection of light and the time delay of radar signals are the classical tests in the first order of General Relativity (GR). However, LISA can possibly test Einstein's theories in the second order and perhaps, it will show some particular feature of non-linearity of gravitational interaction. In the present work we are seeking a method to construct theoretical templates that limit in the first order the tensorial structure of some metric fields, thus the non-linear terms are given by exponential functions of gravitational strength. The Newtonian limit obtained here, in the first order, is equivalent to GR.Comment: Accepted for publication in Astrophysics and Space Science, 17 page

Applications of scalar attractor solutions to Cosmology

We develop a framework to study the phase space of a system consisting of a scalar field rolling down an arbitrary potential with varying slope and a background fluid, in a cosmological setting. We give analytical approximate solutions of the field evolution and discuss applications of its features to the issues of quintessence, moduli stabilisation and quintessential inflation.Comment: 9 pages, 7 figures. Accepted for publication in PR

Influence of uncorrelated overlayers on the magnetism in thin itinerant-electron films

The influence of uncorrelated (nonmagnetic) overlayers on the magnetic properties of thin itinerant-electron films is investigated within the single-band Hubbard model. The Coulomb correlation between the electrons in the ferromagnetic layers is treated by using the spectral density approach (SDA). It is found that the presence of nonmagnetic layers has a strong effect on the magnetic properties of thin films. The Curie temperatures of very thin films are modified by the uncorrelated overlayers. The quasiparticle density of states is used to analyze the results. In addition, the coupling between the ferromagnetic layers and the nonmagnetic layers is discussed in detail. The coupling depends on the band occupation of the nonmagnetic layers, while it is almost independent of the number of the nonmagnetic layers. The induced polarization in the nonmagnetic layers shows a long-range decreasing oscillatory behavior and it depends on the coupling between ferromagnetic and nonmagnetic layers.Comment: 9 pages, RevTex, 6 figures, for related work see: http://orion.physik.hu-berlin.d