The mechanical properties of a CuCr1Zr alloy were measured with the help of a Gleeble 3500 machine at temperatures ranging from 250 °C to 1000 °C. Some tensile tests were also performed at 20 °C. The measured data were first used to determine the thermal linear coefficient at different temperatures. The measured curves were then simulated with an elasto-visco-plastic constitutive model assuming that the material behaviour was isotropic. This law requires the knowledge of temperature dependent parameters that were determined by inverse method. This work represents a part of a PhD study on hot tearing susceptibility during welding.

We present the spatially-resolved polarization measurements for the disk around the Herbig Ae star, AB Aurigae. The images were obtained in J, H, and Ks bands with the coronagraphic camera HiCIAO on the Subaru Telescope. The inner region beyond 30 AU from the star was imaged, which reveals an azimuthal dip, a radial gap at around 80 AU, and complex spiral-like emission in polarized light.

M dwarfs produce explosive flare emission in the near-UV and optical continuum, and the mechanism responsible for this phenomenon is not well-understood. We present a near-UV/optical flare spectrum from the rise phase of a secondary flare, which occurred during the decay of a much larger flare. The newly formed flare emission resembles the spectrum of an early-type star, with the Balmer lines and continuum in absorption. We model this observation phenomenologically as a temperature bump (hot spot) near the photosphere of the M dwarf. The amount of heating implied by our model (ΔTphot ~ 16,000 K) is far more than predicted by chromospheric backwarming in current 1D RHD flare models (ΔTphot ~ 1200 K).

In Wisniewski et al. (2010), paper I, we analyzed 15 years of spectroscopic and spectropolarimetric data from the Ritter and Pine Bluff Observatories of 2 Be stars, 60 Cygni and π Aquarii, when a transition from Be to B star occurred. Here we analyze the intrinsic polarization, where we observe loop-like structures caused by the rise and fall of the polarization Balmer Jump and continuum V-band polarization being mismatched temporaly with polarimetric outbursts. We also see polarization angle deviations from the mean, reported in paper I, which may be indicative of warps in the disk, blobs injected at an inclined orbit, or spiral density waves. We show our ongoing efforts to model time dependent behavior of the disk to constrain the phenomena, using 3D Monte Carlo radiative transfer codes.

We used single crystals of GaN, obtained from high-pressure synthesis, as substrates for Metalorganics Vapor Phase Epitaxy growth of violet and UV laser diodes. The use of high-quality bulk GaN leads to the decrease of the dislocation density to the low level of 105 cm−2, i.e. two orders of magnitude better than typical for the Epitaxial Lateral Overgrowth laser structures fabricated on sapphire. The low density and homogeneous distribution of defects in our structures enables the realization of broad stripe laser diodes. We demonstrate that our laser diodes, having 15 μm wide stripes, are able to emit 1.3-1.9 W per facet (50% reflectivity) in 30 ns long pulses. This result, which is among the best ever reported for nitride lasers, opens the path for the development of a new generation of high power laser diodes.

We used high-pressure grown GaN single crystal substrates to fabricate dislocation free optoelectronic devices like light emitting diodes and laser diodes structures. The latter ones demonstrated laser action under optical pumping condition with the threshold of about 200 kW/cm2 at room temperature. In the present paper we would focus on the specific aspects of the homoepitaxial growth by MOVPE method including epi-ready substrate preparation and surface polarity choice. We believe that our results demonstrate clearly the feasibility of device fabrication based on high-pressure grown GaN bulk crystals.

Using free standing layer of InGaAsN we have succeeded in measuring the optical absorption in very broad spectral range (0.8-2.5eV). This gave us an insight into the conduction band density of states for the energies higher than the energy gap of this compound. By combining Hall effect measurements with determination of plasma edge frequency in infrared reflectivity for differently doped samples we were able to deduce the density of states, conduction band electrons effective mass and dispersion relation. In particular it turned out that both i) experimentally measured dispersion relation of the conduction band shows extremely high degree of nonparabolicity and consistently ii) the effective mass of electrons is few times larger than that corresponding to GaAs of the same electron concentration. So far the obtained experimental results are in line with recently proposed band anticrossing model of the electronic structure of III-N-V alloys.

We have studied an influence of pressure on the emission and absorption spectra measured from various types of InGaN structures such as epilayers, quantum wells and quantum dots. While the known pressure coefficients of GaN and InN bandgaps are in the range 40-25 meV/GPa, the experimental observation for the light emission shift with pressure for InGaN alloys is dramatically different. With the increasing In content and thus decreasing emission energy the observed pressure coefficients become very small eventually reaching zero or even slightly negative values! We have observed a much weaker trend for the decrease of the pressure coefficient for the absorption edges of InGaN. First principle calculations of InGaN band structure and its modification with a pressure are not able to explain the huge effect observed in the emission experiment but are in a good agreement with the results obtained in optical absorption measurements. We discuss here the possible mechanisms which can account for extremely low pressure coefficient of the light emission and the discrepancy between sensitivity light emission and absorption on applied pressure in InGaN alloys.

We propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. Obtained this way, the pressure shift of the luminescence peaks in blue and green InGaN-based emitters should be close to 14meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.

Recent spectroscopic and radiation damage experiments on a series of CexLa1-xF3 crystals suggest that the scintillation light output is limited by an unusual quenching mechanism, which also plays a major role in minimizing radiation-induced damage. The intensity of the radiation-induced absorptions is a strong function ofthe Ce content x, reaching a maximum for x = 0.03 and a minimum for x = 1. This peculiar dependence appears to be due to the influence of deep-lying Ce levels onboth scintillation mechanism and radiation damage. We suggest that various charge transfer processes can explain many aspects of the performance of CexLa1-xF3 scintillators.