Thermal conductivity, dislocation density and GaN device design

The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 10⁵ cm⁻², the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.     

Laserspectroscopic studies of collective properties of neutron deficient Ba Nuclei

Isotope shifts and hyperfine structure of the BaI resonance-line (λ=553.6 nm) have been measured by dye laser induced resonance fluorescence on an atomic beam for^{135m, 129g, 129m, 126}Ba thus extending previous high resolution measurements of neutron deficient Ba nuclides (N<82). The experimental results, now available for 16Ba isotopes and isomers withA=140?126, are used to deduce differences of rms charge radii, magnetic dipole and electric quadrupole moments. While the groundstates display a pronounced odd-even staggering the h 11/2^? isomers^135mBa and^133mBa show a decreased staggering. Conspicuously the isomer shift of theg 7/2⁺ isomer^129m Ba proves to be negative. The nuclear structure information is discussed in the context of gammaspectroscopic studies of transitional nuclei with 50<N,Z<82 and on the basis of a quasi-particle-plus-triaxial rotor model. The isotope shift discrepancy observed is fairly well described by the droplet model.     

The temperature dependence of the quadrupole interaction of100Rh in a Zn lattice

The¹⁰⁰Pd radioactivity was implanted into a pure Zn host with the help of an isotope separator. The temperature dependence of the quadrupole interaction was studied by time-differential measurements of the 84 keVγ- 75 keVK-conversion angular correlation. We conclude that electrons contribute significantly to the electric field gradient at the Rh site, but that local effects due to the 4d electrons of Rh are probably small.     

Phase retrieval for high-numerical-aperture optical systems

We describe a phase retrieval approach for intensity point-spread functions of high-numerical-aperture optical systems such as light microscopes. The method calculates a generalized pupil function defined on a spherical shell, using measured images at several defocus levels. The resultant pupil functionsreproduce measured point-source images significantly better than does an ideal imaging model. Availability of pupil function information will facilitate new approaches to aberration correction in such systems.     

Charge radii and moments of strontium nuclei by laser spectroscopy

The isotope shifts and hyperfine splittings for 11 strontium isotopes (A=80–90) and two isomers for the optical transition λ=293.2 nm were measured. The magnetic dipole and electric quadrupole moments and the changes of the mean square charge radii are derived. The results are discussed with respect to the increasing nuclear deformation and the anomalous coupling scheme of light Sr nuclei.     

Coulomb correction to Delbrück scattering investigated atZ=94

Differential cross sections for the elastic scattering of 2.754 MeV photons by Pu were measured for angles ranging from 45 to 120° and interpreted in terms of Delbrück, nuclear Thomson, Rayleigh and nuclear resonance scattering. It is shown that the largest part of the discrepancy between experiment and the lowest-order Delbrück theory can be removed by introducing a (Zα)⁴-dependent Coulomb correction term. Evidence is obtained for additional Coulomb correction terms of higher order in (Z α).     

Treatment of laser-induced thermal acoustics in the framework of discrete kinetic theory

The physics behind the laser-induced thermal acoustics technique is dealt with on a microscopic level. A discrete velocity model of the Boltzmann equation for inelastically interacting gas mixtures in the presence of two counterpropagating laser beams is established. The collisional scheme for the model is developed by taking into account elastic and inelastic interactions between the gas particles, on the one hand, and the interactions between monochromatic laser photons and gas particles, on the other hand. The formation and evolution of laser-pulse-driven thermal and density gratings are simulated by numerically solving the discrete kinetic equations based on the fractional step method. Numerical results are provided for a wide scope of Knudsen numbers.     

Temperature dependence of the quadrupole interaction of100Rh in a Cd lattice

The quadrupole interaction frequency |v _q| = |eQV _zz /h|0 = 8.71 ± 0.11 MHz was measured for the 75 keV state of¹⁰⁰Rh in a polycrystalline cadmium matrix by use of the γ-e^? and γ-γ time differential perturbed angular correlation (TDPAC) techniques. The temperature dependence of the electric quadrupole coupling constant for¹⁰⁰Rh in Cd is similar to those of other impurities in Cd metal.     

New Discrete Model Boltzmann Equations for Arbitrary Partitions of the Velocity Space

Modified discrete Boltzmann equations for arbitrary partitions of the velocity space are established. The new equations can be derived from the continuous Boltzmann equation and are a generalization of previous discrete-velocity models. They preserve mass, momentum, and energy, and an H-theorem holds. The new model equations are tested by comparing their solutions with the analytical ones of the continuous Boltzmann equation for the Krook–Wu and the very hard particle models.     

The quadrupole moment of176Lu

The intrinsic quadrupole moment of¹⁷⁶Lu has been determined by Coulomb excitation with α-particles to be 6.98±0.10 b. Evidence for a new level at approximately 578 keV with presumably collective structure is presented.