4,902 research outputs found
Adatom Diffusion at GaN (0001) and (000bar1) Surfaces
The diffusion of Ga and N adatoms has been studied for the technologically
relevant wurtzite (000bar1) and (0001) surfaces employing density-functional
theory. Our calculations reveal a very different diffusivity for Ga and N
adatoms on the equilibrium surfaces: While Ga is very mobile at typical growth
temperatures, the diffusion of N is by orders of magnitudes slower. These
results give a very detailed insight of how and under which growth conditions N
adatoms can be stabilized and efficiently incorporated at the surface. We
further find that the presence of excess N strongly increases the Ga diffusion
barrier and discuss the consequences for the growth of GaN.Comment: 4 pages, 4 figures, Appears in Appl. Phys. Lett. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Analytic study of a depressed collector for linear beam microwave amplifiers Final report
Analytic evaluation of depressed collector for linear beam microwave amplifier
Performance of a ten-stage electrostatic depressed collector for klystrons
Computational method for determining performance of ten stage electrostatic depressed collector for klystron
Multistage depressed electrostatic collector for magnetically focused space borne klystrons Final report
Multistage depressed electrostatic collector for magnetically focused spaceborne klystron
High Resolution WFPC2 Imaging of IRAS 09104+4109
With a infrared luminosity of nearly 10^13 Lsuns, IRAS 09104+4109 is the most
luminous galaxy with z<0.5 in the IRAS All Sky Survey. A radio-loud Seyfert 2
type optical spectrum, a cD host galaxy in a rich cluster, and a massive
cooling flow make IRAS 09104+4109 unique among ultraluminous infrared galaxies.
Cannibalized cluster members and the cooling intercluster medium may contribute
both the fuel and the dust needed to re-radiate the power of IRAS 09104+4109
into the far-infrared. We have imaged IRAS 09104+4109 in the WFPC2 F622W,
F814W, and FR680N filters on the HST to obtain rest frame 4300A, 5700A, and
[OIII] emission line images on sub-kpc scales. IRAS 09104+4109 displays a
complex morphology on the smallest scales, with radiating filaments, an
asymmetric [OIII] nebula, and a number of very faint, irregular blue objects
surrounding the cD galaxy. We discuss the nature and possible interplay between
the enshrouded QSO nucleus, the cD host galaxy and the irregular cluster.Comment: LaTex, 6 pages with 2 postscript and 1 jpg figure. To appear in the
proc. of the Ringberg workshop "Ultraluminous Galaxies: Monsters or Babies"
(Ringberg castle, Sept. 1998), Ap&SS, in pres
An Experimental Analysis of Optimal Renewable Resource Management: The Fishery
fishing industry;resource management;decision making;uncertainty;experimental economics
Experimental observation of the influence of furnace temperature profile on convection and segregation in the vertical Bridgman crystal growth technique
Azulene-doped naphthalene was directionally solidified during the vertical Bridgman-Stockbarger technique. Doping homogeneity and convection were determined as a function of the temperature profile in the furnace and the freezing rate. Convection velocities were two orders of magnitude lower when the temperature increased with height. Rarely was the convection pattern axisymmetric, even though the temperature varied less than 0.1 K around the circumference of the growth ampoule. Correspondingly the cross sectional variation in azulene concentration tended to be asymmetric, especially when the temperature increased with height. This cross sectional variation changed dramatically along the ingot, reflecting changes in convection presumably due to the decreasing height of the melt. Although there was large scatter and irreproducibility in the cross sectional variation in doping, this variation tended to be least when the growth rate was low and the convection was vigorous. It is expected that compositional variations would also be small at high growth rates with weak convection and flat interfaces, although this was not investigated in the present experiments. Neither rotation of the ampoule nor deliberate introduction of thermal asymmetries during solidification had a significant influence on cross sectional variations in doping. It is predicted that slow directional solidification under microgravity conditions could produce greater inhomogeneities than on Earth. Combined use of microgravity and magnetic fields would be required to achieve homogeneity when it is necessary to freeze slowly in order to avoid constitutional supercooling
The Post-Newtonian Approximation of the Rigidly Rotating Disc of Dust to Arbitrary Order
Using the analytic, global solution for the rigidly rotating disc of dust as
a starting point, an iteration scheme is presented for the calculation of an
arbitrary coefficient in the post-Newtonian (PN) approximation of this
solution. The coefficients were explicitly calculated up to the 12th PN level
and are listed in this paper up to the 4th PN level. The convergence of the
series is discussed and the approximation is found to be reliable even in
highly relativistic cases. Finally, the ergospheres are calculated at
increasing orders of the approximation and for increasingly relativistic
situations.Comment: 19 pages, 2 tables, 4 figures Accepted for publication in Phys. Rev.
Infrared Astronomy
Several observational programs in infrared astronomy are described and significant findings are briefly discussed. The near infrared work concentrates largely on the use of the 5 m Hale telescope in spectroscopic and photometric studies of extragalactic sources. Observations of the P alpha line profile in a low redshift quasar, X-ray bursters, reflection nebula, and cataclysmic variables are included. Millimeter continuum observations of dust emission from quasars and galactic molecular clouds are also discussed. Finally, improvements to instrumentation are reported
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