24,726 research outputs found
Cross-sectional and plan-view cathodoluminescence of GaN partially coalesced above a nanocolumn array
The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-μm scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of ∼2 μm thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions
Metallization of Fluid Hydrogen
The electrical resistivity of liquid hydrogen has been measured at the high
dynamic pressures, densities and temperatures that can be achieved with a
reverberating shock wave. The resulting data are most naturally interpreted in
terms of a continuous transition from a semiconducting to a metallic, largely
diatomic fluid, the latter at 140 GPa, (ninefold compression) and 3000 K. While
the fluid at these conditions resembles common liquid metals by the scale of
its resistivity of 500 micro-ohm-cm, it differs by retaining a strong pairing
character, and the precise mechanism by which a metallic state might be
attained is still a matter of debate. Some evident possibilities include (i)
physics of a largely one-body character, such as a band-overlap transition,
(ii) physics of a strong-coupling or many-body character,such as a Mott-Hubbard
transition, and (iii) processes in which structural changes are paramount.Comment: 12 pages, RevTeX format. Figures available on request; send mail to:
[email protected] To appear: Philosophical Transaction of the Royal
Society
Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system
In classical Drude theory the conductivity is determined by the mass of the
propagating particles and the mean free path between two scattering events. For
a quantum particle this simple picture of diffusive transport loses relevance
if strong correlations dominate the particle motion. We study a situation where
the propagation of a fermionic particle is possible only through creation and
annihilation of local bosonic excitations. This correlated quantum transport
process is outside the Drude picture, since one cannot distinguish between free
propagation and intermittent scattering. The characterization of transport is
possible using the Drude weight obtained from the f-sum rule, although its
interpretation in terms of free mass and mean free path breaks down. For the
situation studied we calculate the Green's function and Drude weight using a
Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction
Wrongful Death and Disability Awards: Gross or After-Tax Income?
The present value of future wage losses represent an important part of the total economic loss that results from a wrongful death. The econoÂmist\u27s task, in the role of expert witness, is to provide to the interested parties a lump sum amount of income equal to the losses experienced by the decedent\u27s beneficiary. This amount is obtained. by increasing the decedent\u27s wage at the time of death, by an appropriate annual wage growth rate, to generate estimates of what future wages would have been had the decedent been able to continue in his or her employment. Each of the annual future wages is then discounted to determine today\u27s value of the future annual wage. The sum of these discounted future annual wages produces an amount of money, which when invested, will provide an income stream exactly equal to the decedent\u27s lost income stream
Eulerian spectral closures for isotropic turbulence using a time-ordered fluctuation-dissipation relation
Procedures for time-ordering the covariance function, as given in a previous
paper (K. Kiyani and W.D. McComb Phys. Rev. E 70, 066303 (2004)), are extended
and used to show that the response function associated at second order with the
Kraichnan-Wyld perturbation series can be determined by a local (in wavenumber)
energy balance. These time-ordering procedures also allow the two-time
formulation to be reduced to time-independent form by means of exponential
approximations and it is verified that the response equation does not have an
infra-red divergence at infinite Reynolds number. Lastly, single-time
Markovianised closure equations (stated in the previous paper above) are
derived and shown to be compatible with the Kolmogorov distribution without the
need to introduce an ad hoc constant.Comment: 12 page
Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission
Using Keck/HIRES spectra {\Delta}v ~ 7 km/s, we analyze forbidden lines of [O
I] 6300 {\AA}, [O I] 5577 {\AA} and [S II] 6731 {\AA} from 33 T Tauri stars
covering a range of disk evolutionary stages. After removing a high velocity
component (HVC) associated with microjets, we study the properties of the low
velocity component (LVC). The LVC can be attributed to slow disk winds that
could be magnetically (MHD) or thermally (photoevaporative) driven. Both of
these winds play an important role in the evolution and dispersal of
protoplanetary material.
LVC emission is seen in all 30 stars with detected [O I] but only in 2 out of
eight with detected [S II] , so our analysis is largely based on the properties
of the [O I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC)
kinematic components. Both components are found over a wide range of accretion
rates and their luminosity is correlated with the accretion luminosity, but the
NC is proportionately stronger than the BC in transition disks.
The FWHM of both the BC and NC correlates with disk inclination, consistent
with Keplerian broadening from radii of 0.05 to 0.5 AU and 0.5 to 5 AU,
respectively. The velocity centroids of the BC suggest formation in an MHD disk
wind, with the largest blueshifts found in sources with closer to face-on
orientations. The velocity centroids of the NC however, show no dependence on
disk inclination. The origin of this component is less clear and the evidence
for photoevaporation is not conclusive
Research and investigation of geology, mineral, and water resources of Maryland
The authors have identified the following significant results. Field work in Baltimore County revealed that the signature returns of serpentinitic and nonserpentinitic rocks correlates with the vegetation cover and land use pattern. In Maryland Piedmont, bedrock lithology and structure are enhanced only to the extent that land use is geologically dictated. Two prominent sets of linear features are detected on ERTS-1 imagery at N 45 deg E and N 20 deg E. Beaches of Chesapeake Bay are classified as broad and narrow beaches based on the width of the backshore zone. It is shown by comparing historical shorelines of Ocean City, from the inlet to the Maryland-Delaware line that reversal zones of erosion and accretion occur at different locations for different periods. High reflectance levels (high marsh-high topographic areas) for the lower Eastern Shore are found to be distributed as two distinct trending linear ridge systems. Observations of MSS band 5 dated 9 April 1974 exhibited an unique sedimentation pattern for Chesapeake Bay. Following a 1.5 inch rainfall, heavy concentration of suspended sediments is observed on the imagery, particularly in the area of the turbidity maximum
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