13,027 research outputs found
Fabrication of thick structures by sputtering
Deposit, 5500-gram of Cu-0.15 wt % Zr alloy, sputtered onto copper cylinder to average thickness of 12.29 mm. Structure was achieved with high-rate sputter deposition for about 100 hours total sputtering time. Material had twice the strength of unsputtered material at temperatures to 723 K and equivalent strength at nearly 873 K
Stability analysis for the background equations for inflation with dissipation and in a viscous radiation bath
The effects of bulk viscosity are examined for inflationary dynamics in which
dissipation and thermalization are present. A complete stability analysis is
done for the background inflaton evolution equations, which includes both
inflaton dissipation and radiation bulk viscous effects. Three representative
approaches of bulk viscous irreversible thermodynamics are analyzed: the Eckart
noncausal theory, the linear and causal theory of Israel-Stewart and a more
recent nonlinear and causal bulk viscous theory. It is found that the causal
theories allow for larger bulk viscosities before encountering an instability
in comparison to the noncausal Eckart theory. It is also shown that the causal
theories tend to suppress the radiation production due to bulk viscous
pressure, because of the presence of relaxation effects implicit in these
theories. Bulk viscosity coefficients derived from quantum field theory are
applied to warm inflation model building and an analysis is made of the effects
to the duration of inflation. The treatment of bulk pressure would also be
relevant to the reheating phase after inflation in cold inflation dynamics and
during the radiation dominated regime, although very little work in both areas
has been done, the methodology developed in this paper could be extended to
apply to these other problems.Comment: 27 pages, 14 figures, Published version JCA
Gravitational quasinormal modes for Kerr Anti-de Sitter black holes
We investigate the quasinormal modes for gravitational perturbations of
rotating black holes in four dimensional Anti-de Sitter (AdS) spacetime. The
study of the quasinormal frequencies related to these modes is relevant to the
AdS/CFT correspondence. Although results have been obtained for Schwarzschild
and Reissner-Nordstrom AdS black holes, quasinormal frequencies of Kerr-AdS
black holes are computed for the first time. We solve the Teukolsky equations
in AdS spacetime, providing a second order and a Pade approximation for the
angular eigenvalues associated to the Teukolsky angular equation. The
transformation theory and the Regge-Wheeler-Zerilli equations for Kerr-AdS are
obtained.Comment: 20 pages, 13 figures, ReVTe
Combined Gamma Ray/neutron Spectroscopy for Mapping Lunar Resources
Some elements in the Moon can be resources, such as hydrogen and oxygen. Other elements, like Ti or the minerals in which they occur, such as ilmenite, could be used in processing lunar materials. Certain elements can also be used as tracers for other elements or lunar processes, such as hydrogen for mature regoliths with other solar-wind-implanted elements like helium, carbon, and nitrogen. A complete knowledge of the elemental composition of a lunar region is desirable both in identifying lunar resources and in lunar geochemical studies, which also helps in identifying and using lunar resources. The use of gamma ray and neutron spectroscopy together to determine abundances of many elements in the top few tens of centimeters of the lunar surface is discussed. To date, very few discussions of elemental mapping of planetary surfaces considered measurements of both gamma rays and the full range of neutron energies. The theories for gamma ray and neutron spectroscopy of the Moon and calculations of leakage fluxes are presented here with emphasis on why combined gamma ray/neutron spectroscopy is much more powerful than measuring either radiation alone
Black hole formation from massive scalar fields
It is shown that there exists a range of parameters in which gravitational
collapse with a spherically symmetric massive scalar field can be treated as if
it were collapsing dust. This implies a criterion for the formation of black
holes depending on the size and mass of the initial field configuration and the
mass of the scalar field.Comment: 11 pages, RevTeX, 3 eps figures. Submitted to Class. Quantum Gra
Gamma Ray and Neutron Spectrometer for the Lunar Resource Mapper
One of the early Space Exploration Initiatives will be a lunar orbiter to map the elemental composition of the Moon. This mission will support further lunar exploration and habitation and will provide a valuable dataset for understanding lunar geological processes. The proposed payload will consist of the gamma ray and neutron spectrometers which are discussed, an x ray fluorescence imager, and possibly one or two other instruments
Free charges versus excitons: photoluminescence investigation of InGaN/GaN multiple quantum well nanorods and their planar counterparts
InGaN/GaN multiple quantum well (MQW) nanorods have demonstrated significantly improved optical and electronic properties compared to their planar counterparts. However, the exact nature of the processes whereby nanorod structures impact the optical properties of quantum wells is not well understood, even though a variety of mechanisms have been proposed. We performed nanoscale spatially resolved, steady-state, and time-resolved photoluminescence (PL) experiments confirming that photoexcited electrons and holes are strongly bound by Coulomb interactions (i.e., excitons) in planar MQWs due to the large exciton binding energy in InGaN quantum wells. In contrast, free electronâhole recombination becomes the dominant mechanism in nanorods, which is ascribed to efficient exciton dissociation. The nanorod sidewall provides an effective pathway for exciton dissociation that significantly improves the optical performance of InGaN/GaN MQWs. We also confirm that surface treatment of nanorod sidewalls has an impact on exciton dissociation. Our results provide new insights into excitonic and charge carrier dynamics of quantum confined materials as well as the influence of surface states
- âŠ