36,308 research outputs found
First-Principles Thermodynamics of Coherent Interfaces in Samarium-Doped Ceria Nanoscale Superlattices
Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent {100} interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions
Quantum Decoherence in a D-Foam Background
Within the general framework of Liouville string theory, we construct a model
for quantum D-brane fluctuations in the space-time background through which
light closed-string states propagate. The model is based on monopole and vortex
defects on the world sheet, which have been discussed previously in a treatment
of 1+1-dimensional black-hole fluctuations in the space-time background, and
makes use of a T-duality transformation to relate formulations with Neumann and
Dirichlet boundary conditions. In accordance with previous general arguments,
we derive an open quantum-mechanical description of this D-brane foam which
embodies momentum and energy conservation and small mean energy fluctuations.
Quantum decoherence effects appear at a rate consistent with previous
estimates.Comment: 16 pages, Latex, two eps figures include
Effect of field of view and monocular viewing on angular size judgements in an outdoor scene
Observers typically overestimate the angular size of distant objects. Significantly, overestimations are greater in outdoor settings than in aircraft visual-scene simulators. The effect of field of view and monocular and binocular viewing conditions on angular size estimation in an outdoor field was examined. Subjects adjusted the size of a variable triangle to match the angular size of a standard triangle set at three greater distances. Goggles were used to vary the field of view from 11.5 deg to 90 deg for both monocular and binocular viewing. In addition, an unrestricted monocular and binocular viewing condition was used. It is concluded that neither restricted fields of view similar to those present in visual simulators nor the restriction of monocular viewing causes a significant loss in depth perception in outdoor settings. Thus, neither factor should significantly affect the depth realism of visual simulators
Recent advances at NASA in calculating the electronic spectra of diatomic molecules
Advanced entry vehicles, such as the proposed Aero-assisted Orbital Transfer Vehicle, provide new and challenging problems for spectroscopy. Large portions of the flow field about such vehicles will be characterized by chemical and thermal nonequilibrium. Only by considering the actual overlap of the atomic and rotational lines emitted by the species present can the impact of radiative transport within the flow field be assessed correctly. To help make such an assessment, a new computer program is described that can generate high-resolution, line-by-line spectra for any spin-allowed transitions in diatomic molecules. The program includes the matrix elements for the rotational energy and distortion to the fourth order; the spin-orbit, spin-spin, and spin-rotation interactions to first order; and the lambda splitting by a perturbation calculation. An overview of the Computational Chemistry Branch at Ames Research Center is also presented
A supersymmetric D-brane Model of Space-Time Foam
We present a supersymmetric model of space-time foam with two stacks of eight
D8-branes with equal string tensions, separated by a single bulk dimension
containing D0-brane particles that represent quantum fluctuations in the
space-time foam. The ground state configuration with static D-branes has zero
vacuum energy. However, gravitons and other closed-string states propagating
through the bulk may interact with the D0-particles, causing them to recoil and
the vacuum energy to become non zero. This provides a possible origin of dark
energy. Recoil also distorts the background metric felt by energetic massless
string states, which travel at less than the usual (low-energy) velocity of
light. On the other hand, the propagation of chiral matter anchored on the D8
branes is not affected by such space-time foam effects.Comment: 33 pages, latex, five figure
First-principles thermodynamic modeling of lanthanum chromate perovskites
Tendencies toward local atomic ordering in (A,AâČ)(B,BâČ)O_(3âÎŽ) mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La_(1âx),Sr_x)(Cr_(1ây),Fe_y)O_(3âÎŽ) (LSCF) and (La_(1âx),Sr_x)(Cr_(1ây),Ru_y)O_(3âÎŽ) (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature
First-principles thermodynamic modeling of atomic ordering in yttria-stabilized zirconia
Yttria-stabilized zirconia YSZ is modeled using a cluster expansion statistical thermodynamics method
built upon a density-functional theory database. The reliability of cluster expansions in predicting atomic
ordering is explored by comparing with the extensive experimental database. The cluster expansion of YSZ is
utilized in lattice Monte Carlo simulations to compute the ordering of dopant and oxygen vacancies as a
function of concentration. Cation dopants show a strong tendency to aggregate and vacate significantly sized
domains below 9 mol % Y_2O_3, which is likely important for YSZ aging processes in ionic conductivity.
Evolution of vibrational and underlying electronic properties as a function of Y doping is explored
Do Three Dimensions tell us Anything about a Theory of Everything?
It has been conjectured that four-dimensional N=8 supergravity may provide a
suitable framework for a `Theory of Everything', if its composite SU(8) gauge
fields become dynamical. We point out that supersymmetric three-dimensional
coset field theories motivated by lattice models provide toy laboratories for
aspects of this conjecture. They feature dynamical composite supermultiplets
made of constituent holons and spinons. We show how these models may be
extended to include N=1 and N=2 supersymmetry, enabling dynamical conjectures
to be verified more rigorously. We point out some special features of these
three-dimensional models, and mention open questions about their relevance to
the dynamics of N=8 supergravity.Comment: 20 pages Latex, 2 eps figure
Brany Liouville Inflation
We present a specific model for cosmological inflation driven by the
Liouville field in a non-critical supersymmetric string framework, in which the
departure from criticality is due to open strings stretched between the two
moving Type-II 5-branes. We use WMAP and other data on fluctuations in the
cosmic microwave background to fix parameters of the model, such as the
relative separation and velocity of the 5-branes, respecting also the
constraints imposed by data on light propagation from distant gamma-ray
bursters. The model also suggests a small, relaxing component in the present
vacuum energy that may accommodate the breaking of supersymmetry.Comment: 23 pages LATEX, two eps figures incorporated; version accepted for
publication in NJ
CP-Violating MSSM Higgs Bosons in the Light of LEP 2
In the MSSM, the CP parities of the neutral Higgs bosons may be mixed by
radiative effects induced by explicit CP violation in the third generation of
squarks. To allow for this possibility, we argue that the charged Higgs-boson
mass and tan(beta) should be used to parametrize the MSSM Higgs sector. We
introduce a new benchmark scenario of maximal CP violation appropriate for
direct searches of CP-violating MSSM Higgs bosons. We show that the bounds
established by LEP 2 on the MSSM Higgs sector may be substantially relaxed at
low and intermediate values of tan(beta) in the presence of CP violation, and
comment on possible Higgs boson signatures at LEP 2 within this framework.Comment: 16 pages, LaTeX, 4 encapsulated figure
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