16,999 research outputs found
Analytical comparisons of ablative nozzle materials final report, jun. 20, 1963 - nov. 20, 1964
Reaction kinetics ablation program to predict ablation performance of rocket nozzle heat protection material
Topics concerning state variable feedback in automatic control systems. Part 1 - Specification. Part 2 - Sensitivity. Part 3 - Intentional nonlinearities. Part 4 - Unavailable states
Specifications, sensitivity, intentional nonlinearities, and unavailable states concerned with state variable feedback in automatic control system
The Effect of 45{\deg} Grain Boundaries and associated Fe particles on Jc and resistivity in Ba(Fe0.9Co0.1)2As2 Thin Films
The anisotropy of the critical current density Jc depends in general on both
the properties of the flux lines (such as line tension, coherence length and
penetration depth) and the properties of the defects (such as density, shape,
orientation etc.). Whereas the Jc anisotropy in microstructurally clean films
can be scaled to an effective magnetic field containing the Ginzburg-Landau
anisotropy term, it is in general not possible (or only in a limited field
range) for samples containing extended defects. Here, the Jc anisotropy of a
Co-doped BaFe2As2 sample with 45{\deg} [001] tilt grain boundaries (GBs), i.e.
grain boundaries created by 45{\deg} in-plane rotated grains, as well as
extended Fe particles is investigated. This microstructure leads to c-axis
correlated pinning, both due to the GBs and the Fe particles and manifests in a
c-axis peak in the Jc anisotropy at low magnetic fields and a deviation from
the anisotropic Ginzburg-Landau scaling at higher fields. Strong pinning at
ellipsoidal extended defects, i.e. the Fe particles, is discussed, and the full
Jc anisotropy is fitted successfully with the vortex path model. The results
are compared to a sample without GBs and Fe particles. 45{\deg} GBs seem to be
good pinning centers rather than detrimental to current flow.Comment: 8 pages, 7 figures, CEC-ICMC 2013 proceeding, accepted for
publication in Advances in Cryogenic Engineering (Materials
Chemical NOx budget in the upper troposphere over the tropical South Pacific
The chemical NOx budget in the upper troposphere over the tropical South Pacific is analyzed using aircraft measurements made at 6-12 km altitude in September 1996 during the Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A campaign. Chemical loss and production rates of NOx along the aircraft flight tracks are calculated with a photochemical model constrained by observations. Calculations using a standard chemical mechanism show a large missing source for NOx; chemical loss exceeds chemical production by a factor of 2.4 on average. Similar or greater NOx budget imbalances have been reported in analyses of data from previous field studies. Ammonium aerosol concentrations in PEM-Tropics A generally exceeded sulfate on a charge equivalent basis, and relative humidities were low (median 25% relative to ice). This implies that the aerosol could be dry in which case N2O5 hydrolysis would be suppressed as a sink for NOx. Suppression of N2O5 hydrolysis and adoption of new measurements of the reaction rate constants for NO2 + OH + M and HNO3 + OH reduces the median chemical imbalance in the NOx budget for PEM-Tropics A from 2.4 to 1.9. The remaining imbalance cannot be easily explained from known chemistry or long-range transport of primary NOx and may imply a major gap in our understanding of the chemical cycling of NOx in the free troposphere. Copyright 2000 by the American Geophysical Union
Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields
We apply Density Functional Theory (DFT) and the DFT+U technique to study the
adsorption of transition metal porphine molecules on atomistically flat Au(111)
surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange
correlation functional correctly predict the palladium porphine (PdP) low-spin
ground state. PdP is found to adsorb preferentially on gold in a flat geometry,
not in an edgewise geometry, in qualitative agreement with experiments on
substituted porphyrins. It exhibits no covalent bonding to Au(111), and the
binding energy is a small fraction of an eV. The DFT+U technique, parameterized
to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be
crucial for reproducing the correct magnetic moment and geometry of the
isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111)
substantially alters the Mn ion spin state. Its interaction with the gold
substrate is stronger and more site-specific than PdP. The binding can be
partially reversed by applying an electric potential, which leads to
significant changes in the electronic and magnetic properties of adsorbed MnP,
and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine
macrocycle. We conjecture that this DFT+U approach may be a useful general
method for modeling first row transition metal ion complexes in a
condensed-matter setting.Comment: 14 pages, 6 figure
Rugged Metropolis Sampling with Simultaneous Updating of Two Dynamical Variables
The Rugged Metropolis (RM) algorithm is a biased updating scheme, which aims
at directly hitting the most likely configurations in a rugged free energy
landscape. Details of the one-variable (RM) implementation of this
algorithm are presented. This is followed by an extension to simultaneous
updating of two dynamical variables (RM). In a test with Met-Enkephalin in
vacuum RM improves conventional Metropolis simulations by a factor of about
four. Correlations between three or more dihedral angles appear to prevent
larger improvements at low temperatures. We also investigate a multi-hit
Metropolis scheme, which spends more CPU time on variables with large
autocorrelation times.Comment: 8 pages, 5 figures. Revisions after referee reports. Additional
simulations for temperatures down to 220
Epitaxial LaFeAsOF thin films grown by pulsed laser deposition
Superconducting and epitaxially grown LaFeAsOF thin films were successfully
prepared on (001)-oriented LaAlO3 substrates using pulsed laser deposition. The
prepared thin films show exclusively a single in-plane orientation with
epitaxial relation (001)[100] parallel to (001)[100] and a FWHM value of 1deg.
Furthermore, resistive measurement of the superconducting transition
temperature revealed a Tc90 of 25K with a high residual resistive ratio of 6.8.
The applied preparation technique, standard thin film pulsed laser deposition
at room temperature in combination with a subsequent post annealing process, is
suitable for fabrication of high quality LaFeAsO1-xFx thin films. A high upper
critical field of 76.2 T was evaluated for magnetic fields applied
perpendicular to the c-axis and the anisotropy was calculated to be 3.3
assuming single band superconductivity.Comment: 6 pages, 4 Figure
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Wigner crystallization in a polarizable medium
We present a variational study of the 2D and 3D Wigner crystal phase of large
polarons. The method generalizes that introduced by S. Fratini,P.\
Qu{\'{e}}merais [Mod. Phys. Lett. B {\bf 12} 1003 (1998)]. We take into account
the Wigner crystal normal modes rather than a single mean frequency in the
minimization procedure of the variational free energy. We calculate the
renormalized modes of the crystal as well as the charge polarization
correlation function and polaron radius. The solid phase boundaries are
determined via a Lindemann criterion, suitably generalized to take into account
the classical-to-quantum cross-over.
In the weak electron-phonon coupling limit, the Wigner crystal parameters are
renormalized by the electron-phonon interaction leading to a stabilization of
the solid phase for low polarizability of the medium. Conversely, at
intermediate and strong coupling, the behavior of the system depends strongly
on the polarizability of the medium.
For weakly polarizable media, a density crossover occurs inside the solid
phase when the renormalized plasma frequency approaches the phonon frequency.
At low density, we have a renormalized polaron Wigner crystal, while at higher
densities the electron-phonon interaction is weakened irrespective of the {\it
bare} electron-phonon coupling.
For strongly polarizable media, the system behaves as a Lorentz lattice of
dipoles. The abrupt softening of the internal polaronic frequency predicted by
Fratini and Quemerais is observed near the actual melting point only at very
strong coupling, leading to a possible liquid polaronic phase for a wider range
of parameters.Comment: 24 pages, 13 figures v1.
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