12,530 research outputs found
M\"{o}ssbauer study of the '11' iron-based superconductors parent compound Fe(1+x)Te
57Fe Moessbauer spectroscopy was applied to investigate the superconductor
parent compound Fe(1+x)Te for x=0.06, 0.10, 0.14, 0.18 within the temperature
range 4.2 K - 300 K. A spin density wave (SDW) within the iron atoms occupying
regular tetrahedral sites was observed with the square root of the mean square
amplitude at 4.2 K varying between 9.7 T and 15.7 T with increasing x. Three
additional magnetic spectral components appeared due to the interstitial iron
distributed over available sites between the Fe-Te layers. The excess iron
showed hyperfine fields at approximately 16 T, 21 T and 49 T for three
respective components at 4.2 K. The component with a large field of 49 T
indicated the presence of isolated iron atoms with large localized magnetic
moment in interstitial positions. Magnetic ordering of the interstitial iron
disappeared in accordance with the fallout of the SDW with the increasing
temperature
Effect of Strain Relaxation on Magnetotransport properties of epitaxial La_0.7Ca_0.3MnO_3 films
In this paper, we have studied the effect of strain relaxation on
magneto-transport properties of La_0.7Ca_0.3MnO_3 epitaxial films (200 nm
thick), which were deposited by pulsed laser deposition technique under
identical conditions. All the films are epitaxial and have cubic unit cell. The
amount of strain relaxation has been varied by taking three different single
crystal substrates of SrTiO_3, LaAlO_3 and MgO. It has been found that for
thicker films the strain gets relaxed and produces variable amount of disorder
depending on the strength of strain relaxation. The magnitude of lattice
relaxation has been found to be 0.384, 3.057 and 6.411 percent for film
deposited on SrTiO_3, LaAlO_3 and MgO respectively. The films on LaAlO_3 and
SrTiO_3 show higher T_{IM} of 243 K and 217 K respectively as compared to
T_{IM} of 191 K for the film on MgO. Similarly T_C of the films on SrTiO_3 and
LaAlO_3 is sharper and has value of 245 K and 220 K respectively whereas the TC
of the film on MgO is 175 K. Higher degree of relaxation creates more defects
and hence TIM (T_C) of the film on MgO is significantly lower than of SrTiO_3
and LaAlO_3. We have adopted a different approach to correlate the effect of
strain relaxation on magneto-transport properties of LCMO films by evaluating
the resistivity variation through Mott's VRH model. The variable presence of
disorder in these thick films due to lattice relaxation which have been
analyzed through Mott's VRH model provides a strong additional evidence that
the strength of lattice relaxation produces disorder dominantly by increase in
density of defects such as stacking faults, dislocations, etc. which affect the
magneto-transport properties of thick epitaxial La_0.7Ca_0.3MnO_3 films
Improved silicon nitride for advanced heat engines
The AiResearch Casting Company baseline silicon nitride (92 percent GTE SN-502 Si sub 3 N sub 4 plus 6 percent Y sub 2 O sub 3 plus 2 percent Al sub 2 O sub 3) was characterized with methods that included chemical analysis, oxygen content determination, electrophoresis, particle size distribution analysis, surface area determination, and analysis of the degree of agglomeration and maximum particle size of elutriated powder. Test bars were injection molded and processed through sintering at 0.68 MPa (100 psi) of nitrogen. The as-sintered test bars were evaluated by X-ray phase analysis, room and elevated temperature modulus of rupture strength, Weibull modulus, stress rupture, strength after oxidation, fracture origins, microstructure, and density from quantities of samples sufficiently large to generate statistically valid results. A series of small test matrices were conducted to study the effects and interactions of processing parameters which included raw materials, binder systems, binder removal cycles, injection molding temperatures, particle size distribution, sintering additives, and sintering cycle parameters
Backscatter Inversion in Spherically Asymmetric Ionosphere
It is Well Known that Various Diurnal and Morphological Features of the Ionosphere Reveal Substantial Departures from the Normally Simplified Assumption of Spherical Symmetry at Certain Times of the Day or at Certain Geographic Locations. a Radio Ray Passing through Such an Ionosphere Must Bear Information About its Horizontal Gradients. the Leading Edge of a Backscatter Ionogram is Formed by Obliquely Propagated Radio Rays of Minimum Time Delay and Hence is Useful in Deducing Information About the Ionospheric Horizontal Gradients. in This Regard, the Ionospheric Electron Density Distribution is Modeled by a Locally Quasi‐parabolic Layer with Six Parameters. This Six‐parameter Space is Known as the \u27ion state.\u27 Our Object is to Seek the \u27best\u27 Ion state in the Sense that the Corresponding Mean Square Error in Group Delay is a Minimum. a Computer Program Has Been Written to Carry Out the Specified Procedure. a Number of Sample Calculations Are Presented and Discussed. Copyright 1979 by the American Geophysical Union
Casimir force between designed materials: what is possible and what not
We establish strict upper limits for the Casimir interaction between
multilayered structures of arbitrary dielectric or diamagnetic materials. We
discuss the appearance of different power laws due to frequency-dependent
material constants. Simple analytical expressions are in good agreement with
numerical calculations based on Lifshitz theory. We discuss the improvements
required for current (meta) materials to achieve a repulsive Casimir force.Comment: 9 pages, 4 figures, graphicx, v4: Europhysics Letters, in pres
Optimizing omnidirectional reflection by multilayer mirrors
Periodic layered media can reflect strongly for all incident angles and
polarizations in a given frequency range. Quarter-wave stacks at normal
incidence are commonplace in the design of such omnidirectional reflectors. We
discuss alternative design criteria to optimize these systems.Comment: 9 pages, 6 figures. To be published in J. Opt. A: Pure and Applied
Optic
Radiation pattern of a classical dipole in a photonic crystal: photon focusing
The asymptotic analysis of the radiation pattern of a classical dipole in a
photonic crystal possessing an incomplete photonic bandgap is presented. The
far-field radiation pattern demonstrates a strong modification with respect to
the dipole radiation pattern in vacuum. Radiated power is suppressed in the
direction of the spatial stopband and strongly enhanced in the direction of the
group velocity, which is stationary with respect to a small variation of the
wave vector. An effect of radiated power enhancement is explained in terms of
\emph{photon focusing}. Numerical example is given for a square-lattice
two-dimensional photonic crystal. Predictions of asymptotic analysis are
substantiated with finite-difference time-domain calculations, revealing a
reasonable agreement.Comment: Submitted to Phys. Rev.
Robust and scalable rf spectroscopy in first-order magnetic sensitive states at second-long coherence time
Trapped-ion quantum sensors have become highly sensitive tools for the search
of physics beyond the Standard Model. Recently, stringent tests of local
Lorentz-invariance (LLI) have been conducted with precision spectroscopy in
trapped ions. We here elaborate on robust and scalable radio-frequency
composite-pulse spectroscopy at second long coherence times in the magnetic
sublevels of the long-lived state of a trapped Yb
ion. We compare two Ramsey-type composite rf pulse sequences, a generalized
spin-echo (GSE) sequence and a sequence based on universal rotations with 10
rephasing pulses (UR10) that decouple the energy levels from magnetic field
noise, enabling robust and accurate spectroscopy. Both sequences are
characterized theoretically and experimentally in the spin-$\
^{2}S_{1/2}^{172}^{+}^2F_{7/2}^2F_{7/2}2.5\,^{2}F_{7/2}\Theta\,=\,-0.0298(38)\,ea^{2}_{0}$
which is in agreement with the value deduced from clock measurements.Comment: 19 pages, 7 figure
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