8,075 research outputs found
Field theoretic calculation of scalar turbulence
The cascade rate of passive scalar and Bachelor's constant in scalar
turbulence are calculated using the flux formula. This calculation is done to
first order in perturbation series. Batchelor's constant in three dimension is
found to be approximately 1.25. In higher dimension, the constant increases as
.Comment: RevTex4, publ. in Int. J. Mod. Phy. B, v.15, p.3419, 200
Long-range Ni/Mn structural order in epitaxial double perovskite La2NiMnO6 thin films
We report and compare the structural, magnetic, and optical properties of
ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An
x-ray diffraction study reveals that the B-site Ni/Mn ordering induces
additional XRD reflections as the crystal symmetry is transformed from a
pseudocubic perovskite unit cell in the disordered phase to a monoclinic form
with larger lattice parameters for the ordered phase. Polarized Raman
spectroscopy studies reveal that the ordered samples are characterized by
additional phonon excitations that are absent in the disordered phase. The
appearance of these additional phonon excitations is interpreted as the
clearest signature of Brillouin zone folding as a result of the long-range
Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a
single ferromagnetic-to-paramagnetic transition. The ordered films display also
a saturation magnetization close to 4.8 mB/f.u. and a transition temperature
(FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u.
saturation magnetization and a FM-TC around 138 K. The differences in their
magnetic behaviours are understood based on the distinct local electronic
configurations of their Ni/Mn cations.Comment: 15 pages, 5 fig
Phase formation, phonon behavior, and magnetic properties of novel ferromagnetic La3BAlMnO9 (B = Co or Ni) triple perovskites
In the quest for novel magnetoelectric materials, we have grown, stabilized
and explored the properties of La3BAlMnO9 (B = Co or Mn) thin films. In this
paper, we report the influence of the growth parameters that promote B/Al/Mn
ordering in the pseudo-cubic unit cell and their likely influence on the
magnetic and multiferroic properties. The temperature dependence of the
magnetization shows that La3CoAlMnO9 is ferromagnetic up to 190 K while
La3NiAlMnO9 shows a TC of 130 K. The behavior of these films are compared and
contrasted with related La2BMnO6 double perovskites. It is observed that the
insertion of AlO6 octahedra between CoO6 and MnO6 suppresses significantly the
strength of the superexchange interaction, spin-phonon and spin-polar coupling.Comment: 13 pages, 3 fig
Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality and Defects
We calculate the interaction between two spherical colloidal particles
embedded in the isotropic phase of a nematogenic liquid. The surface of the
particles induces wetting nematic coronas that mediate an elastic interaction.
In the weak wetting regime, we obtain exact results for the interaction energy
and the texture, showing that defects and biaxiality arise, although they are
not topologically required. We evidence rich behaviors, including the
possibility of reversible colloidal aggregation and dispersion. Complex
anisotropic self-assembled phases might be formed in dense suspensions.Comment: 4 pages, 6 figure
Investigation of phonon behavior in Pr2NiMnO6 by micro-Raman spectroscopy
The temperature dependence of phonon excitations and the presence of spin
phonon coupling in polycrystalline Pr2NiMnO6 samples were studied using
micro-Raman spectroscopy and magnetometry. Magnetic properties show a single
ferromagnetic-to-paramagnetic transition at 228 K and a saturation
magnetization close to 4.95 \muB/f.u.. Three distinct Raman modes at 657, 642,
and 511 cm-1 are observed. The phonon excitations show a clear hardening due to
anharmonicity from 300 K down to 10 K. Further, temperature dependence of the
657 cm-1 mode shows only a small softening. This reflects the presence of a
relatively weak spin-phonon coupling in Pr2NiMnO6 contrary to other double
perovskites previously studied.Comment: 10 pages, 4 fig
Laboratory astrophysics on ASDEX Upgrade: Measurements and analysis of K-shell O, F, and Ne spectra in the 9 - 20 A region
High-resolution measurements of K-shell emission from O, F, and Ne have been performed at the ASDEX Upgrade tokamak in Garching, Germany. Independently measured temperature and density profiles of the plasma provide a unique test bed for model validation. We present comparisons of measured spectra with calculations based on transport and collisional-radiative models and discuss the reliability of commonly used diagnostic line ratios
Reversibility of Superconducting Nb Weak Links Driven by the Proximity Effect in a Quantum Interference Device
We demonstrate the role of proximity effect in the thermal hysteresis of
superconducting constrictions. From the analysis of successive thermal
instabilities in the transport characteristics of micron-size superconducting
quantum interference devices with a well-controlled geometry, we obtain a
complete picture of the different thermal regimes. These determine whether the
junctions are hysteretic or not. Below the superconductor critical temperature,
the critical current switches from a classical weak-link behavior to one driven
by the proximity effect. The associated small amplitude of the critical current
makes it robust with respect to the heat generation by phase-slips, leading to
a non-hysteretic behavior.Comment: 6 pages, 5 figures, including Supplementary Informatio
Geometrically-controlled twist transitions in nematic cells
We study geometrically-controlled twist transitions of a nematic confined
between a sinusoidal grating and a flat substrate. In these cells the
transition to the twisted state is driven by surface effects. We have
identified the mechanisms responsible for the transition analytically and used
exact numerical calculations to study the range of surface parameters where the
twist instability occurs. Close to these values the cell operates under minimal
external fields or temperature variations
Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
This paper introduces a new modular Fish Bone Active Camber morphing wing with novel 3D printed skin panels. These skin panels are printed using two different Thermoplastic Polyurethane (TPU) formulations: a soft, high strain formulation for the deformable membrane of the skin, reinforced with a stiffer formulation for the stringers and mounting tabs. Additionally, this is the first FishBAC device designed to be modular in its installation and actuation. Therefore, all components can be removed and replaced for maintenance purposes without having to remove or disassemble other parts. A 1 m span, 0.27 m chord morphing wing with a 25% chord FishBAC was built and tested mechanically and in a low-speed wind tunnel. Results show that the new design is capable of achieving the same large changes in airfoil lift coefficient (approximate ΔCL≈0.55) with a low drag penalty seen in previous FishBAC work, but with a much simpler, practical and modular design. Additionally, the device shows a change in the pitching moment coefficient of ΔCM≈0.1, which shows the potential that the FishBAC has as a control surface
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