2,000 research outputs found

### Thermodynamic ground states of platinum metal nitrides

The thermodynamic stabilities of various phases of the nitrides of the
platinum metal elements are systematically studied using density functional
theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal
structures, in which the metal ions occupy simple tetragonal lattice sites,
have lower formation enthalpies at ambient conditions than any previously
proposed structures. The region of stability with respect to those structures
extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal
structures at this pressure are thermodynamically stable also with respect to
phase separation. The fact that the local density and generalized gradient
approximations predict different values of the absolute formation enthalpies as
well different relative stabilities between simple tetragonal and the pyrite or
marcasite structures are further discussed.Comment: 5 pages, 4 figure

### Gravitational Waves in the Nonsymmetric Gravitational Theory

We prove that the flux of gravitational radiation from an isolated source in
the Nonsymmetric Gravitational Theory is identical to that found in Einstein's
General Theory of Relativity.Comment: 10 Page

### Deposit Growth in the Wetting of an Angular Region with Uniform Evaporation

Solvent loss due to evaporation in a drying drop can drive capillary flows
and solute migration. The flow is controlled by the evaporation profile and the
geometry of the drop. We predict the flow and solute migration near a sharp
corner of the perimeter under the conditions of uniform evaporation. This
extends the study of Ref. 6, which considered a singular evaporation profile,
characteristic of a dry surrounding surface. We find the rate of the deposit
growth along contact lines in early and intermediate time regimes. Compared to
the dry-surface evaporation profile of Ref. 6, uniform evaporation yields more
singular deposition in the early time regime, and nearly uniform deposition
profile is obtained for a wide range of opening angles in the intermediate time
regime. Uniform evaporation also shows a more pronounced contrast between acute
opening angles and obtuse opening angles.Comment: 12 figures, submitted to Physical Review

### Can the Copernican principle be tested by cosmic neutrino background?

The Copernican principle, stating that we do not occupy any special place in
our universe, is usually taken for granted in modern cosmology. However recent
observational data of supernova indicate that we may live in the under-dense
center of our universe, which makes the Copernican principle challenged. It
thus becomes urgent and important to test the Copernican principle via
cosmological observations. Taking into account that unlike the cosmic photons,
the cosmic neutrinos of different energies come from the different places to us
along the different worldlines, we here propose cosmic neutrino background as a
test of the Copernican principle. It is shown that from the theoretical
perspective cosmic neutrino background can allow one to determine whether the
Copernican principle is valid or not, but to implement such an observation the
larger neutrino detectors are called for.Comment: JHEP style, 10 pages, 4 figures, version to appear in JCA

### Linearisation Instabilities of the Massive Nonsymmetric Gravitational Theory

The massive nonsymmetric gravitational theory is shown to posses a
linearisation instability at purely GR field configurations, disallowing the
use of the linear approximation in these situations. It is also shown that
arbitrarily small antisymmetric sector Cauchy data leads to singular evolution
unless an ad hoc condition is imposed on the initial data hypersurface.Comment: 14 pages, IOP style for submission to CQG. Minor changes and
additional background material adde

### Cascade time-scales for energy and helicity in homogeneous isotropic turbulence

We extend the Kolmogorov phenomenology for the scaling of energy spectra in
high-Reynolds number turbulence, to explicitly include the effect of helicity.
There exists a time-scale $\tau_H$ for helicity transfer in homogeneous,
isotropic turbulence with helicity. We arrive at this timescale using the
phenomenological arguments used by Kraichnan to derive the timescale $\tau_E$
for energy transfer (J. Fluid Mech. {\bf 47}, 525--535 (1971)). We show that in
general $\tau_H$ may not be neglected compared to $\tau_E$, even for rather low
relative helicity. We then deduce an inertial range joint cascade of energy and
helicity in which the dynamics are dominated by $\tau_E$ in the low wavenumbers
with both energy and helicity spectra scaling as $k^{-5/3}$; and by $\tau_H$ at
larger wavenumbers with spectra scaling as $k^{-4/3}$. We demonstrate how,
within this phenomenology, the commonly observed ``bottleneck'' in the energy
spectrum might be explained. We derive a wavenumber $k_h$ which is less than
the Kolmogorov dissipation wavenumber, at which both energy and helicity
cascades terminate due to dissipation effects. Data from direct numerical
simulations are used to check our predictions.Comment: 14 pages, 5 figures, accepted to Physical Review

### A Unified treatment of small and large- scale dynamos in helical turbulence

Helical turbulence is thought to provide the key to the generation of
large-scale magnetic fields. Turbulence also generically leads to rapidly
growing small-scale magnetic fields correlated on the turbulence scales. These
two processes are usually studied separately. We give here a unified treatment
of both processes, in the case of random fields, incorporating also a simple
model non-linear drift. In the process we uncover an interesting plausible
saturated state of the small-scale dynamo and a novel analogy between quantum
mechanical (QM) tunneling and the generation of large scale fields. The steady
state problem of the combined small/large scale dynamo, is mapped to a
zero-energy, QM potential problem; but a potential which, for non-zero mean
helicity, allows tunneling of bound states. A field generated by the
small-scale dynamo, can 'tunnel' to produce large-scale correlations, which in
steady state, correspond to a force-free 'mean' field.Comment: 4 pages, 1 figure, Physical Review Letters, in pres

### Solving the Flatness and Quasi-flatness Problems in Brans-Dicke Cosmologies with a Varying Light Speed

We define the flatness and quasi-flatness problems in cosmological models. We
seek solutions to both problems in homogeneous and isotropic Brans-Dicke
cosmologies with varying speed of light. We formulate this theory and find
perturbative, non-perturbative, and asymptotic solutions using both numerical
and analytical methods. For a particular range of variations of the speed of
light the flatness problem can be solved. Under other conditions there exists a
late-time attractor with a constant value of \Omega that is smaller than, but
of order, unity. Thus these theories may solve the quasi-flatness problem, a
considerably more challenging problem than the flatness problem. We also
discuss the related \Lambda and quasi-\Lambda problem in these theories. We
conclude with an appraisal of the difficulties these theories may face.Comment: 21 pages, 6 figure

### Energies of knot diagrams

We introduce and begin the study of new knot energies defined on knot
diagrams. Physically, they model the internal energy of thin metallic solid
tori squeezed between two parallel planes. Thus the knots considered can
perform the second and third Reidemeister moves, but not the first one. The
energy functionals considered are the sum of two terms, the uniformization term
(which tends to make the curvature of the knot uniform) and the resistance term
(which, in particular, forbids crossing changes). We define an infinite family
of uniformization functionals, depending on an arbitrary smooth function $f$
and study the simplest nontrivial case $f(x)=x^2$, obtaining neat normal forms
(corresponding to minima of the functional) by making use of the Gauss
representation of immersed curves, of the phase space of the pendulum, and of
elliptic functions

### Experimental evidence of chaotic advection in a convective flow

Lagrangian chaos is experimentally investigated in a convective flow by means
of Particle Tracking Velocimetry. The Fnite Size Lyapunov Exponent analysis is
applied to quantify dispersion properties at different scales. In the range of
parameters of the experiment, Lagrangian motion is found to be chaotic.
Moreover, the Lyapunov depends on the Rayleigh number as ${\cal R}a^{1/2}$. A
simple dimensional argument for explaining the observed power law scaling is
proposed.Comment: 7 pages, 3 figur

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