7,900 research outputs found
Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds
Magnesium dihydride (\mgh) stores 7.7 weight % hydrogen, but it suffers
from a high thermodynamic stability and slow (de)hydrogenation kinetics.
Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at
improving the thermodynamic and kinetic properties. We study the structure and
stability of MgTMH compounds, -1], by first-principles
calculations at the level of density functional theory. We find that the
experimentally observed sharp decrease in hydrogenation rates for
correlates with a phase transition of MgTMH from a fluorite to
a rutile phase. The stability of these compounds decreases along the series Sc,
Ti, V, Cr. Varying the transition metal (TM) and the composition , the
formation enthalpy of MgTMH can be tuned over the substantial
range 0-2 eV/f.u. Assuming however that the alloy MgTM does not
decompose upon dehydrogenation, the enthalpy associated with reversible
hydrogenation of compounds with a high magnesium content () is close to
that of pure Mg.Comment: 8 pages, 5 figure
Rotating light, OAM paradox and relativistic complex scalar field
Recent studies show that the angular momentum, both spin and orbital, of
rotating light beams possesses counter-intuitive characteristics. We present a
new approach to the question of orbital angular momentum of light based on the
complex massless scalar field representation of light. The covariant equation
for the scalar field is treated in rotating system using the general
relativistic framework. First we show the equivalence of the U(1) gauge current
for the scalar field with the Poynting vector continuity equation for paraxial
light, and then apply the formalism to the calculation of the orbital angular
momentum of rotating light beams. If the difference between the co-, contra-,
and physical quantities is properly accounted for there does not result any
paradox in the orbital angular momentum of rotating light. An artificial
analogue of the paradoxical situation could be constructed but it is wrong
within the present formalism. It is shown that the orbital angular momentum of
rotating beam comprising of modes with opposite azimuthal indices corresponds
to that of rigid rotation. A short review on the electromagnetism in
noninertial systems is presented to motivate a fully covariant Maxwell field
approach in rotating system to address the rotating light phenomenon.Comment: No figure
Relativity and EPR Entanglement: Comments
Recent experiment by Zhinden et al (Phys. Rev {\bf A} 63 02111, 2001)
purports to test compatibility between relativity and quantum mechanics in the
classic EPR setting. We argue that relativity has no role in the EPR argument
based solely on non-relativistic quantum formalism. It is suggested that this
interesting experiment may have significance to address fundamental questions
on quantum probability.Comment: 6 pages, no figure; Submitted to Phys. Rev.
Frontier Orbital and Conformational Analysis of a Nematic Liquid Crystal: 4-n-heptyloxy-4´-cyanobiphenyl
687-689The 4-n-heptyloxy-4´-cyanobiphenyl (7OCB) is a member of 4-n-alkoxy-4´-cyanobiphenyl homologous series. This liquid crystal series is well known for its electro-optical properties. The 7OCB compound transforms from crystal to nematic at 53.5°C and nematic to isotropic phase at 75.0°C. Using Hartree-Fock and DFT methods with 6-31G(d,p) basis set, molecular geometry, conformational analysis, electronic structure, MEP, HOMO-LUMO surfaces and molecular parameters of 7OCB molecular system have been investigated. Efforts have been made to elucidate the physico-chemical properties of the 7OCB liquid crystal
Controlling the Ultrafast Dynamics of HD+ by the Carrier-Envelope Phases of an Ultrashort Laser Pulse: A Quasi-Classical Dynamics Study
A theoretical study on the coupled electron-nuclear dynamics of HD+ molecular ions under ultrashort, intense laser pulses is performed by employing a well-established quasi-classical model. The influence of the laser carrier-envelope phase on various channel (H + D+, D + H+, and H+ + D+) probabilities is investigated at different laser field intensities. The carrier-envelope phase is found to govern the dissociation (H + D+ and D + H+) and Coulomb explosion (H+ + D+) channel probabilities. The kinetic energy release distributions of the fragments are also found to be sensitive to the carrier-envelope phase of the laser pulse. Our results are in agreement with the previously reported quantum dynamics studies and experiments
Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves
We describe a simple method of including dissipation in the spin wave band
structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz
equation for the magnetization with the Gilbert damping term. We use this
approach to calculate the band structure of square and triangular arrays of Ni
nanocylinders embedded in an Fe host. The results show that there are certain
bands and special directions in the Brillouin zone where the spin wave lifetime
is increased by more than an order of magnitude above its average value. Thus,
it may be possible to generate spin waves in such composites decay especially
slowly, and propagate especially large distances, for certain frequencies and
directions in -space.Comment: 13 pages, 4 figures, submitted to Phys Rev
Stochastic Lag Time in Nucleated Linear Self-Assembly
Protein aggregation is of great importance in biology, e.g., in amyloid
fibrillation. The aggregation processes that occur at the cellular scale must
be highly stochastic in nature because of the statistical number fluctuations
that arise on account of the small system size at the cellular scale. We study
the nucleated reversible self-assembly of monomeric building blocks into
polymer-like aggregates using the method of kinetic Monte Carlo. Kinetic Monte
Carlo, being inherently stochastic, allows us to study the impact of
fluctuations on the polymerisation reactions. One of the most important
characteristic features in this kind of problem is the existence of a lag phase
before self-assembly takes off, which is what we focus attention on. We study
the associated lag time as a function of the system size and kinetic pathway.
We find that the leading order stochastic contribution to the lag time before
polymerisation commences is inversely proportional to the system volume for
large-enough system size for all nine reaction pathways tested. Finite-size
corrections to this do depend on the kinetic pathway
Relativistic anisotropic charged fluid spheres with varying cosmological constant
Static spherically symmetric anisotropic source has been studied for the
Einstein-Maxwell field equations assuming the erstwhile cosmological constant to be a space-variable scalar, viz., . Two
cases have been examined out of which one reduces to isotropic sphere. The
solutions thus obtained are shown to be electromagnetic in origin as a
particular case. It is also shown that the generally used pure charge
condition, viz., is not always required for constructing
electromagnetic mass models.Comment: 15 pages, 3 eps figure
Diffusion in bcc Transition Metals
Diffusion in metals and alloys is a defect contro-lled process. It occurs as a result of solvent (solute) atom-vacency exchange in the matrix. Diffusion can also occur along short circuiting paths like dislocations or grain boundaries. In all these cases the temperature dep-endence of diffusivity follows an Arrhanious relationship i.e.log D vs l/t plot is linear. The diffusivity is writt-en as D=Do exp(-Q/rt) where D0 and Q are frequency factor and activation energy for diffusion
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