15,905 research outputs found
New theory of the gamma-alpha phase transition in Ce: quadrupolar ordering
We present a theoretical model of the "isostructural" \gamma-\alpha phase
transition in Ce which is based on quadrupolar interactions due to coupled
charge density fluctuations of 4f electrons and of conduction electrons.
Conduction electrons are treated in tight-binding approximation. The
\gamma-\alpha transition is described as an orientational ordering of
quadrupolar electronic densities in a Pa3 structure. The quadrupolar order of
the conduction electron densities is complementary to the quadrupolar order of
4f electron densities. The inclusion of conduction electrons leads to an
increase of the lattice contraction at the \gamma-\alpha transition in
comparison to the sole effect of 4f electrons. We calculate the Bragg
scattering law and suggest synchrotron radiation experiments in order to check
the Pa3 structure. The theory is capable of accounting for transitions to
phases of non-cubic symmetry, but it is not sufficient to describe the magnetic
phenomena which we ascribe to the Kondo mechanism. We also present a
microscopic derivation of multipolar interactions and discuss the crystal field
of \gamma-Ce.Comment: 19 pages, 4 figures, full version to be published in Eur. Phys. J.
Parallelization of implicit finite difference schemes in computational fluid dynamics
Implicit finite difference schemes are often the preferred numerical schemes in computational fluid dynamics, requiring less stringent stability bounds than the explicit schemes. Each iteration in an implicit scheme involves global data dependencies in the form of second and higher order recurrences. Efficient parallel implementations of such iterative methods are considerably more difficult and non-intuitive. The parallelization of the implicit schemes that are used for solving the Euler and the thin layer Navier-Stokes equations and that require inversions of large linear systems in the form of block tri-diagonal and/or block penta-diagonal matrices is discussed. Three-dimensional cases are emphasized and schemes that minimize the total execution time are presented. Partitioning and scheduling schemes for alleviating the effects of the global data dependencies are described. An analysis of the communication and the computation aspects of these methods is presented. The effect of the boundary conditions on the parallel schemes is also discussed
Crystal Structures of Polymerized Fullerides AC60, A=K, Rb, Cs and Alkali-mediated Interactions
Starting from a model of rigid interacting C60 polymer chains on an
orthorhombic lattice, we study the mutual orientation of the chains and the
stability of the crystalline structures Pmnn and I2/m. We take into account i)
van der Waals interactions and electric quadrupole interactions between C60
monomers on different chains as well as ii) interactions of the monomers with
the surrounding alkali atoms. The direct interactions i) always lead to an
antiferrorotational structure Pmnn with alternate orientation of the C60 chains
in planes (001). The interactions ii) with the alkalis consist of two parts:
translation-rotation (TR) coupling where the orientations of the chains
interact with displacements of the alkalis, and quadrupolar electronic
polarizability (ep) coupling, where the electric quadrupoles on the C60
monomers interact with induced quadrupoles due to excited electronic d states
of the alkalis. Both interactions ii) lead to an effective
orientation-orientation interaction between the C60 chains and always favor the
ferrorotational structure I2/m where C60 chains have a same orientation. The
structures Pmnn for KC60 and I2/m for Rb- and CsC60 are the result of a
competition between the direct interaction i) and the alkali-mediated
interactions ii). In Rb- and CsC60 the latter are found to be dominant, the
preponderant role being played by the quadrupolar electronic polarizability of
the alkali ions.Comment: J.Chem.Phys., in press, 14 pages, 3 figures, 8 table
Inert states of spin-S systems
We present a simple but efficient geometrical method for determining the
inert states of spin-S systems. It can be used if the system is described by a
spin vector of a spin-S particle and its energy is invariant in spin rotations
and phase changes. Our method is applicable to an arbitrary S and it is based
on the representation of a pure spin state of a spin-S particle in terms of 2S
points on the surface of a sphere. We use this method to find candidates for
some of the ground states of spinor Bose-Einstein condensates.Comment: 4 pages, 2 figures, minor changes, references added, typos correcte
Induced polarization and electronic properties of carbon doped boron-nitride nanoribbons
The electronic properties of boron-nitride nanoribbons (BNNRs) doped with a
line of carbon atoms are investigated by using density functional calculations.
Three different configurations are possible: the carbon atoms may replace a
line of boron or nitrogen atoms or a line of alternating B and N atoms which
results in very different electronic properties. We found that: i) the NCB
arrangement is strongly polarized with a large dipole moment having an
unexpected direction, ii) the BCB and NCN arrangement are non-polar with zero
dipole moment, iii) the doping by a carbon line reduces the band gap
independent of the local arrangement of boron and nitrogen around the carbon
line, iv) an electric field parallel to the carbon line polarizes the BN sheet
and is found to be sensitive to the presence of carbon dopants, and v) the
energy gap between the highest occupied molecular orbital and the lowest
unoccupied molecular orbital decreases linearly with increasing applied
electric field directed parallel to the carbon line. We show that the
polarization and energy gap of carbon doped BNNRs can be tuned by an electric
field applied parallel along the carbon line.Comment: 11 pages, 6 figure
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