837 research outputs found
On orientational relief of inter-molecular potential and the structure of domain walls in fullerite C60
A simple planar model for an orientational ordering of threefold molecules on
a triangular lattice modelling a close-packed (111) plane of fullerite is
considered. The system has 3-sublattice ordered ground state which includes 3
different molecular orientations. There exist 6 kinds of orientational domains,
which are related with a permutation or a mirror symmetry. Interdomain walls
are found to be rather narrow.
The model molecules have two-well orientational potential profiles, which are
slightly effected by a presence of a straight domain wall. The reason is a
stronger correlation between neighbour molecules in triangular lattice versus
previously considered square lattice
A considerable reduction (up to one order) of orientational interwell
potential barrier is found in the core regions of essentially two-dimentional
potential defects, such as a three-domain boundary or a kink in the domain
wall. For ultimately uncorrelated nearest neighbours the height of the
interwell barrier can be reduced even by a factor of 100.Comment: 11 pages, 13 figures, LaTeX, to appear in Low Temperature Physic
Low-temperature orientational order and possible domain structures in C(_{60}) fullerite
Based on a simple model for ordering of hexagons on square planar lattice, an
attempt has been made to consider possible structure of C(_{60}) fullerite in
its low temperature phase. It is shown that hexagons, imitating fullerens
oriented along (C_{3}) axes of \emph{sc} lattice, can be ordered into an ideal
structure with four non-equivalent molecules in unit cell. Then the energy
degeneracy for each hexagon rotations by (\pi /3) around its (C_{3}) axis
leaves the translational and orientational order in this structure, but leads
to a random distribution of (\pi /3) rotations and hence to {}``averaged{}''
unit cell with two molecules. However the most relevant structural defects are
not these intrinsic \char`\"{}misorientations\char`\"{} but certain walls
between the domains with different sequencies of the above-mentioned two
(non-ideal) sublattices. Numeric estimates have been made for the anisotropic
inter-molecular potential showing that the anisotropy is noticeably smaller for
molecules in walls than in domains
Efimov states in atom-molecular collisions
We analyse scattering of a heavy atom off a weakly bound molecule comprising
an identical heavy and a light atom in the Born-Oppenheimer approximation. We
focus on the situation where the heavy atoms are bosons, which was realized in
several experiments. The elastic and inelastic cross sections for the
atom-molecular scattering exhibit a series of resonances corresponding to
three-body Efimov states. Resonances in elastic collisions are accessible
experimentally through thermalization rates, and thus constitute an alternative
way of observing Efimov states.Comment: 4 pages, 2 figure
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