39,815 research outputs found
Defects and Lattice Instability in Doped Lead-Based Perovskite Antiferroelectrics: Revisited
This paper is a summary of earlier results that have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZrO3 single crystals doped with niobium and Pb(Zr0.70Ti0.30)O3 ceramics doped with different concentration of Bi2O3. It was found that these crystals undergo new phase transitions never observed before. To investigate all phase transitions, different experimental methods were used to characterize the crystal properties. Temperature and time dependencies have been tentatively measured in a wide range, including a region above Tc, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices. Also, coexistence of antiferroelectric phase and one of the intermediate phases could be observed in a wide temperature range. The phase transition mechanism in PbZrO3 is discussed, taking into account the local breaking of the crystal symmetry above Tc and the defects of crystal lattices, i.e., those generated during crystal growth, and intentionally introduced by preheating in a vacuum or doping with hetero-valent dopant
Total Chiral Symmetry Breaking during Crystallization: Who needs a "Mother Crystal"?
Processes that can produce states of broken chiral symmetry are of particular
interest to physics, chemistry and biology. Chiral symmetry breaking during
crystallization of sodium chlorate occurs via the production of secondary
crystals of the same handedness from a single "mother crystal" that seeds the
solution. Here we report that a large and "symmetric" population of D- and
L-crystals moves into complete chiral purity disappearing one of the
enantiomers. This result shows: (i) a new symmetry breaking process
incompatible with the hypothesis of a single "mother crystal"; (ii) that
complete symmetry breaking and chiral purity can be achieved from an initial
system with both enantiomers. These findings demand a new explanation to the
process of total symmetry breaking in crystallization without the intervention
of a "mother crystal" and open the debate on this fascinating phenomenon. We
present arguments to show that our experimental data can been explained with a
new model of "complete chiral purity induced by nonlinear autocatalysis and
recycling".Comment: 5 pages, 4 figures, Added reference
Chiral symmetry breaking via crystallization of the glycine and \alpha-amino acid system: a mathematical model
We introduce and numerically solve a mathematical model of the experimentally
established mechanisms responsible for the symmetry breaking transition
observed in the chiral crystallization experiments reported by I. Weissbuch, L.
Addadi, L. Leiserowitz and M. Lahav, J. Am. Chem. Soc. 110 (1988), 561-567. The
mathematical model is based on five basic processes: (1) The formation of
achiral glycine clusters in solution, (2) The nucleation of oriented glycine
crystals at the air/water interface in the presence of hydrophobic amino acids,
(3) A kinetic orienting effect which inhibits crystal growth, (4) The
enantioselective occlusion of the amino acids from solution, and (5) The growth
of oriented host glycine crystals at the interface. We translate these
processes into differential rate equations. We first study the model with the
orienting process (2) without (3) and then combine both allowing us to make
detailed comparisons of both orienting effects which actually act in unison in
the experiment. Numerical results indicate that the model can yield a high
percentage orientation of the mixed crystals at the interface and the
consequent resolution of the initially racemic mixture of amino acids in
solution. The model thus leads to separation of enantiomeric territories, the
generation and amplification of optical activity by enantioselective occlusion
of chiral additives through chiral surfaces of glycine crystals
An Investigation of Orientational Symmetry-Breaking Mechanisms in High Landau Levels
The principal axes of the recently discovered anisotropic phases of 2D
electron systems at high Landau level occupancy are consistently oriented
relative to the crystal axes of the host semiconductor. The nature of the
native rotational symmetry breaking field responsible for this preferential
orientation remains unknown. Here we report on experiments designed to
investigate the origin and magnitude of this symmetry breaking field. Our
results suggest that neither micron-scale surface roughness features nor the
precise symmetry of the quantum well potential confining the 2D system are
important factors. By combining tilted field transport measurements with
detailed self-consistent calculations we estimate that the native anisotropy
energy, whatever its origin, is typically ~ 1 mK per electron.Comment: Reference added, minor notational changes; final published versio
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