161 research outputs found
Origin of second-harmonic generation in the incommensurate phase of K2SeO4
We show that a ferroelectric phase transition takes place in the
incommensurate phase of the K2SeO4 crystal. The ferroelectric character of the
IC phase explains the second-harmonic generation observed in the corresponding
temperature range.Comment: 5 pages, 1 figur
Comment on "On the -Anomaly in Betaine Calcium Chloride Dihydrate"
Recently, Hlinka and Ishibashi [J. Phys. Soc. Jpn. 67, 495 (1998)] discussed
the -anomaly in betaine calcium chloride dihydrate (BCCD) in a Landau-type
approach. We comment on the shortcomings of this approach and discuss the
-anomaly in the framework of a microscopical pseudo spin model based on a
realistic description of BCCD in terms of symmetry-adapted local modes.Comment: 2 pages, RevTex, submitted to J. Phys. Soc. Jp
Landau Model for Commensurate-Commensurate Phase Transitions in Uniaxial Improper Ferroelectric Crystals
We propose the Landau model for lock-in phase transitions in uniaxially
modulated improper ferroelectric incommensurate-commensurate systems of class
I. It includes Umklapp terms of third and fourth order and secondary order
parameter representing the local polarization. The corresponding phase diagram
has the structure of harmless staircase, with the allowed wave numbers obeying
the Farey tree algorithm. Among the stable commensurate phases only those with
periods equal to odd number of lattice constants have finite macroscopic
polarizations. These results are in excellent agreement with experimental
findings in some A2BX4 compounds.Comment: 9 pages, 5 figures, revtex, to be published in Journal of Physics:
Cond. Matter as a Letter to the Edito
Collective modes in uniaxial incommensurate-commensurate systems with the real order parameter
The basic Landau model for uniaxial systems of the II class is nonintegrable,
and allows for various stable and metastable periodic configurations, beside
that representing the uniform (or dimerized) ordering. In the present paper we
complete the analysis of this model by performing the second order variational
procedure, and formulating the combined Floquet-Bloch approach to the ensuing
nonstandard linear eigenvalue problem. This approach enables an analytic
derivation of some general conclusions on the stability of particular states,
and on the nature of accompanied collective excitations. Furthermore, we
calculate numerically the spectra of collective modes for all states
participating in the phase diagram, and analyze critical properties of
Goldstone modes at all second order and first order transitions between
disordered, uniform and periodic states. In particular it is shown that the
Goldstone mode softens as the underlying soliton lattice becomes more and more
dilute.Comment: 19 pages, 16 figures, REVTeX, to be published in Journal of Physics
A: Mathematical and Genera
Landau model for uniaxial systems with complex order parameter
We study the Landau model for uniaxial incommensurate-commensurate systems of
the I class by keeping Umklapp terms of third and fourth order in the expansion
of the free energy. It applies to systems in which the soft mode minimum lies
between the corresponding commensurate wave numbers. The minimization of the
Landau functional leads to the sine-Gordon equation with two nonlinear terms,
equivalent to the equation of motion for the well-known classical mechanical
problem of two mixing resonances. We calculate the average free energies for
periodic, quasiperiodic and chaotic solutions of this equation, and show that
in the regime of finite strengths of Umklapp terms only periodic solutions are
absolute minima of the free energy, so that the phase diagram contains only
commensurate configurations. The phase transitions between neighboring
configurations are of the first order, and the wave number of ordering goes
through harmless staircase with a finite number of steps. These results are the
basis for the interpretation of phase diagrams for some materials from the I
class of incommensurate-commensurate systems, in particular of those for
ABX and BCCD compounds. Also, we argue that chaotic barriers which
separate metastable periodic solutions represent an intrinsic mechanism for
observed memory effects and thermal hystereses.Comment: 12 pages, 14 figures, LaTeX, to be published in Phys. Rev.
Universal mechanism of discontinuity of commensurate-incommensurate transitions in three-dimensional solids: Strain dependence of soliton self-energy
We show that there exists a universal mechanism of long-range soliton
attraction in three-dimensional solids and, therefore, of discontinuity of any
commensurate-incommensurate (C-IC) phase transition. This mechanism is due to
the strain dependence of the soliton self-energy and specific features of the
solid-state elasticity. The role of this mechanism is studied in detail for a
class of C-IC transitions where the IC modulation is one-dimensional, the
anisotropy in the order parameter space is small, and the symmetry of the
systems allows the existence of the Lifshitz invariant. Two other mechanisms of
soliton attraction are operative here but the universal mechanism considered in
this paper is found to be the most important one in some cases. Comparison with
the most extensively studied C-IC transition in shows that the
experimentally observed thermal anomalies can be understood as a result of the
smearing of the theoretically predicted discontinuous transition.Comment: 8 pages (extended version, title changed
Diversity-Oriented Synthesis of Novel Trihalomethyl-Containing Spirochromeno[3,4- a ](thia)pyrrolizidines and Spirochromeno-[3,4- a ]indolizidines by One-Pot, Three-Component [3+2]-Cyclo addition Reaction
Regio- and stereoselective methods for the synthesis of 6′-trifluoro(trichloro)methyl substituted spiro[acenaphthylene-1,11′-chromeno[3,4- a ](thia)pyrrolizidin]-2-ones and spiro[acenaphthylene-1,12′-chromeno[3,4- a ]indolizidin]-2-ones have been developed based on the three-component reaction of 3-nitro-2-trifluoro(trichloro)methyl-2 H -chromenes with azomethine ylides generated in situ from acenaphthenequinone and cyclic α-amino acids. The cycloaddition proceeds under mild conditions in ethanol or DMSO, and only endo -isomers of the products with cis -arrangement of nitro and trifluoromethyl groups are formed. The relative configuration of cycloadducts is reliably confirmed by X-ray diffraction analysis and by 2D NOESY spectroscopy. © 2021 Georg Thieme Verlag. All rights reserved.The work was financially supported by the Russian Foundation for Basic Research (grant 20-03-00716) and by the Ministry of Science and Higher Education of the Russian Federation (project FEUZ-2020-0052)
DOPING NANOSTRUCTURED ALUMINA BY MANGANESE AND INVESTIGATION OF ITS LUMINESCENT PROPERTIES
The nanostructured aluminium oxide doped manganese are synthesized at different annealing temperature and concentration of impurity. Pulse cathodoluminescence of obtained samples are measured. An increase in the concentration of dopant causes quenching of the luminescence of chromium ions
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