202 research outputs found
Universal Properties of Ferroelectric Domains
Basing on Ginzburg-Landau approach we generalize the Kittel theory and derive
the interpolation formula for the temperature evolution of a multi-domain
polarization profile P(x,z). We resolve the long-standing problem of the
near-surface polarization behavior in ferroelectric domains and demonstrate the
polarization vanishing instead of usually assumed fractal domain branching. We
propose an effective scaling approach to compare the properties of different
domain-containing ferroelectric plates and films.Comment: Phys. Rev. Lett. to be publishe
Landau model for the phase diagrams of the orthorhombic rare-earth manganites RMnO3
The present work aims to describe, within a single phenomenological approach,
the specific sequence of phase transitions observed in the rare-earth
manganites RMnO3 at zero magnetic field. It is shown that a single integrated
description of the temperature versus composition phase diagrams of these
compounds and related solid solutions can be obtained within the scope of
Landau theory by adopting the so called type-II description of the modulated
phases.Comment: 37 pages, 7 figures, 4 table
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
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
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 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
Shaping potential landscape for organic polariton condensates in double-dye cavities
We investigate active spatial control of polariton condensates independently
of the polariton-, gain-inducing excitation profile. This is achieved by
introducing an extra intracavity semiconductor layer, non-resonant to the
cavity mode. Saturation of the optical absorption in the uncoupled layer
enables the ultra-fast modulation of the effective refractive index and,
through excited-state absorption, the polariton dissipation. Utilising these
mechanisms, we demonstrate control over the spatial profile and density of a
polariton condensate at room temperature
Towards a microscopic theory of toroidal moments in bulk periodic crystals
We present a theoretical analysis of magnetic toroidal moments in periodic
systems, in the limit in which the toroidal moments are caused by a time and
space reversal symmetry breaking arrangement of localized magnetic dipole
moments. We summarize the basic definitions for finite systems and address the
question of how to generalize these definitions to the bulk periodic case. We
define the toroidization as the toroidal moment per unit cell volume, and we
show that periodic boundary conditions lead to a multivaluedness of the
toroidization, which suggests that only differences in toroidization are
meaningful observable quantities. Our analysis bears strong analogy to the
modern theory of electric polarization in bulk periodic systems, but we also
point out some important differences between the two cases. We then discuss the
instructive example of a one-dimensional chain of magnetic moments, and we show
how to properly calculate changes of the toroidization for this system.
Finally, we evaluate and discuss the toroidization (in the local dipole limit)
of four important example materials: BaNiF_4, LiCoPO_4, GaFeO_3, and BiFeO_3.Comment: replaced with final (published) version, which includes some changes
in the text to improve the clarity of presentatio
Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO
Neutron scattering studies of single-crystal LiNiPO reveal a spontaneous
first-order commensurate-incommensurate magnetic phase transition. Short- and
long-range incommensurate phases are intermediate between the high temperature
paramagnetic and the low temperature antiferromagnetic phases. The modulated
structure has a predominant antiferromagnetic component, giving rise to
satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg
reflection, and a ferromagnetic component giving rise to peaks at small
momentum-transfers around the origin at . The wavelength of the
modulated magnetic structure varies continuously with temperature. It is argued
that the incommensurate short- and long-range phases are due to
spin-dimensionality crossover from a continuous to the discrete Ising state.
These observations explain the anomalous first-order transition seen in the
magnetoelectric effect of this system
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