796 research outputs found
Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution
The presence of superlattice reflections and detailed analyses of the powder
neutron and x-ray diffraction data reveal that La rich
(BF-LF)-(PT) (BF-LF-PT) has ferroelectric
rhombohedral crystal structure with space group \textit{} at ambient
conditions. The temperature dependence of lattice parameters, tilt angle,
calculated polarization , volume, and integrated intensity of
superlattice and magnetic reflections show an anomaly around 170 K. Impedance
spectroscopy, dielectric and ac conductivity measurements were performed in
temperature range to probe the origin of large remnant
polarization and frequency dependent broad transitions with large dielectric
constant near . Results of impedance spectroscopy measurements
clearly show contributions of both grain and grain boundaries throughout the
frequency range ( Hz Hz). It could be concluded
that the grain boundaries are more resistive and capacitive as compared to the
grains, resulting in inhomogeneities in the sample causing broad frequency
dependent dielectric anomalies. Enhancement in dielectric constant and remnant
polarization values are possibly due to space charge polarization caused by
piling of charges at the interface of grains and grain boundaries. The
imaginary parts of dielectric constant () Vs frequency
data were fitted using Maxwell-Wagner model at K) and model
fits very well with the data up to Hz. Magnetodielectric measurements
prove that the sample starts exhibiting magnetoelectric coupling at
K, which is also validated by neutron diffraction data.Comment: 20 pages, 10 figure
Ultracold heteronuclear molecules and ferroelectric superfluids
We analyze the possibility of a ferroelectric transition in heteronuclear
molecules consisting of Bose-Bose, Bose-Fermi or Fermi-Fermi atom pairs. This
transition is characterized by the appearance of a spontaneous electric
polarization below a critical temperature. We discuss the existence of a
ferroelectric Fermi liquid phase for Fermi molecules and the existence of a
ferroelectric superfluid phase for Bose molecules characterized by the
coexistence of ferroelectric and superfluid orders. Lastly, we propose an
experiment to detect ferroelectric correlations through the observation of
coherent dipole radiation pulses during time of flight.Comment: 4 pages and 3 figure
Generarized Cubic Model for BaTiO-like Ferroelectric Substance
We propose an order-disorder type microscopic model for BaTiO-like
Ferroelectric Substance. Our model has three phase transitions and four phases.
The symmetry and directions of the polarizations of the ordered phases agree
with the experimental results of BaTiO. The intermediate phases in our
model are known as an incompletely ordered phase, which appears in a
generalized clock model.Comment: 6 pages, 4figure
Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films
The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO),
PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via
first-principles computational methods. The effects of epitaxial strain on the
atomic structure, ferroelectric response, barrier to coherent domain reversal,
domain-wall energy, and barrier to domain-wall translation are studied. It is
found that PTO has a larger polarization, but smaller energy barrier to domain
reversal, than PZO. Consequentially the idealized coercive field is over two
times smaller in PTO than PZO. The Ti--O bond length is more sensitive to
strain than the other bonds in the crystals. This results in the polarization
and domain-wall energy in PTO having greater sensitivity to strain than in PZO.
Two ordered phases of PZT are considered, the rock-salt structure and a (100)
PTO/PZO superlattice. In these simple structures we find that the ferroelectric
properties do not obey Vergard's law, but instead can be approximated as an
average over individual 5-atom unit cells.Comment: 9 pages, 13 figure
Comment on "On the importance of the free energy for elasticity under pressure"
Marcus et al. (Marcus P, Ma H and Qiu S L 2002 J. Phys.: Condens. Matter 14
L525) claim that thermodynamic properties of materials under pressure must be
computed using the Gibbs free energy , rather than the internal energy .
Marcus et al. state that ``The minima of , but not of , give the
equilibrium structure; the second derivatives of , but not of , with
respect to strains at the equilibrium structure give the equilibrium elastic
constants.'' Both statements are incorrect.Comment: Commen
Vortex-to-Polarization Phase Transformation Path in Pb(ZrTi)O Nanoparticles
Phase transformation in finite-size ferroelectrics is of fundamental
relevance for understanding collective behaviors and balance of competing
interactions in low-dimensional systems. We report a first-principles effective
Hamiltonian study of vortex-to-polarization transformation in
Pb(ZrTi)O nanoparticles, caused by homogeneous electric
fields normal to the vortex plane. The transformation is shown to (1) follow an
unusual {\it macroscopic} path that is symmetry non-conforming and
characterized by the occurrence of a previously unknown structure as the
bridging phase; (2) lead to the discovery of a striking collective phenomenon,
revealing how ferroelectric vortex is annihilated {\it microscopically}.
Interactions underlying these behaviors are discussed
Chaos in effective classical and quantum dynamics
We investigate the dynamics of classical and quantum N-component phi^4
oscillators in the presence of an external field. In the large N limit the
effective dynamics is described by two-degree-of-freedom classical Hamiltonian
systems. In the classical model we observe chaotic orbits for any value of the
external field, while in the quantum case chaos is strongly suppressed. A
simple explanation of this behaviour is found in the change in the structure of
the orbits induced by quantum corrections. Consistently with Heisenberg's
principle, quantum fluctuations are forced away from zero, removing in the
effective quantum dynamics a hyperbolic fixed point that is a major source of
chaos in the classical model.Comment: 6 pages, RevTeX, 5 figures, uses psfig, changed indroduction and
conclusions, added reference
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