199 research outputs found
Effects of ac-field amplitude on the dielectric susceptibility of relaxors
The thermally activated flips of the local spontaneous polarization in
relaxors were simulated to investigate the effects of the applied-ac-field
amplitude on the dielectric susceptibility. It was observed that the
susceptibility increases with increasing the amplitude at low temperatures. At
high temperatures, the susceptibility experiences a plateau and then drops. The
maximum in the temperature dependence of susceptibility shifts to lower
temperatures when the amplitude increases. A similarity was found between the
effects of the amplitude and frequency on the susceptibility.Comment: 8 pages, 7 figures, Phys. Rev. B (in July 1st
Interplay between static and dynamic polar correlations in relaxor Pb(Mg_{1/3}Nb_{2/3})O_{3}
We have characterized the dynamics of the polar nanoregions in
Pb(MgNb)O (PMN) through high-resolution neutron
backscattering and spin-echo measurements of the diffuse scattering cross
section. We find that the diffuse scattering intensity consists of \emph{both}
static and dynamic components. The static component first appears at the Curie
temperature K, while the dynamic component freezes completely
at the temperature T K; together, these components account for
all of the observed spectral weight contributing to the diffuse scattering
cross section. The integrated intensity of the dynamic component peaks near the
temperature at which the frequency-dependent dielectric constant reaches a
maximum (T) when measured at 1 GHz, i. e. on a timescale of
ns. Our neutron scattering results can thus be directly related to dielectric
and infra-red measurements of the polar nanoregions. Finally, the global
temperature dependence of the diffuse scattering can be understood in terms of
just two temperature scales, which is consistent with random field models.Comment: (8 pages, 5 figures, submitted to Phys. Rev. B
Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3
Neutron and x-ray diffraction techniques have been used to study the
competing long and short-range polar order in the relaxor ferroelectric
Pb(MgNb)O (PMN) under a [111] applied electric field.
Despite reports of a structural transition from a cubic phase to a rhombohedral
phase for fields E 1.7 kV/cm, we find that the bulk unit cell remains cubic
(within a sensitivity of 90- =0.03)for fields up to
8 kV/cm. Furthermore, we observe a structural transition confined to the near
surface volume or `skin' of the crystal where the cubic cell is transformed to
a rhombohedral unit cell at T=210 K for E 4 kV/cm, for which
90-=0.08 0.03 below 50 K. While the bulk unit
cell remains cubic, a suppression of the diffuse scattering and concomitant
enhancement of the Bragg peak intensity is observed below T=210 K,
indicating a more ordered structure with increasing electric field yet an
absence of a long-range ferroelectric ground state in the bulk. The electric
field strength has little effect on the diffuse scattering above T,
however below T the diffuse scattering is reduced in intensity and adopts
an asymmetric lineshape in reciprocal space. The absence of hysteresis in our
neutron measurements (on the bulk) and the presence of two distinct temperature
scales suggests that the ground state of PMN is not a frozen glassy phase as
suggested by some theories but is better understood in terms of random fields
introduced through the presence of structural disorder. Based on these results,
we also suggest that PMN represents an extreme example of the two-length scale
problem, and that the presence of a distinct skin maybe necessary for a relaxor
ground state.Comment: 12 pages, 9 figure
Transport and cooling of singly-charged noble gas ion beams
The transport and cooling of noble gas singly-charged ion beams by means of a
Radio Frequency Quadrupole Cooler Buncher (RFQCB) have been studied at the
LIMBE low energy beam line of the GANIL facility. Ions as light as
have been cooled and stored before their extraction in bunches using as
buffer gas. Bunches characteristics have been studied as a function of the
parameters of the device. Sizeable transmissions of up to 10 have been
obtained. A detailed study of the lifetime of ions inside the buncher has been
performed giving an estimate of the charge exchange cross-section. Results of a
microscopic Monte-Carlo transport code show reasonable agreement with
experimental data.Comment: 13 figure
Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping
We use neutron scattering to characterize the acoustic phonons in the relaxor
PMN and demonstrate the presence of an anisotropic damping mechanism directly
related to short-range, polar correlations. For a large range of temperatures
above Tc ~ 210, K, where dynamic polar correlations exist, acoustic phonons
propagating along [1\bar{1}0] and polarized along [110] (TA2 phonons) are
overdamped and softened across most of the Brillouin zone. By contrast,
acoustic phonons propagating along [100] and polarized along [001] (TA1
phonons) are overdamped and softened for only a limited range of wavevectors.
The anisotropy and temperature dependence of the acoustic phonon energy
linewidth are directly correlated with the elastic diffuse scattering,
indicating that polar nanoregions are the cause of the anomalous behavior. The
damping and softening vanish for q -> 0, i.e. for long-wavelength acoustic
phonons, which supports the notion that the anomalous damping is a result of
the coupling between the relaxational component of the diffuse scattering and
the harmonic TA phonons. Therefore, these effects are not due to large changes
in the elastic constants with temperature because the elastic constants
correspond to the long-wavelength limit. We compare the elastic constants we
measure to those from Brillouin scattering and to values reported for pure PT.
We show that while the values of C44 are quite similar, those for C11 and C12
are significantly less in PMN and result in a softening of (C11-C12) over PT.
There is also an increased elastic anisotropy (2C44/(C11-C12)) versus that in
PT. These results suggest an instability to TA2 acoustic fluctuations in
relaxors. We discuss our results in the context of the debate over the
"waterfall" effect and show that they are inconsistent with TA-TO phonon
coupling or other models that invoke the presence of a second optic mode.Comment: (21 pages, 16 figures, to be published in Physical Review B
Low temperature superlattice in monoclinic PZT
TEM has shown that the strongly piezoelectric material Pb(Zr0.52Ti0.48)O3
separates into two phases at low temperatures. The majority phase is the
monoclinic phase previously found by x-ray diffraction. The minority phase,
with a nanoscale coherence length, is a slightly distorted variant of the first
resulting from the anti-phase rotation of the oxygen octahedra about [111].
This work clears up a recent controversy about the origin of superlattice peaks
in these materials, and supports recent theoretical results predicting the
coexistence of ferroelectric and rotational instabilities.Comment: REVTeX4, 4 eps figures embedded. JPG version of figs. 2&4 is also
include
A Universal Phase Diagram for PMN-xPT and PZN-xPT
The phase diagram of the Pb(Mg1/3Nb2/3)O3 and PbTiO3 solid solution (PMN-xPT)
indicates a rhombohedral ground state for x < 0.32. X-ray powder measurements
by Dkhil et al. show a rhombohedrally split (222) Bragg peak for PMN-10%PT at
80 K. Remarkably, neutron data taken on a single crystal of the same compound
with comparable q-resolution reveal a single resolution-limited (111) peak down
to 50 K, and thus no rhombohedral distortion. Our results suggest that the
structure of the outer layer of these relaxors differs from that of the bulk,
which is nearly cubic, as observed in PZN by Xu et al.Comment: Replaced Fig. 3 with better versio
Defect-induced condensation and central peak at elastic phase transitions
Static and dynamical properties of elastic phase transitions under the
influence of short--range defects, which locally increase the transition
temperature, are investigated. Our approach is based on a Ginzburg--Landau
theory for three--dimensional crystals with one--, two-- or three--dimensional
soft sectors, respectively. Systems with a finite concentration of
quenched, randomly placed defects display a phase transition at a temperature
, which can be considerably above the transition temperature
of the pure system. The phonon correlation function is calculated in
single--site approximation. For a dynamical central peak
appears; upon approaching , its height diverges and its width
vanishes. Using an appropriate self--consistent method, we calculate the
spatially inhomogeneous order parameter, the free energy and the specific heat,
as well as the dynamical correlation function in the ordered phase. The
dynamical central peak disappears again as the temperatur is lowered below
. The inhomogeneous order parameter causes a static central
peak in the scattering cross section, with a finite width depending on the
orientation of the external wave vector relative to the soft sector.
The jump in the specific heat at the transition temperatur of the pure system
is smeared out by the influence of the defects, leading to a distinct maximum
instead. In addition, there emerges a tiny discontinuity of the specific heat
at . We also discuss the range of validity of the mean--field
approach, and provide a more realistic estimate for the transition temperature.Comment: 11 pages, 11 ps-figures, to appear in PR
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