24,781 research outputs found
Structural phase transition and dielectric relaxation in Pb(Zn1/3Nb2/3)O3 single crystals
The structure and the dielectric properties of Pb(Zn1/3Nb2/3)O3 (PZN) crystal
have been investigated by means of high-resolution synchrotron x-ray
diffraction (with an x-ray energy of 32 keV) and dielectric spectroscopy (in
the frequency range of 100 Hz - 1 MHz). At high temperatures, the PZN crystal
exhibits a cubic symmetry and polar nanoregions inherent to relaxor
ferroelectrics are present, as evidenced by the single (222) Bragg peak and by
the noticeable tails at the basis of the peak. At low temperatures, in addition
to the well-known rhombohedral phase, another low-symmetry, probably
ferroelectric, phase is found. The two phases coexist in the form of mesoscopic
domains. The para- to ferroelectric phase transition is diffused and observed
between 325 and 390 K, where the concentration of the low-temperature phases
gradually increases and the cubic phase disappears upon cooling. However, no
dielectric anomalies can be detected in the temperature range of diffuse phase
transition. The temperature dependence of the dielectric constant show the
maximum at higher temperature (Tm = 417 - 429 K, depending on frequency) with
the typical relaxor dispersion at T < Tm and the frequency dependence of Tm
fitted to the Vogel-Fulcher relation. Application of an electric field upon
cooling from the cubic phase or poling the crystal in the ferroelectric phase
gives rise to a sharp anomaly of the dielectric constant at T 390 K and
diminishes greatly the dispersion at lower temperatures, but the dielectric
relaxation process around Tm remains qualitatively unchanged. The results are
discussed in the framework of the present models of relaxors and in comparison
with the prototypical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3.Comment: PDF file, 13 pages, 6 figures collected on pp.12-1
Ferroelectric Dynamics in the Perovskite Relaxor PMN
We review results obtained from recent neutron scattering studies of the
lead-oxide class of perovskite relaxors PMN and PZN. A ferroelectric soft mode
has been identified in PMN at 1100 K that becomes overdamped near 620 K. This
is the same temperature at which polar nanoregions (PNR) begin to form, denoted
by Td, and suggests that a direct connection exists between the soft mode and
the PNR. The appearance of diffuse scattering intensity at Td reported by
Naberezhnov et al. lends further support to this picture. At lower temperature
the soft mode in PMN reappears close to Tc = 213 K (defined only for E > Ec).
These results are provocative because the dynamics below Tc are characteristic
of an ordered ferroelectric state, yet they occur in a system that remains
cubic on average at all temperatures. We discuss a coupled-mode model that
successfully describes these data as well as those from earlier lattice
dynamical studies of other perovskites such as BaTiO3.Comment: 10 pages, 7 figures, Conference - Fundamental Physics of
Ferroelectrics 200
Monoclinic phase in the relaxor-based piezo-/ ferroelectric Pb(MgNb-PbTiO system
A ferroelectric monoclinic phase of space group ( type) has been
discovered in 0.65Pb(MgNb-0.35PbTiO by means of high
resolution synchrotron X-ray diffraction. It appears at room temperature in a
single crystal previously poled under an electric field of 43 kV/cm applied
along the pseudocubic [001] direction, in the region of the phase diagram
around the morphotropic phase boundary between the rhombohedral (R3m) and the
tetragonal (P4mm) phases. The monoclinic phase has lattice parameters a = 5.692
A, b = 5.679 A, c = 4.050 A and = , with the b-axis
oriented along the pseudo-cubic [110] direction . It is similar to the
monoclinic phase observed in PbZrTiO, but different from that
recently found in Pb(ZnNb-PbTiO, which is of space
group ( type).Comment: Revised version after referees' comments. PDF file. 6 pages, 4
figures embedde
An Anomalous Phase in the Relaxor Ferroelectric Pb(ZnNb)O
X-ray diffraction studies on a Pb(ZnNb)O (PZN) single
crystal sample show the presence of two different structures. An outer-layer
exists in the outer most 10 to 50~m of the crystal, and undergoes a
structural phase transition at the Curie temperature K. The
inside phase is however, very different. The lattice inside the crystal
maintains a cubic unit cell, while ferroelectric polarization develops below
. The lattice parameter of the cubic unit cell remains virtually a
constant, i.e., much less variations compared to that of a typical relaxor
ferroelectric, in a wide temperature range of 15 K to 750 K. On the other hand,
broadening of Bragg peaks and change of Bragg profile line-shapes in both
longitudinal and transverse directions at clearly indicate a structural
phase transition occurring.Comment: to be submitted for PR
Hydrostatic pressure effects on the static magnetism in Eu(FeCo)As
The effects of hydrostatic pressure on the static magnetism in
Eu(FeCo)As are investigated by complementary
electrical resistivity, ac magnetic susceptibility and single-crystal neutron
diffraction measurements. A specific pressure-temperature phase diagram of
Eu(FeCo)As is established. The structural phase
transition, as well as the spin-density-wave order of Fe sublattice, is
suppressed gradually with increasing pressure and disappears completely above
2.0 GPa. In contrast, the magnetic order of Eu sublattice persists over the
whole investigated pressure range up to 14 GPa, yet displaying a non-monotonic
variation with pressure. With the increase of the hydrostatic pressure, the
magnetic state of Eu evolves from the canted antiferromagnetic structure in the
ground state, via a pure ferromagnetic structure under the intermediate
pressure, finally to a possible "novel" antiferromagnetic structure under the
high pressure. The strong ferromagnetism of Eu coexists with the
pressure-induced superconductivity around 2 GPa. The change of the magnetic
state of Eu in Eu(FeCo)As upon the application
of hydrostatic pressure probably arises from the modification of the indirect
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu moments
tuned by external pressure.Comment: 9 pages, 6 figure
A model of rotating hotspots for 3:2 frequency ratio of HFQPOs in black hole X-ray binaries
We propose a model to explain a puzzling 3:2 frequency ratio of high
frequency quasi-periodic oscillations (HFQPOs) in black hole (BH) X-ray
binaries, GRO J1655-40, GRS 1915+105 and XTE J1550-564. In our model a
non-axisymmetric magnetic coupling (MC) of a rotating black hole (BH) with its
surrounding accretion disc coexists with the Blandford-Znajek (BZ) process. The
upper frequency is fitted by a rotating hotspot near the inner edge of the
disc, which is produced by the energy transferred from the BH to the disc, and
the lower frequency is fitted by another rotating hotspot somewhere away from
the inner edge of the disc, which arises from the screw instability of the
magnetic field on the disc. It turns out that the 3:2 frequency ratio of HFQPOs
in these X-ray binaries could be well fitted to the observational data with a
much narrower range of the BH spin. In addition, the spectral properties of
HFQPOs are discussed. The correlation of HFQPOs with jets from microquasars is
contained naturally in our model.Comment: 8 pages, 4 figures. accepted by MNRA
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