414 research outputs found
Symmetry of high-piezoelectric Pb-based complex perovskites at the morphotropic phase boundary II. Theoretical treatment
The structural characteristics of the perovskite- based ferroelectric
Pb(Zn1/3Nb2/3)O3-9%PbTiO3 at the morphotropic phase boundary (MPB) region
(x≃0.09) have been analyzed. The analysis is based on the symmetry
adapted free energy functions under the assumption that the total polarization
and the unit cell volume are conserved during the transformations between
various morphotropic phases. Overall features of the relationships between the
observed lattice constants at various conditions have been consistently
explained. The origin of the anomalous physical properties at MPB is discussed
Quantum criticality at cryogenic melting of polar bubble lattices
Quantum fluctuations (QFs) caused by zero-point phonon vibrations (ZPPVs) are
known to prevent the occurrence of polar phases in bulk incipient
ferroelectrics down to 0K1-3. On the other hand, little is known about the
effects of QFs on the recently discovered topological patterns in ferroelectric
nanostructures4-9. Here, by using an atomistic effective Hamiltonian within
classical Monte Carlo (CMC) and path integral quantum Monte Carlo
(PI-QMC)1,3,10,11, we unveil how QFs affect the topology of several dipolar
phases in ultrathin Pb(Zr0.4Ti0.6)O3 (PZT) films. In particular, our PI-QMC
simulations show that the ZPPVs do not suppress polar patterns but rather
stabilize the labyrinth4, bimeron5 and bubble phases12,13 within a wider range
of bias field magnitudes. Moreover, we reveal that quantum fluctuations induce
a quantum critical point (QCP) separating a hexagonal bubble lattice from a
liquid-like state characterized by spontaneous motion, creation and
annihilation of polar bubbles at cryogenic temperatures. Finally, we show that
the discovered quantum melting is associated with anomalous physical response,
as, e.g., demonstrated by a negative longitudinal piezoelectric coefficient.Comment: Nature communication, accepted, 21 pages, 4 Fig
Origin of sawtooth domain walls in ferroelectrics
Domains and domain walls are among the key factors that determine the
performance of ferroelectric materials. In recent years, a unique type of
domain walls, i.e., the sawtooth-shaped domain walls, has been observed in
BiFeO and PbTiO. Here, we build a minimal model to reveal the
origin of these sawtooth-shaped domain walls. Incorporating this model into
Monte-Carlo simulations shows that (i) the competition between the long-range
Coulomb interaction (due to bound charges) and short-range interaction (due to
opposite dipoles) is responsible for the formation of these peculiar domain
walls and (ii) their relative strength is critical in determining the
periodicity of these sawtooth-shaped domain walls. Necessary conditions to form
such domain walls are also discussed
Nonlinear phonon Hall effects in ferroelectrics: its existence and non-volatile electrical control
Nonlinear Hall effects have been previously investigated in
non-centrosymmetric systems for electronic systems. However, they only exist in
metallic systems and are not compatible with ferroelectrics since these latter
are insulators, hence limiting their applications. On the other hand,
ferroelectrics naturally break inversion symmetry and can induce a non-zero
Berry curvature. Here, we show that a non-volatile electric-field control of
heat current can be realized in ferroelectrics through the nonlinear phonon
Hall effects. More precisely, based on Boltzmann equation under the
relaxation-time approximation, we derive the equation for nonlinear phonon Hall
effects, and further show that the behaviors of nonlinear phonon (Boson) Hall
effects are very different from nonlinear Hall effects for electrons (Fermion).
Our work provides a route for electric-field control of thermal Hall current in
ferroelectrics.Comment: 16 pages, 2 figure
Giant electrocaloric response in the prototypical Pb(Mg,Nb)O relaxor ferroelectric from atomistic simulations
An atomistic effective Hamiltonian is used to investigate electrocaloric (EC)
effects of Pb(MgNb)O (PMN) relaxor ferroelectrics in its
ergodic regime, and subject to electric fields applied along the pseudocubic
[111] direction. Such Hamiltonian qualitatively reproduces (i) the electric
field-versus-temperature phase diagram, including the existence of a critical
point where first-order and second-order transitions meet each other; and (ii)
a giant EC response near such critical point. It also reveals that such giant
response around this critical point is microscopically induced by field-induced
percolation of polar nanoregions. Moreover, it is also found that, for any
temperature above the critical point, the EC coefficient-versus-electric field
curve adopts a maximum (and thus larger electrocaloric response too), that can
be well described by the general Landau-like model proposed in [Jiang et al,
Phys. Rev. B 96, 014114 (2017)] and that is further correlated with specific
microscopic features related to dipoles lying along different rhombohedral
directions. Furthermore, for temperatures being at least 40 K higher than the
critical temperature, the (electric field, temperature) line associated with
this maximal EC coefficient is below both the Widom line and the line
representing percolation of polar nanoregions.Comment: 6 pages, 5 figure
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