386 research outputs found
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Avalanches from charged domain wall motion in BaTiO<inf>3</inf> during ferroelectric switching
© 2020 Author(s). We report two methods for direct observations of avalanches in ferroelectric materials during the motion of domain walls. In the first method, we use optical imaging techniques to derive changes in domain structures under an electric field. All changes occur through small jumps (jerks) that obey avalanche statistics. In the second method, we analyze jerks by their displacement current. Both methods reveal a power law distribution with an energy exponent of 1.6, in agreement with previous acoustic emission measurements, and integrated mean field theory. This new combination of methods allows us to probe both polarization and strain variations during the motion of domain walls and can be used for a much wider class of ferroelectrics, including ceramic samples, than acoustic emission
Tin telluride: a weakly co-elastic metal
We report resonant ultrasound spectroscopy (RUS),
dilatometry/magnetostriction, magnetotransport, magnetization, specific heat,
and Sn M\"ossbauer spectroscopy measurements on SnTe and
SnCrTe. Hall measurements at K indicate that our
Bridgman-grown single crystals have a -type carrier concentration of cm and that our Cr-doped crystals have an -type
concentration of cm. Although our SnTe crystals are
diamagnetic over the temperature range , the Cr-doped crystals are room temperature ferromagnets with a Curie
temperature of 294 K. For each sample type, three-terminal capacitive
dilatometry measurements detect a subtle 0.5 micron distortion at K. Whereas our RUS measurements on SnTe show elastic hardening near the
structural transition, pointing to co-elastic behavior, similar measurements on
SnCrTe show a pronounced softening, pointing to
ferroelastic behavior. Effective Debye temperature, , values of SnTe
obtained from Sn M\"ossbauer studies show a hardening of phonons in the
range 60--115K ( = 162K) as compared with the 100--300K range
( = 150K). In addition, a precursor softening extending over
approximately 100 K anticipates this collapse at the critical temperature, and
quantitative analysis over three decades of its reduced modulus finds with , a value
indicating a three-dimensional softening of phonon branches at a temperature
K, considerably below . We suggest that the differences in
these two types of elastic behaviors lie in the absence of elastic domain wall
motion in the one case and their nucleation in the other
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Multiple Avalanche Processes in Acoustic Emission Spectroscopy: Multibranching of the Energy−Amplitude Scaling
Several physical processes can conspire to generate avalanches in materials. Such processes include avalanche mechanisms like dislocation movements, friction processes by pinning magnetic domain walls, moving dislocation tangles, hole collapse in porous materials, collisions of ferroelectric and ferroelastic domain boundaries, kinks in interfaces, and many more. Known methods to distinguish between these species which allow the physical identification of multiavalanche processes are reviewed. A new approach where the scaling relationship between the avalanche energies E and amplitudes A is considered is then described. Avalanches with single mechanisms scale experimentally as E = SiAi2. The energy E reflects the duration D of the avalanche and A(t), the temporal amplitude. The scaling prefactor S depends explicitly on the duration of the avalanche and on details of the avalanche profiles. It is reported that S is not a universal constant but assumes different values depending on the avalanche mechanism. If avalanches coincide, they can still show multivalued scaling between E and A with different S-values for each branch. Examples for this multibranching effect in low-Ni 316L stainless steel, 316L stainless steel, polycrystalline Ni, TC21 titanium alloy, and a Fe40Mn40Co10Cr10 high-entropy alloy are shown
Low-temperature phase transformations of PZT in the morphotropic phase-boundary region
We present anelastic and dielectric spectroscopy measurements of
PbZr(1-x)Ti(x)O(3) with 0.455 < x < 0.53, which provide new information on the
low temperature phase transitions. The tetragonal-to-monoclinic transformation
is first-order for x < 0.48 and causes a softening of the polycrystal Young's
modulus whose amplitude may exceed the one at the cubic-to-tetragonal
transformation; this is explainable in terms of linear coupling between shear
strain components and tilting angle of polarization in the monoclinic phase.
The transition involving rotations of the octahedra below 200 K is visible both
in the dielectric and anelastic losses, and it extends within the tetragonal
phase, as predicted by recent first-principle calculations.Comment: 4 pages, 4 figure
Octahedral tilting, monoclinic phase and the phase diagram of PZT
Anelastic and dielectric spectroscopy measurements on PZT close to the
morphotropic (MPB) and antiferroelectric boundaries provide new insight in some
controversial aspects of its phase diagram. No evidence is found of a border
separating monoclinic (M) from rhombohedral (R) phases, in agreement with
recent structural studies supporting a coexistence of the two phases over a
broad composition range x < 0.5, with the fraction of M increasing toward the
MPB. It is also discussed why the observed maximum of elastic compliance
appears to be due to a rotational instability of the polarisation and therefore
cannot be explained by extrinsic softening from finely twinned R phase alone,
but indicates the presence also of M phase, not necessarily homogeneous.
A new diffuse transition is found within the ferroelectric phase near x ~
0.1, at a temperature T_IT higher than the well established boundary T_T to the
phase with tilted octahedra. It is proposed that around T_IT the octahedra
start rotating in a disordered manner and finally become ordered below T_T. In
this interpretation, the onset temperature for octahedral tilting monotonically
increases up to the antiferroelectric transition of PbZrO3, and the depression
of T_T(x) below x = 0.18 would be a consequence of the partial relieve of the
mismatch between the cation radii with the initial stage of tilting below T_IT.Comment: submitted to J. Phys.: Condens. Matte
Observation of a continuous phase transition in a shape-memory alloy
Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and
pressure-dependent electrical transport measurements have been made on single
crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic
transition temperatures (T_M=64K and 45K, respectively). In each composition,
elastic neutron scattering detects new commensurate Bragg peaks (modulation)
appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's
T_M. Although the new Bragg peaks appear in a discontinuous manner in the
Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn.
Surprising us, the temperature dependence of the AuZn Bragg peak intensity and
the specific-heat jump near the transition temperature are in favorable accord
with a mean-field approximation. A Landau-theory-based fit to the pressure
dependence of the transition temperature suggests the presence of a critical
endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a
quantum saturation temperature \theta_s=48.3 +/- 3.7K.Comment: 6 figure
The quantum paraelectric behavior of SrTiO_{3} revisited: relevance of the structural phase transition temperature
It has been known for a long time that the low temperature behavior shown by
the dielectric constant of quantum paraelectric can not be fitted
properly by Barrett's formula using a single zero point energy or saturation
temperature (). As it was originally shown [K. A. M\"{u}ller and H.
Burkard, Phys. Rev. B {\bf 19}, 3593 (1979)] a crossover between two different
saturation temperatures (=77.8K and =80K) at is
needed to explain the low and high temperature behavior of the dielectric
constant. However, the physical reason for the crossover between these two
particular values of the saturation temperature at is unknown. In
this work we show that the crossover between these two values of the saturation
temperature at can be taken as a direct consequence of (i) the
quantum distribution of frequencies associated
with the complete set of low-lying modes and (ii) the existence of a definite
maximum phonon frequency given by the structural transition critical
temperature .Comment: 8 pages, 3 figure
Anharmonic effects in the A15 compounds induced by sublattice distortions
We demonstrate that elastic anomalies and lattice instabilities in the the
A15 compounds are describable in terms of first-principles LDA electronic
structure calculations. We show that at T=0 V_3Si, V_3Ge, and Nb_3Sn are
intrinsically unstable against shears with elastic moduli C_11-C_12 and C_44,
and that the zone center phonons, Gamma_2 and Gamma_12, are either unstable or
extremely soft. We demonstrate that sublattice relaxation (internal strain)
effects are key to understanding the behavior of the A15 materials.Comment: 5 pages, RevTex, 3 postscript figures, Submitted to Phys. Rev. Lett.
Apr. 23, 1997 July 7, 1997: minor corrections, final accepted versio
Exploring the vicinity of the Bogomol'nyi-Prasad-Sommerfield bound
We investigate systems of real scalar fields in bidimensional spacetime,
dealing with potentials that are small modifications of potentials that admit
supersymmetric extensions. The modifications are controlled by a real
parameter, which allows implementing a perturbation procedure when such
parameter is small. The approach allows obtaining the energy and topological
charge in closed forms, up to first order in the parameter. We illustrate the
procedure with some examples. In particular, we show how to remove the
degeneracy in energy for the one-field and the two-field solutions that appear
in a model of two real scalar fields.Comment: Revtex, 9 pages, To be published in J. Phys.
Modelling avalanches in martensites
Solids subject to continuous changes of temperature or mechanical load often
exhibit discontinuous avalanche-like responses. For instance, avalanche
dynamics have been observed during plastic deformation, fracture, domain
switching in ferroic materials or martensitic transformations. The statistical
analysis of avalanches reveals a very complex scenario with a distinctive lack
of characteristic scales. Much effort has been devoted in the last decades to
understand the origin and ubiquity of scale-free behaviour in solids and many
other systems. This chapter reviews some efforts to understand the
characteristics of avalanches in martensites through mathematical modelling.Comment: Chapter in the book "Avalanches in Functional Materials and
Geophysics", edited by E. K. H. Salje, A. Saxena, and A. Planes. The final
publication is available at Springer via
http://dx.doi.org/10.1007/978-3-319-45612-6_
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