279 research outputs found
Photoferroelectric oxides
Giant photovoltaic effect due to bulk photovoltaic effect observed in
multiferroic BiFeO3 thin films has triggered a renewed interest on
photoferroelectric materials for photovoltaic applications. Tremendous advance
has been done to improve power conversion efficiency (up to up to 8.1%) in
photoferroelectrics via absorption increase using narrow bandgap
ferroelectrics. Other strategies, as it is the more efficient use of
ferroelectric internal electric field, are ongoing. Moreover, as a by-product,
several progress have been also achieved on photostriction that is the
photo-induced deformation phenomenon. Here, we review ongoing and promising
routes to improve ferroelectrics photoresponse
Strong enhancement of direct magnetoelectric effect in strained ferroelectric-ferromagnetic thin-film heterostructures
The direct magnetoelectric (ME) effect resulting from the polarization
changes induced in a ferroelectric film by the application of a magnetic field
to a ferromagnetic substrate is described using the nonlinear thermodynamic
theory. It is shown that the ME response strongly depends on the initial strain
state of the film. The ME polarization coefficient of the heterostructures
involving Terfenol-D substrates and compressively strained lead zirconate
titanate (PZT) films, which stabilize in the out-of-plane polarization state,
is found to be comparable to that of bulk PZT/Terfenol-D laminate composites.
At the same time, the ME voltage coefficient reaches a giant value of 50 V/(cm
Oe), which greatly exceeds the maximum observed static ME coefficients of bulk
composites. This remarkable feature is explained by a favorable combination of
considerable strain sensitivity of polarization and a low electric permittivity
in compressively strained PZT films. The theory also predicts a further
dramatic increase of ME coefficients at the strain-induced transitions between
different ferroelectric phases.Comment: 7 pages, 3 figure
Uniaxial-stress induced phase transitions in [001]c-poled 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3
First-order, rhombohedral to orthorhombic, stress-induced phase transitions
have been evidenced by bulk charge-stress measurements and X-ray diffraction
derived lattice strain measurements in [001]c-poled PZN-4.5PT. The transitions
are induced by uniaxial, compressive loads applied either along or
perpendicular to the poling direction. In each case, they occur via rotation of
the polar vector in the Cm monoclinic plane and the induced lattice strain is
hysteretic yet reversible. Although no depoling is observed in the transverse
mode, net depolarization is observed under longitudinal stress which is
important for the use of [001]c-poled PZN-PT and PMN-PT single crystals in
Tonpilz-type underwater projectors.Comment: To be published in Applied Physics Letters, 16 pages, 3 figure
Phase transitions and ferroelectrics: revival and the future in the field
It appeared worthwhile to us to present a state-of-the-art look at the field
of ferroelectrics. We are certainly not attempting to provide a complete review
of all aspects of the field of ferroelectrics over the last years but we wish
to transport a flavour of the current excitement in the field through the
(subjective) choice of four specific examples of current interest: (i)
Piezoelectrics and the morphotropic phase boundary, (ii) Multiferroics, (iii)
The effect of high pressure on ferroelectrics and (iv) Strain-engineering in
ferroelectric oxide thin films. For each topic we will try to work out both
current interesting approaches and an outlook into future challenges.
Throughout our discussion, the reader is referred to a list of significant
review articles, books and papers in the field.Comment: Editorial overview, 31 pages, 4 Figures, 315 Reference
The SrTiO displacive transition revisited by Coherent X-ray Diffraction
We present a Coherent X-ray Diffraction study of the antiferrodistortive
displacive transition of SrTiO, a prototypical example of a phase
transition for which the critical fluctuations exhibit two length scales and
two time scales. From the microbeam x-ray coherent diffraction patterns, we
show that the broad (short-length scale) and the narrow (long-length scale)
components can be spatially disentangled, due to 100 m-scale spatial
variations of the latter. Moreover, both components exhibit a speckle pattern,
which is static on a 10 mn time-scale. This gives evidence that the
narrow component corresponds to static ordered domains. We interpret the
speckles in the broad component as due to a very slow dynamical process,
corresponding to the well-known \emph{central} peak seen in inelastic neutron
scattering.Comment: 4 pages, 3 figures, accepted in PR
Constitutive modeling of the anisotropic behavior of Mullins softened filled rubbers
Original constitutive modeling is proposed for filled rubber materials in order to capture the anisotropic softened behavior induced by general non-proportional pre-loading histo-ries. The hyperelastic framework is grounded on a thorough analysis of cyclic experimental data. The strain energy density is based on a directional approach. The model leans on the strain amplification factor concept applied over material directions according to the Mul-lins softening evolution. In order to provide a model versatile that applies for a wide range of materials, the proposed framework does not require to postulate the mathematical forms of the elementary directional strain energy density and of the Mullins softening evo-lution rule. A computational procedure is defined to build both functions incrementally from experimental data obtained during cyclic uniaxial tensile tests. Successful compari-sons between the model and the experiments demonstrate the model abilities. Moreover, the model is shown to accurately predict the non-proportional uniaxial stress-stretch responses for uniaxially and biaxially pre-stretched samples. Finally, the model is effi-ciently tested on several materials and proves to provide a quantitative estimate of the anisotropy induced by the Mullins softening for a wide range of filled rubbers
Two-step phase changes in cubic relaxor ferroelectrics
The field-driven conversion between the zero-field-cooled frozen relaxor
state and a ferroelectric state of several cubic relaxors is found to occur in
at least two distinct steps, after a period of creep, as a function of time.
The relaxation of this state back to a relaxor state under warming in zero
field also occurs via two or more sharp steps, in contrast to a one-step
relaxation of the ferroelectric state formed by field-cooling. An intermediate
state can be trapped by interrupting the polarization. Giant pyroelectric noise
appears in some of the non-equilibrium regimes. It is suggested that two
coupled types of order, one ferroelectric and the other glassy, may be required
to account for these data.Comment: 27 pages with 8 figures to appear in Phys. Rev.
Phase diagram of Pb(Zr,Ti)O3 solid solutions from first principles
A first-principles-derived scheme, that incorporates ferroelectric and
antiferrodistortive degrees of freedom, is developed to study
finite-temperature properties of PbZr1-xTixO3 solid solutions near its
morphotropic phase boundary. The use of this numerical technique (i) resolves
controversies about the monoclinic ground-state for some Ti compositions, (ii)
leads to the discovery of an overlooked phase, and (iii) yields three
multiphase points, that are each associated with four phases. Additional
neutron diffraction measurements strongly support some of these predictions.Comment: 10 pages, 2 figure
Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films
epitaxially grown on cubic substrates is developed using the Landau-Devonshire
approach. The eighth-order thermodynamic potential for BT single crystal and
modified fourth-order potential for ST single crystal were used as starting
potentials for the end-members of the solid solution with the aim to develop
potential of BST- solid solution valid at high temperatures. Several
coefficients of these potentials for BT were changed to obtain reasonable
agreement between theory and experimental phase diagram for BST-x (x > 0.2)
solid solutions. For low Ba content we constructed the specific phase diagram
where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and
all transitions are of the second order. The "concentration-misfit strain"
phase diagrams for BST-x thin films at room temperature and "temperature-misfit
strain" phase diagrams for particular concentrations are constructed and
discussed. Near T_N2 coupling between polarization and structural order
parameter in the epitaxial film is modified considerably and large number of
new phases not present in the bulk materials appear on the phase diagram.Comment: 8 pages 5 figure
Photostriction in Ferroelectrics from Density Functional Theory
International audienceAn ab initio procedure allowing the computation of the deformation of ferroelectric-based materials under light is presented. This numerical scheme consists in structurally relaxing the system under the constraint of a fixed n e concentration of electrons photoexcited into a specific conduction band edge state from a chosen valence band state, via the use of a constrained density functional theory method. The resulting change in lattice constant along a selected crystallographic direction is then calculated for a reasonable estimate of n e. This method is applied to bulk multiferroic BiFeO 3 and predicts a photostriction effect of the same order of magnitude than the ones recently observed. A strong dependence of photostrictive response on both the reached conduction state and the crystallographic direction (along which this effect is determined) is also revealed. Furthermore, analysis of the results demonstrates that the photostriction mechanism mostly originates from the screening of the spontaneous polarization by the photoexcited electrons in combination with the inverse piezoelectric effect. The coupling of ferroelectric or multiferroic materials with light is currently attracting a lot of attention [1], as, e.g., demonstrated by the above-band-gap photovoltages found in BiFeO 3 (BFO) thin films [2], the search of low band gap materials for photovoltaic applications [3], or the recent development in the so-called hybrid perovskite solar cells [4]. Beyond the photovoltaic effect, there is another coupling between light and properties of ferroelectrics or multiferroics that is of current interest, namely, the so-called photostriction effect, a deformation of the material under illumination [5]. The photostriction phenomenon opens new perspectives for combining several functionalities in future generations of remote switchable devices and is promising for the realization of light-induced actuators [5]. It has been recently observed in BFO under visible light [6,7]. A giant shear strain generated by femtosecond laser pulses was also reported [8,9], and time-resolved synchrotron diffraction reported a shift of the Bragg peak on a picosecond time scale in both bismuth ferrite [10] and lead titanate [11]. However, the microscopic mechanism responsible for photostriction is poorly understood [8,9]. Obviously, having accurate numerical techniques able to tackle photostriction will allow us to " shed some light " on this effect. However, to the best of our knowledge, such numerical tools allowing a systematic study of the photostriction phenomenon and its atomistic origin are not available yet, despite recent attempts to use Density Functional Theory (DFT) as a tool to fit x-ray absorption spectra in pump-probe photostriction experiments [12]. Here, we report the development of an ab initio procedure to compute photostriction from first principles. This procedure not only reproduces the order of magnitude of the observed change of lattice constant in BFO [6], but also reveals that photostriction mostly originates from the combination of the screening of the polarization by the electrons photoejected in the conduction band and the inverse piezoelectric effect. It is also found that photo-striction depends on the precise conduction state the electron is excited into, and on the crystallographic direction along which the effect is studied. In order to realize the difficulty in mimicking photo-striction, let us start by recalling that the Kohn-Sham (KS) implementation of DFT [13] reformulates the many-body problem of interacting electrons into many single-body problems, and " only " guarantees that the model noninter-acting KS Hamiltonian yields the same ground state density and energy as the real interacting Hamiltonian. Such a fact, therefore, leaves the description of unoccupied states within traditional DFT an unanswered question, and the determination of excitation energies remains the privilege of rather costly techniques, such as time-dependent DFT [14] or the GW approximation [15]. However, an alternative formulation of DFT that treats ground and excited states on the same footing has been proposed [16]. In particular, Ref. [16] connected each eigenstate of a real interacting Hamiltonian with the eigenstate of a model noninteracting Hamiltonian through a generalized adiabatic connection (GAC) scheme. The so-called ΔSCF method [17] takes advantage of this GAC scheme, and assumes an one-to-one correspondence between the excited states of a single Kohn-Sham system and the real system [16]. This ΔSCF scheme has proved successful and computationall
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