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

Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions

The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in details that there exists a critical thickness at which an unusual ferroelectric-\'\' polar non-ferroelectric\rq\rq phase transition occurs in asymmetric ferroelectric tunnel junctions. In the \'\' polar non-ferroelectric\rq\rq phase, there is only one non-switchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closure-like domains are proposed to form to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field does not only result in smearing of phase transition but also forces the transition to take place at a reduced temperature. Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric tunnel junction-type or ferroelectric capacitor-type devices.Comment: 9 pages, 8 figures, submitted to PR

Partial decoupling between strain and polarization in mono-oriented Pb(Zr0.2Ti0.8)O3 thin film

The structural evolution of epitaxial mono-oriented (i.e. with the c-axis perpendicular to the interface) ferroelectric Pb(Zr0.2,Ti0.8)O3 thin film has been investigated, using high resolution, temperature dependent, X-ray diffraction. The full set of lattice parameters was obtained, it allowed to estimate the variation of the polarization as a function of temperature, underlying the difference between the polarization-induced tetragonality and the elastic one. The temperature evolution of the misfit strain has been calculated and found to be in good agreement with the theoretical temperature-misfit strain phase diagramComment: 11 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

The SrTiO3_3 displacive transition revisited by Coherent X-ray Diffraction

We present a Coherent X-ray Diffraction study of the antiferrodistortive displacive transition of SrTiO$_3$, 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 $\mu$m-scale spatial variations of the latter. Moreover, both components exhibit a speckle pattern, which is static on a $\sim$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

Worst-Case Communication Overhead in a Many-Core based Shared-Memory Model

National audienceWith emerging many-core architectures, using on-chip shared memories is an interesting approach because it provides high bandwidth and high throughput data exchange. Such a feature is usually implemented as a multi-bus multi-banked memory. Since predicting timing behavior is key to efficient design and verification of embedded real-time systems, the question that arises is how to evaluate the access time for one memory access of a given task while others may concurrently access the same memory-bank at t the same time. In this paper, we give the answers for a subset of streaming applications modeled like CSDF Model of Computation and implemented in Kalray’s MPPA chip