References of "Kreisel, Jens 50031730"
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See detailIn memory of Ihor Stasyuk
Kreisel, Jens UL; Roleder, Krystian

in PHASE TRANSITIONS (2020), 93(10-11), 945-945

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See detailDomain-wall engineering and topological defects in ferroelectric and ferroelastic materials
Nataf, G. F.; Guennou, Mael UL; Gregg, J. M. et al

in NATURE REVIEWS PHYSICS (2020), 2(11), 634-648

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional ... [more ▼]

Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent new and exciting objects in matter. In this Review, we discuss the exotic polarization profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain-wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions that have been postulated as fundamental mechanisms responsible for domain-wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo- and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today's topological systems. Ferroelectric and ferroelastic domain walls are 2D topological defects with thicknesses approaching the unit cell level and emergent functional properties. This Review discusses the exotic polarization profiles that arise at domain walls and the fundamental mechanisms responsible for domain-wall conduction. [less ▲]

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See detailRole of the ferroelastic strain in the optical absorption of BiVO4
Hill, Christina; Weber, Mads C.; Lehmann, Jannis et al

in APL MATERIALS (2020), 8(8),

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical ... [more ▼]

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical bandgap is found to be very sensitive to the temperature, and to the tetragonal-to-monoclinic ferroelastic transition at 523 K. The anisotropy, as measured by the difference in the absorption edge for the light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth vanadate-based light absorbers in photocatalytic devices. [less ▲]

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See detailVibrational properties of LaNiO3 films in the ultrathin regime
Schober, Alexander; Fowlie, Jennifer; Guennou, Mael UL et al

in APL MATERIALS (2020), 8(6),

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films ... [more ▼]

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (similar to 1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit. [less ▲]

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See detailLattice dynamics and Raman spectrum of BaZrO3 single crystals
Toulouse, Constance UL; Amoroso, Danila; Xin, Cong et al

in PHYSICAL REVIEW B (2019), 100

BaZrO3 is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with ... [more ▼]

BaZrO3 is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of BaZrO3 single crystals in a broad temperature range (4-1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of BaZrO3, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra. [less ▲]

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See detailIncreasing bulk photovoltaic current by strain tuning
Nadupalli, Shankari; Kreisel, Jens UL; Granzow, Torsten

in SCIENCE ADVANCES (2019), 5(3),

Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based ... [more ▼]

Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO3 bulk single crystal the existence of a purely intrinsic ``piezophotovoltaic'' effect that leads to a linear increase in photovoltaic current density. The increase reaches 75 under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005\%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented. [less ▲]

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See detailCrossover in the pressure evolution of elementary distortions in RFeO3 perovskites and its impact on their phase transition
Vilarinho, Rui; Bouvier, Pierre; Guennou, Mael UL et al

in Physical Review. B (2019), 99

This work reports on the pressure dependence of the octahedral tilts and mean Fe-O bond lengths in RFeO3 (R = Nd, Sm, Eu, Gd, Tb, and Dy), determined through synchrotron x-ray diffraction and Raman ... [more ▼]

This work reports on the pressure dependence of the octahedral tilts and mean Fe-O bond lengths in RFeO3 (R = Nd, Sm, Eu, Gd, Tb, and Dy), determined through synchrotron x-ray diffraction and Raman scattering, and their role on the pressure-induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedral tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). EuFeO3 stands at the borderline, with nearly pressure-independent tilt angles. For the compounds where the tilts increase with pressure, the FeO6 octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed in relation to the general rules proposed from different theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, points out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedral compressibility and critical pressure is ascertained. [less ▲]

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See detailAccelerated Ionic Motion in Amorphous Memristor Oxides for Nonvolatile Memories and Neuromorphic Computing
Schmitt, Rafael; Kubicek, Markus; Sediva, Eva et al

in ADVANCED FUNCTIONAL MATERIALS (2019), 29(5),

Memristive devices based on mixed ionic-electronic resistive switches have an enormous potential to replace today's transistor-based memories and Von Neumann computing architectures thanks to their ... [more ▼]

Memristive devices based on mixed ionic-electronic resistive switches have an enormous potential to replace today's transistor-based memories and Von Neumann computing architectures thanks to their ability for nonvolatile information storage and neuromorphic computing. It still remains unclear however how ionic carriers are propagated in amorphous oxide films at high local electric fields. By using memristive model devices based on LaFeO3 with either amorphous or epitaxial nanostructures, we engineer the structural local bonding units and increase the oxygen-ionic diffusion coefficient by one order of magnitude for the amorphous oxide, affecting the resistive switching operation. We show that only devices based on amorphous LaFeO3 films reveal memristive behavior due to their increased oxygen vacancy concentration. We achieved stable resistive switching with switching times down to microseconds and confirm that it is predominantly the oxygen-ionic diffusion character and not electronic defect state changes that modulate the resistive switching device response. Ultimately, these results show that the local arrangement of structural bonding units in amorphous perovskite films at room temperature can be used to largely tune the oxygen vacancy (defect) kinetics for resistive switches (memristors) that are both theoretically challenging to predict and promising for future memory and neuromorphic computing applications. [less ▲]

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See detailSingle crystal growth of BaZrO3 from the melt at 2700 degrees C using optical floating zone technique and growth prospects from BaB2O4 flux at 1350 degrees C
Xin, Cong; Veber, Philippe; Guennou, Mael UL et al

in CRYSTENGCOMM (2019), 21(3), 502-512

We report the growth of BaZrO3 single crystals by the optical floating zone technique and the investigation on its flux growth using BaB2O4 as a solvent. 6 mm long colorless and transparent single ... [more ▼]

We report the growth of BaZrO3 single crystals by the optical floating zone technique and the investigation on its flux growth using BaB2O4 as a solvent. 6 mm long colorless and transparent single crystals were obtained with a mirror furnace without the need for post-treatment annealing. Its properties are determined and compared with those of two commercial crystals grown by the tri-arc Czochralski method. The chemical composition was investigated using glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS), which indicate minor impurities of Sr, Hf, Ca and Ti, with maximal concentrations for Sr and Hf in the range of 0.3-0.5 at. The optical band gap determined by UV-visible spectroscopy is found to be similar to 4.8 eV and indicates the high quality of the BaZrO3 crystals grown by the optical floating zone technique. Raman spectroscopy at ambient conditions and at low temperatures down to 4.2 K reveals a relatively sharp second-order spectrum and does not reveal any structural phase transition. Prospective high-temperature solution growth using BaB2O4 self-flux was investigated and led to 150-200 mu m BaZrO3 crystals. This solvent opens the way to grow BaZrO3 at half its melting point by the flux method. [less ▲]

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See detailOptical spectroscopy on the photo-response in multiferroic BiFeO3 at high pressure
Meggle, F.; Ebad-Allah, J.; Kreisel, Jens UL et al

in JOURNAL OF APPLIED PHYSICS (2019), 126(16),

The pressure dependence of light-induced effects in single-crystalline BiFeO3 is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just ... [more ▼]

The pressure dependence of light-induced effects in single-crystalline BiFeO3 is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just below the two crystal-field excitations and the absorption onset, respectively. These absorption features were previously ascribed to excitons, possibly connected with the ultrafast photostriction effect in BiFeO3. The pressure-induced redshift of the absorption features follows the pressure dependence of the corresponding crystal-field excitations and absorption onset, suggesting the link between them. Above the structural phase transition at P-c1 approximate to 3.5 GPa, the three absorption features disappear, suggesting their connection to the polar phase in BiFeO3. The pressure-induced disappearance of the photoinduced features is irreversible upon pressure release. Published under license by AIP Publishing. [less ▲]

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See detailTemperature-dependent photo-response in multiferroic BiFeO3 revealed by transmission measurements
Meggle, F.; Viret, M.; Kreisel, Jens UL et al

in JOURNAL OF APPLIED PHYSICS (2019), 125(11),

We studied the light-induced effects in BiFeO3 single crystals as a function of temperature by means of optical spectroscopy. Here, we report the observation of several light-induced absorption features ... [more ▼]

We studied the light-induced effects in BiFeO3 single crystals as a function of temperature by means of optical spectroscopy. Here, we report the observation of several light-induced absorption features which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO3. Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO3. Published under license by AIP Publishing. [less ▲]

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See detailRaman signatures of ferroic domain walls captured by principal component analysis
Nataf, G. F.; Barrett, N.; Kreisel, Jens UL et al

in JOURNAL OF PHYSICS-CONDENSED MATTER (2018), 30(3),

Ferroic domain walls are currently investigated by several state-of-the art techniques in order to get a better understanding of their distinct functional properties. Here, principal component analysis ... [more ▼]

Ferroic domain walls are currently investigated by several state-of-the art techniques in order to get a better understanding of their distinct functional properties. Here, principal component analysis (PCA) of Raman maps is used to study ferroelectric domain walls (DWs) in LiNbO3 and ferroelastic DWs in NdGaO3. It is shown that PCA allows us to quickly and reliably identify small Raman peak variations at ferroelectric DWs and that the value of a peak shift can be deduced-accurately and without a priori-from a first order Taylor expansion of the spectra. The ability of PCA to separate the contribution of ferroelastic domains and DWs to Raman spectra is emphasized. More generally, our results provide a novel route for the statistical analysis of any property mapped across a DW. [less ▲]

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See detailRare-earth nickelates RNiO3: thin films and heterostructures
Catalano, S.; Gibert, M.; Fowlie, J. et al

in REPORTS ON PROGRESS IN PHYSICS (2018), 81(4),

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the La, Pr, Nd Sm,., Lu. Nickelates are characterized by a rich ... [more ▼]

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the La, Pr, Nd Sm,., Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices. [less ▲]

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See detailRules and mechanisms governing octahedral tilts in perovskites under pressure
Xiang, H. J.; Guennou, Mael UL; Iniguez, Jorge et al

in PHYSICAL REVIEW B (2017), 96(5),

The rotation of octahedra (octahedral tilting) is common in ABO(3) perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to ... [more ▼]

The rotation of octahedra (octahedral tilting) is common in ABO(3) perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this paper we discover several new mechanisms and formulate a set of simple rules that allow us to understand how pressure affects oxygen octahedral tiltings via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A ion by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered, namely that (i) the octahedral rotations in ABO(3) perovskites with empty low-lying d states on the B site are greatly enhanced by pressure, in order to lower the electronic kinetic energy; (ii) dR/dP is enhanced when the system possesses weak tilt instabilities, and (iii) for the most common phase exhibited by perovskites-the orthorhombic Pbnm state-the in-phase and antiphase octahedral rotations are not automatically both suppressed or both enhanced by the application of pressure because of a trilinear coupling between these two rotation types and an antipolar mode involving the A ions. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure by indirectly controlling the amplitude of octahedral rotations. [less ▲]

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See detailFinite-strain Landau theory applied to the high-pressure phase transition of lead titanate
Troester, A.; Ehsan, S.; Belbase, K. et al

in PHYSICAL REVIEW B (2017), 95(6),

Standard Landau theory coupled to infinitesimal strain allows a concise description of the temperature-driven ferroelectric tetragonal-to-cubic phase transition in PbTiO3 at ambient pressure ... [more ▼]

Standard Landau theory coupled to infinitesimal strain allows a concise description of the temperature-driven ferroelectric tetragonal-to-cubic phase transition in PbTiO3 at ambient pressure. Unfortunately, it fails to cover its high-pressure counterpart at ambient temperature. For example, the experimental transition pressure is vastly underestimated and neither the change from first to second order with increasing pressure nor the unusual pressure dependence of the tetragonal unit cell parameters observed in experiment are reproduced. Here we demonstrate that a combination of density functional theory and a recently constructed finite-strain extension of Landau theory provides a natural mechanism for resolving these discrepancies between theory and experiment. Our approach also allows us to determine the full tetragonal-cubic phase boundary in the (P,T) plane including an estimate of the tricritical point. We show that a careful analysis of the thermal elastic baseline is an essential ingredient to the success of this theory. [less ▲]

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See detailConductivity and Local Structure of LaNiO3 Thin Films
Fowlie, Jennifer; Gibert, Marta; Tieri, Giulio et al

in ADVANCED MATERIALS (2017), 29(18),

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with ... [more ▼]

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth. [less ▲]

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See detailControl of surface potential at polar domain walls in a nonpolar oxide
Nataf, G. F.; Guennou, Mael UL; Kreisel, Jens UL et al

in PHYSICAL REVIEW MATERIALS (2017), 1(7),

Ferroic domain walls could play an important role in microelectronics given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on ... [more ▼]

Ferroic domain walls could play an important role in microelectronics given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential. [less ▲]

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See detailEvolution of defect signatures at ferroelectric domain walls in Mg-doped LiNbO3
Nataf, Guillaume F.; Guennou, Mael UL; Haussmann, Alexander et al

in PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS (2016), 10(3), 222-226

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is ... [more ▼]

The domain structure of uniaxial ferroelectric lithium niobate single crystals is investigated using Raman spectroscopy mapping. The influence of doping with magnesium and poling at room temperature is studied by analysing frequency shifts at domain walls and their variations with dopant concentration and annealing conditions. It is shown that defects are stabilized at domain walls and that changes in the defect structures with Mg concentration can be probed by the shift of Raman modes. We show that the signatures of polar defects in the bulk and at the domain walls differ. (C) 2016 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim [less ▲]

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See detailPhotovoltaics with Ferroelectrics: Current Status and Beyond
Paillard, Charles; Bai, Xiaofei; Infante, Ingrid C. et al

in ADVANCED MATERIALS (2016), 28(26), 5153-5168

Ferroelectrics carry a switchable spontaneous electric polarization. This polarization is usually coupled to strain, making ferroelectrics good piezoelectrics. When coupled to magnetism, they become so ... [more ▼]

Ferroelectrics carry a switchable spontaneous electric polarization. This polarization is usually coupled to strain, making ferroelectrics good piezoelectrics. When coupled to magnetism, they become so-called multiferroic systems, a fi eld that has been widely investigated since 2003. While ferroelectrics are birefringent and non-linear optically transparent materials, the coupling of polarization with optical properties has received, since 2009, renewed attention, triggered notably by low-bandgap ferroelectrics suitable for sunlight spectrum absorption and original photovoltaic effects. Consequently, power conversion effi ciencies up to 8.1 were recently achieved and values of 19.5 were predicted, making photoferroelectrics promising photovoltaic alternatives. This article aims at providing an up-to-date review on this emerging and rapidly progressing fi eld by highlighting several important issues and parameters, such as the role of domain walls, ways to tune the bandgap, consequences arising from the polarization switchability, and the role of defects and contact electrodes, as well as the downscaling effects. Beyond photovoltaicity other polarization-related processes are also described, like light-induced deformation (photostriction) or light-assisted chemical reaction (photostriction). It is hoped that this overview will encourage further avenues to be explored and challenged and, as a byproduct, will inspire other research communities in material science, e.g., so-called hybrid halide perovskites. [less ▲]

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See detailDirect observation of polar tweed in LaAlO3
Salje, Ekhard K. H.; Alexe, Marin; Kustov, Sergey et al

in SCIENTIFIC REPORTS (2016), 6

Polar tweed was discovered in mechanically stressed LaAlO3. Local patches of strained material (diameter ca. 5 mu m) form interwoven patterns seen in birefringence images, Piezo-Force Microscopy (PFM) and ... [more ▼]

Polar tweed was discovered in mechanically stressed LaAlO3. Local patches of strained material (diameter ca. 5 mu m) form interwoven patterns seen in birefringence images, Piezo-Force Microscopy (PFM) and Resonant Piezoelectric Spectroscopy (RPS). PFM and RPS observations prove unequivocally that electrical polarity exists inside the tweed patterns of LaAlO3. The local piezoelectric effect varies greatly within the tweed patterns and reaches magnitudes similar to quartz. The patterns were mapped by the shift of the E-g soft-mode frequency by Raman spectroscopy. [less ▲]

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