Flexoelectric effect in finite samples

Static flexoelectric effect in a finite sample of a solid is addressed in terms of phenomenological theory for the case of a thin plate subjected to bending. It has been shown that despite an explicit asymmetry inherent to the bulk constitutive electromechanical equations which take into account the flexoelectric coupling, the electromechanical response for a finite sample is "symmetric". "Symmetric" means that if a sensor and an actuator are made of a flexoelectric element, performance of such devices can be characterized by the same effective piezoelectric coefficient. This behavior is consistent with the thermodynamic arguments offered earlier, being in conflict with the current point of view on the matter in literature. This result was obtained using standard mechanical boundary conditions valid for the case where the polarization vanishes at the surface. It was shown that, for the case where there is the polarization is nonzero at the surface, the aforementioned symmetry of electromechanical response may be violated if standard mechanical boundary conditions are used, leading to a conflict with the thermodynamic arguments. It was argued that this conflict may be resolved when using modified mechanical boundary conditions. It was also shown that the contribution of surface piezoelectricity to the flexoelectric response of a finite sample is expected to be comparable to that of the static bulk contribution (including the material with high values of the dielectric constant) and to scale as the bulk value of the dielectric constant (similar to the bulk contribution). This finding implies that if the experimentally measured flexoelectric coefficient scales as the dielectric constant of the material, this does not imply that the measured flexoelectric response is controlled by the static bulk contribution to the flexoelectric effect

Surface polar states and pyroelectricity in ferroelastics induced by flexo-roto field

Theoretical analysis based on the Landau-Ginzburg-Devonshire (LGD) theory is used to show that the joint action of flexoelectric effect and rotostriction leads to a large spontaneous in-plane polarization (~ 1-5 muC/cm2) and pyroelectric coefficient (~10^-3 C/m2K) in the vicinity of surfaces of otherwise non-ferroelectric ferroelastics, such as SrTiO3, with static octahedral rotations. The origin of the improper polarization and pyroelectricity is an electric field we name flexo-roto field whose strength is proportional to the convolution of the flexoelectric and rotostriction tensors with octahedral tilts and their gradients. Flexo-roto field should exist at surfaces and interfaces in all structures with static octahedral rotations, and thus it can induce surface polar states and pyroelectricity in a large class of otherwise nonpolar materials.Comment: 16 pages, 3 figures, 1 table, supplementary material

Origin Of the enhanced flexoelectricity of relaxor ferroelectrics

We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals with compositions at the relaxor-ferroelectric phase boundary. The crystals display very large flexoelectricity, with flexocoupling coefficients an order of magnitude bigger than the theoretical upper limit set by the theories of Kogan and Tagantsev. This enhancement persists in the paraphrase up to a temperature T* that coincides with the start of elastic softening in the crystals. Analysis of the temperature dependence and cross-correlation between flexoelectric, dielectric and elastic properties indicates that the large bendinginduced polarization of relaxor ferroelectrics is not caused by intrinsically giant flexoelectricity, but by the reorientation of polar nanotwins that become ferroelastically active below T*

Spin singlet small bipolarons in Nb-doped BaTiO3

The magnetic susceptibility and electrical resistivity of n-type BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the electrical resistivity around 15 K indicates a crossover from band to hopping transport of the charge carriers, a phenomenon expected but rarely observed in real polaronic systems.Comment: 5 pages, 4 figure

Interfacial Polarization and Pyroelectricity in Antiferrodistortive Structures Induced by a Flexoelectric Effect and Rotostriction

Theoretical analysis based on the Landau-Ginzburg-Devonshire (LGD) theory is used to show that the product effect of flexoelectricity and rotostriction can lead to a spontaneous polarization in the vicinity of antiphase boundaries, ferroelastic twin walls, surfaces and interfaces in the octahedrally tilted phase of otherwise non-ferroelectric perovskites such as CaTiO3, SrTiO3, and EuTiO3. As an example, we numerically demonstrate a large spontaneous polarization on the order of 1-5muC/cm2 at the SrTiO3 antidistortive phase boundaries at temperatures lower than the antiferrodistortive structural phase transition temperature of ~105 KComment: 61 page, 24 figures, 1 table; Extended version including twin domain wall

Comment on ``Conduction states in oxide perovskites: Three manifestations of Ti3+^{3 +} Jahn-Teller polarons in barium titanate''

In this comment to [S. Lenjer, O. F. Schirmer, H. Hesse, and Th. W. Kool, Phys. Rev. B {\bf 66}, 165106 (2002)] we discuss the electronic structure of oxygen vacancies in perovskites. First principles computations are in favour of rather deep levels in these vacancies, and Lenjer et al suggest that the electrons' interaction energy is negative, but data on electroconductivity are against.Comment: 2 pages, no figure

Role of gallium diffusion in the formation of a magnetically dead layer at the Y3Fe5O12/Gd3Ga5O12 epitaxial interface

We have clarified the origin of a magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700 degrees C onto a gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG/GGG interface is demonstrated by means of composition depth profiling performed by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and x-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of a Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and x-ray reflectometry studies of the YIG/GGG epitaxial system

Giant Enhancement of Surface Second Harmonic Generation in BaTiO_3 due to Photorefractive Surface Wave Excitation

We report observation of strongly enhanced surface SHG in BaTiO_3 due to excitation of a photorefractive surface electromagnetic wave. Surface SH intensity may reach 10^{-2} of the incident fundamental light intensity. Angular, crystal orientation and polarization dependencies of this SHG are presented. Possible applications of this effect in nonlinear surface spectroscopy are discussed.Comment: 5 pages, 6 figures, submitted to Physical Review Letters on the 3/29/199