Anomalous orientation of ferroelectric liquid crystal films in an electric field

An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric polarization, 10⁴ times greater than that in bulk liquid-crystal samples, in the film. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 414–418 (25 September 1997)     

Scaling effect of flexoelectric (Ba,Sr)TiO3 microcantilevers

The flexoelectric microcantilever offers an alternative approach for the development of micro/nano‐sensors. The transverse flexoelectric coefficients µ₁₂ of barium strontium titanate microcantilevers were measured at room temperature, and found to keep the same value of 8.5 µC/m for microcantilevers with thickness ranging from 30 µm to 1.4 mm. The calculated effective piezoelectric coefficient and electrical energy density of flexoelectric cantilevers are superior to those of their piezoelectric counterparts, suggesting that the flexoelectricity‐induced polarization can be significantly increased as structures are scaled down due to the scaling effect of strain gradient, holding promise for flexoelectric micro/nano cantilever sensing applications. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

Effect of GMR and Magnetization Reversal on Microwave Absorption

Low-field microwave absorption has been measured as a function of magnetic field in a series of thin film structures exhibiting or not exhibiting the giant magnetoresistance effect (GMR). Although a close correlation has been found between the microwave absorption and GMR, an additional absorption due to magnetization reversal is shown to have substantial effect on the overall microwave response.     

Giant flexoelectricity of bent-core nematic liquid crystals

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.     

Prediction of flexoelectricity in BaTiO3 using molecular dynamics simulations

Flexoelectric effect, referring to the strain gradient induced polarization, widely exists in dielectric materials, but its molecular dynamics has not been studied so much so far. In this work, the radial distribution function of BaTiO₃ and the phase transition temperatures have been investigated, and the results show that the core-shell potential model is effective and the structure of BaTiO₃ is stable in a temperature range of 10 K-150 K. Molecular dynamics simulated hysteresis loops of BaTiO₃ show that anisotropy can play an important role in the coercive field. Based on the rational simulation process, the effects of cantilever beam bent angle and fixed length on the polarization are analyzed. It is found that the small bent angle of the curved cantilever beam can give a proportional relationship with a fixed end length and a non-linear relationship is presented when the bent angle is much larger. The prediction of flexoelectric coefficient in BaTiO₃ is 18.5 nC/m. This work provides a computational framework for the study of flexoelectric effect by using molecular dynamics.     

Detection of the contribution of the inverse flexoelectric effect to the photorefractive response in a bismuth titanium oxide single crystal

Relations determining the contribution of the inverse flexoelectric effect to the photorefractive response have been determined for the case of the interaction of a steady pump wave conserving left-hand circular polarization with a phase-modulated signal wave that has circular polarization of the opposite sign on the input face of a crystal that has the (100) orientation and symmetry group 23. The experimental studies of such an interaction in a Bi₁₂TiO₂0:Fe,Cu crystal make it possible to estimate its flexoelectric coefficient as f ₁₁ = 5.3 nC/m.     

Flexoelectric blue phases

We describe the occurrence and properties of liquid crystal phases showing two dimensional splay and bend distortions which are stabilized by flexoelectric interactions. These phases are characterized by regions of locally double splayed order separated by topological defects and are thus analogous to the blue phases of cholesteric liquid crystals. We present a mean-field analysis based upon the Landau-de Gennes Q-tensor theory and construct a phase diagram for flexoelectric structures using analytic and numerical results. We stress the similarities and discrepancies between the cholesteric and flexoelectric cases.     

Increase in the electric polarization in a multiferroic induced by the flexoelectric effect

It has been shown that the electric polarization in multiferroic crystals exhibiting the magnetoelectric effect can be increased in the presence of the flexoelectric effect. The polarization in manganite-based perovskite-like multiferroics can be increased if the long axes of the rhombohedra formed upon the rhombohedral distortion of MnO₆ octahedra can undergo fan spatial rotations, leading to the flexoelectric effect.     

Threshold effects in homeotropically oriented nematic liquid crystals in an external electric field

The electric field-induced orientational transition in a homeotropically oriented nematic liquid crystal cell is investigated. The interaction with the field as a result of anisotropy of the permittivity and flexoelectric polarization is taken into account. For an arbitrary energy of interaction of the nematic with the substrate simple relations are derived for determining the threshold characteristics of the phase transition. It is shown that, in contrast with planar orientation, in fields above a critical value a periodic structure can occur only by virtue of the flexoelectric effect. The resulting dependences for the threshold parameters in the given geometry are exceptionally useful for determining experimentally the surface energy and the difference in the flexoelectric coefficients. Zh. éksp. Teor. Fiz. 116, 543–550 (August 1999)