Electromechanical phenomena in non-centrosymmetric materials subject to strain gradients

Abstract

The flexoelectric effect is a ubiquitous electromechanical phenomenon arising in any dielectric material. Despite of its ubiquitousness, it is relatively insignificant in the macroscopic scale because applying a large strain gradient without fracturing or cracking is a hard task. In nanoscale systems, however, huge gradients as much as $10^5$ ~ $10^7 m^{-1}$ are often present, and the flexoelectric effect becomes one of the major electromechanical couplings. In this work, the photocurrent enhancement at the interfaces bearing large strain gradients due to the flexoelectric effect is addressed. The morphotropic phase boundary of the bismuth ferrite thin film system is used as the model system. The vector piezoresponse force microscopy technique is developed to visualize the complex domain structures in the vicinity of the phase interfaces. The interfaces under large strain gradients show the dipolar feature implying the presence of the depolarization field originated by the flexoelectric effect. Based on the experimental observations and the phase field simulation, the non-linear flexoelectric effect arising at the non-centrosymmetric system is proposed. To deepen the understanding of the flexoelectricity, the ionic chain network model reproducing the flexoelectric effect is introduced. The conventional linear flexoelectric effect and the quadratic flexoelectric effect are manipulated by tuning the model parameters. In the model system, the difference in the interaction strengths among the same kind ions is responsible for the linear flexoelectric effect. Also the broken inversion symmetry generates the quadratic flexoelectric effect, and the marginal strain gradient that the linear and quadratic flexoelectric effects give the same magnitude of polarization is estimated.