Techniques to handle defects in ferroelectric nanostructures
Researchers in the Department of Physics at KAIST have recently developed an innovational technique to manipulate topological defects in ferroelectric nanoplates. Topological defects in ferroelectric materials have been rarely observed because it requires a considerable amount of energy to deform the lattice coupled with the electric polarization. The research team discovered that an inhomogeneous strain field successfully stabilizes topological defects in ferroelectric nanostructures. Using angle-resolved piezo-response force microscopy, they directly identified the existence of topological defects. Moreover, the total “winding number” of the topological texture can be configured from -1 to 3 by partial domain switching, offering a useful concept for electric topological “defectronics”.
The topological defects in condensed matter have attracted a tremendous amount of scientific and technological attention. Further study will allow for more applications to be discovered based on the understanding of novel functional properties. In particular, the discovery of quasiparticles, which are localized and energetically quantized in real space, allows for the conception of new information storage media and efficient logic devices as carriers of energy and information.
These quasiparticles are inevitable due to the topological structures inherent in materials and their existence can be stable and protected from external perturbations. In principle, ferroelectrics, in which spontaneous electric dipoles are aligned in parallel, can have topological defects at a smaller size and can be controlled with less energy than magnetic materials.
However, experimental research has not been carried out sufficiently on how to stabilize and control electric topological defects. Topological defects in ferroelectric materials have been rarely observed because it requires a considerable energy to deform the lattice coupled with the ferroelectric property through the mechanoelectric effect.
Moreover, direct real-space detection techniques have been lacking. Observing the behavior of electric vortices at spatial resolution on the nanoscale in a non-destructive manner is required.
Recently, a research team in the Department of Physics at KAIST, led by Prof. Chan-Ho Yang, has discovered that the strain-gradient-induced flexoelectric effect can stabilize topologically non-trivial ferroelectric textures. His team, through significant contributions by Mr. Kwang-Eun Kim, found that an epitaxial ferroelectric square nanoplate of bismuth ferrite enables five discrete levels for the ferroelectric topological invariant of the entire system. This is because of its peculiar radial quadrant domain texture and its inherent domain wall chirality. The nanoplate was subjected to a large strain gradient (as much as 105 m−1) and this is associated with misfit strain relaxation.
The total winding number of the topological texture can be configured from −1 to 3 by selective non-local electric switching of the quadrant domains. By using angle-resolved piezoresponse force microscopy in conjunction with local winding number analysis, they directly identify the existence of vortices and anti-vortices, observe pair creation and annihilation, and manipulate the net number of vortices.
The findings of this research offer not only a useful concept for multi-level topological defect memory, but also useful insights into topological defect pair creation, electric frustration and programmable charged domain walls. This study was performed in collaboration with researchers from POSTECH, Pennsylvania State University, and the University of California at Berkeley.
The work was supported by the National Research Foundation of Korea via the Creative Research Initiative Center for Lattice Defectronics and the Center for Quantum Coherence in Condensed Matter. The result was recently published in Nature Communications (January 26, 2018).
* Website: https://www.nature.com/articles/s41467-017-02813-5 (paper link)
* lab webpage : http://oxide.kaist.ac.kr/