Polar topology has great application potential in the fields of high-density memory and new electronic devices, which is a research hotspot of ferroelectric materials. Exploring the influence of external fields such as electric field and stress excitation on topological polar structure and the dynamic process is very important for their practical application. In recent years, thanks to the development of material growth technology and microscopic characterization technology, by regulating the stress boundary conditions and electric field of ferroelectric materials, researchers have prepared a variety of polar topological structures in complex oxide superlattices and ferroelectric thin films, such as polar vortex (Polar Vortex), polar vertex (Polar Vertex), polar Skyrmion (Polar Meron) and so on, these polar topologies are promising for ultra-high density memory devices. Relevant research results:

1, Nat. Commun. 2020, 11, 1840

2, Proc. Natl. Acad. Sci. 2020, 117, 18954

3, Phys. Rev. Lett. 2022, 129, 107601

4, Nature Materials, 2020,19 (8): 881-886

Recently, professor Gao Peng of Peking University and his collaborators studied the dynamic characteristics of a single polar topological structure using the triple polar vertex structure as a model. Triple polar vertices are generally stable on an insulating substrate, but the insulating substrate makes it impossible to effectively apply an electric field and thus cannot achieve the study of electric field regulation. They use the spontaneous interdiffusion of interface elements to adjust the degree of polarization charge shielding of the interface, which can stabilize this polar topology and apply an electric field. The crystal structure and element distribution at the PbTiO3/SrRuO3 interface indicate that the diffusion of Ti into the SrRuO3 partially replaces the Ru element, resulting in a SrTiO3 insulating layer (Fig. 2) of about 2 unit cell thickness at the PbTiO3/SrTiO3 interface, providing the required electric field boundary conditions for the formation of the triple polarized vertex structure. At the same time, the thin SrTiO3 layer provides the necessary conditions for applying an electric field to the ferroelectric thin film through the probe and the SrRuO3 bottom electrode. In addition, from the perspective of lattice mismatch, they believe that the tensile stress from the DyScO3 substrate also provides a stress boundary condition for the formation of the triple polar vertex. The effect of SrTiO3 layer is verified by the phase field simulation results.

Then they used in situ transmission electron microscopy to study the generation and movement of isolated tripolar vertices under the action of electric field. As shown in the figure below, using the in-situ transmission electron microscope electrical measurement system (PicoFemto produced by Anhui Zeyou Technology Co., Ltd.®Scanning probe type transmission electron mirror in situ electrical measurement series products), tungsten tip as the upper electrode to apply a bias on PbTiO3, the bottom electrode SrRuO3 grounded, real-time observation of the evolution of the domain structure under the action of the electric field (Fig. 3). They found that when the electric field-induced 180 ° ferroelectric domain wall extends to the interface, the triplet polar vertex structure begins to nucleate. Then the isolated triple polar vertex is driven by an external electric field with the 180 ° domain wall moving back and forth, reaching a high speed of about 629 nm/s. Phase-field simulations reproduce the process of nucleation and movement of an isolated tripolar vertex structure driven by an electric field. The results of this study show the ability of the isolated triple polar vertex structure to move under the electric field, and provide useful information for the dynamic characteristics and applications of the single polar topology.

The research result was published in Nature Communications under the title of "Electric-field control of the nucleation and motion of isolated three-fold polar vertices. Professor Gao Peng of the School of Physics of Peking University is the corresponding author of the paper, and Li Mingqiang, a graduate student of Peking University, is the first author of the paper. The cooperative researchers include the team of Professor Chu, Ying-Hao of Yangming Jiaotong University in Taiwan, the team of Professor Chen, Long-Qing of Pennsylvania State University in the United States, and the team of Bai Xuedong, researcher of Institute of Physics, Chinese Academy of Sciences.

Original Link: https://www.nature.com/articles/s41467-022-33973-8