Electronic “Cyclones” at the Nanoscale

New state of matter holds promise for ultracompact data storage and processing.

Image courtesy of R. Ramesh, Lawrence Berkeley National Laboratory

The Science

It has long been thought that spiral electronic structures called “polar vortices” would not be produced in electrically ordered materials. The idea persisted although scientists observed a similar phenomenon on the surface of some magnetic materials. However, through highly controlled multilayer thin film synthesis, scientists controlled various competing atomic forces to allow these regions to form.

The Impact

Polar vortices can serve as a precursor to new phenomena in materials. The materials could be vital for ultra-low energy electronic devices. The vortices in electrically ordered, or ferroelectric, materials are similar to phenomena in magnetic materials — local, rotating magnetic regions called skyrmions.

Summary

Recently, scientists discovered that in some magnetic materials, magnetic spirals, called skyrmions, could form on the material’s surface. These skyrmions can retain their organization as they are moved over macroscopic distances, which makes them excellent candidates for ultra-low power electronics applications. Until now, scientists assumed this phenomenon was unique to magnetic materials. Ferroelectrics are a class of non-magnetic materials that have local regions where the atomic charge orients spontaneously (below a critical temperature) to produce regions where the electric “poles” of the atoms (like the “poles” of the Earth) align, resulting in electrical polarization. The direction of this aligned orientation, called polarization, in a ferroelectric can be modified by application of an electric field. However, scientists did not think that regions of rotating electrical polarization in ferroelectrics (similar to the skyrmions observed in magnetic materials) could be formed. By careful synthesis of ultrafine layered structures or superlattices, built from layers of lead titanate and strontium titanate (controlled down to a thickness of 4 angstroms (for comparison, a cold virus is 300 angstroms across)), Lawrence Berkeley National Laboratory researchers discovered they could create electrical spirals, called polar vortices, similar to the rotating vortices observed in magnetic systems. Changing the superlattice constituent layer thicknesses allows one to tune the characteristics of the rotating regions and produce entirely new phenomena in these materials. The fact that these polar vortices can display emergent behavior in their electronic, optical, magnetic, and other properties, such as a sense of handedness (chirality) suggests the possibility of new properties and potential applications for the material.

Contact

R. Ramesh
Lawrence Berkeley National Laboratory
[email protected]

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Lawrence Berkeley National Laboratory); National Science Foundation (University of California Berkeley and Pennsylvania State University); and Army Research Office (University of California Berkeley). This research used resources at the Advanced Light Source, Advanced Photon Source, and Molecular Foundry, U.S. Department of Energy Office of Science user facilities.

Publications

A.K. Yadav, C.T. Nelson, S.L. Hsu, Z. Hong, J.D. Clarkson, C.M. Schlepütz, A.R. Damodaran, P. Shafer, E. Arenholz, L.R. Dedon, D. Chen, A. Vishwanath, A.M. Minor, L.Q. Chen, J.F. Scott, L.W. Martin, and R. Ramesh, “Observation of polar vortices in oxide superlattices.” Nature 530, 198-201 (2016). [DOI: 10.1038/nature16463]