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Posted: Apr 20, 2017

Electronic 'cyclones' at the nanoscale

(Nanowerk News) 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.
By controlling the layered structure, scientists observed discrete spiral regions in the ionic structure of multiple layered materials. The image shows an atomic-scale, high-resolution scanning transmission electron micrograph of the layered structures, with an overlay (yellow arrows) showing the local ionic movement, demonstrating the spiral configurations or “polar vortices.” The materials are made of very thin layers of electrically ordered lead titanate and electrically disordered strontium titanate. (Image: R. Ramesh, Lawrence Berkeley National Laboratory)
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.
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 (Nature, "Observation of polar vortices in oxide superlattices").
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.
Source: Department of Energy, Office of Science
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