Imaging the ultra-small with x-ray nanovision: first observation of 3D topological vortex in a single ferroelectric nanoparticle
(Left) Volume of a ferroelectric nanoparticle with the topological vortex structural phase. (Right) Projections of the toroidal moment of the ferroelectric displacement and 3D rendering of ferroelectric polarization under the application of an external electric field. The lack of mirror symmetry of each spatial component of the toroidal moment and their reproducible evolution under external electric field indicates the electrically controllable chirality which suggests potential applications in novel nanoelectronic devices.
Looking for future technology for integrated electronics? Maybe topology can help. Researchers at Los Alamos National Laboratory (LANL) have observed a three-dimensional vortex structure in a single ferroelectric nanoparticle of Barium Titanate. These results were published in Nature Communications, and exhibit topological structure formed by subtle but long-ordered shifts in the displacement fields. The group led by LANL and‑New Mexico State University (NMSU) Rosen Professor Edwin Fohtung was able to show that the topological vortex can be controlled with external electric field, which opens exciting opportunities in next generation integrated electronics and nanoscale devices.
Dr. Fohtung stated: ‘the visualization of the whole volume behavior under applied perturbations is extremely challenging due to such fundamental limitations of the probes as penetrability, resolution, noninvasiveness etc. But we took on this challenge by combining the unique technique of Bragg coherent x-ray diffractive imaging, available at synchrotron radiation sources and X-ray free electron laser facilities, and a functional sample environment. It all provides us with greater control of the experimental conditions while the high flux of coherent x-rays propagating through and scattering from the sample’s extremities permits to study dynamical responses.’
The experimental work was done at station 34-ID-C of the Advanced Photon Source, which is a synchrotron radiation facility located at Argonne National Laboratory. Bragg coherent diffractive imaging (BCDI) is a lensless imaging technique where the physical lens is replaced by computational phase retrieval algorithms serving as a virtual lens. Only, this virtual lens in not diffraction-limited. When the coherent waves of X-rays are scattered by the sample it forms a reciprocal image in the far-field. The diffraction intensities can be iteratively inverted using algorithms based on Fourier optics and kinematic diffraction theory. A major benefit of this approach is the ability to achieve diffraction limited resolution. In the case of the current published paper, the authors were able to achieve up to 18 nm spatial resolution which allowed them to track the subtle structural changes in the nanoparticle undergoing phase transitions.
The interpretation of experimental results was possible through the aid of theoretical simulations provided by the group of Dr. Turab Lookman in the Theoretical Division of LANL. The group used Landau theory and phase-field models to confirm the experimentally observed behavior of the topological vortex-structure, allowing to decipher complex interactions of the crystal structure, structural phase transitions and the competing energy contributions to the formation of the stable vortex structural phase within a single nanoparticle. Dr. Lookman is also working with Dr. Fohtung to design a form of a theoretically guided phase retrieval algorithm. Dmitry Karpov graduate student from NMSU-TPU, who performed the experiments, said that “this approach may help in providing physically realistic solutions to a host of nanoscale material systems studied using BCDI.”
Published results demonstrate that the vortex core is mobile under an external field and represents 2D projections of a 3D paraelectric nanorod spanning the volume of the nanoparticle. This 3D nanorod can be displaced, erased and created within the monolith of an individual Barium Titanate nanoparticle. Thus making the nanorod a conductive channel in an otherwise insulating nanocrystal.
Both Dr. Fohtung and Dr. Lookman share their anticipation that this research direction will likely open a new avenue in information technology where quantum bits can be designed by quantizing the energy spectra of such nanorod and using not only the orientation of the axial polarization under an applied electric field but also the chirality of the 3D toroidal moment. Other applications include nanomotors, nanoswitches, and materials where controllable optical chiral behavior is desired. The results are also crucial for future studies of the dynamical behavior of material properties under external perturbations such as electromagnetic waves, shock waves especially in light of current LANL plans for its signature XFEL facility known as MaRIE (Matter-Radiation Interactions in Extremes).
About Los Alamos National Laboratory (http://www.lanl.gov)
Los Alamos National Laboratory (LANL), a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, other team members included Z. Liu, D. Xue and P. Balachandran from LANL, T. dos Santos Rollo from Karlsruhe Institute of Technology and R. Harder from Argonne National Laboratory. The work published in the discussed manuscript was supported by the Air Force Office of Scientific Research under Award No. FA9550-14-1-0363 (Program Manager: Dr. Ali Sayir), the Laboratory Directed Research and Development program at Los Alamos National Laboratory (LANL) and by MaRIE. Authors also acknowledge support, in part from the LANSCE Professorship sponsored by the National Security Education Center at LANL under subcontract No. 257827. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (ANL) under contract no. DE-AC02-06CH11357.
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