Switchable chirality

Chirality, a property that describes systems that differ from their mirror images, is one of the most intriguing fundamental phenomena in nature. The very word "chirality" comes from ancient Greek. “Hand” reflects the essence of this phenomenon, indicating the impossibility of combining the right and left palms. Sunflower flowers, seashells, DNA molecules are all examples of chiral systems found in nature. Even our Universe, according to the hypothesis of modern science, can be asymmetrical with respect to the replacement of the right with the left. Materials consisting of chiral molecules are widely used in various fields, from nonlinear optics to biology and pharmaceuticals. Chiral structures form the basis for a new generation of artificial chiral nanomaterials based on magnetic compounds, liquid crystals, and carbon nanotubes. However, chirality is usually a permanent inherent property of a given material.

The team of researchers from University of Picardie (France), Southern Federal University (Russia), pharmaceutical company Life Chemicals Inc (Ukraine), Argonne National Laboratory (USA), supported by the H2020-MSCA actions MELON, MANIC and ENGIMA, in collaboration with several other French and USA colleagues, for the first time showed the possibility of switching chirality in nanosystems. For this, special materials, ferroelectrics, which have the unique property of preserving electrical polarization, were used.

The essence of the discovery lies in the fact that polarization in a ferroelectric particle made in the form of a nanotablet forms an unusual structure, a skyrmion, in which the distribution of fields can be imagined as a kind of polarization fountain swirling clockwise or counterclockwise, causing the chirality of the system. The skyrmions themselves were predicted in the 60s of the XX century in elementary particle physics as topological formations with extraordinary stability. It is this stability that made it possible to propose the use of skyrmions, also found in magnetic systems, as information carriers in computers of the future. Generally speaking, there are four possible states of chiral skyrmions, which can correspond to four bits of information. These states are characterized by both chirality and directions of polarization in the center of the skyrmion and it is more convenient to represent them as different positions of the right and left palms with different directions of the thumb (see figure).

Fig. 1.a. Switching between different skyrmion states is carried out by instantaneous impulses of electric voltage applied to the capacitor plates. b. The various states of chiral skyrmions can be imagined using the fingers of the right and left palms. The direction of the thumb, up or down, corresponds to the polarization in the center of the skyrmion, and the direction of the remaining bent fingers, clockwise or counterclockwise, indicates the rotation of the polarization in the equatorial part of the skyrmion.

A feature of ferroelectric skyrmions is that, being placed in a capacitor, they interact with an electric field and can switch incredibly quickly with an electric pulse. The research team has shown that a properly selected set of impulses makes it possible to implement various sequences of states in such structures. Moreover, the switching dynamics have been found to involve a number of other intermediate states and are surprisingly similar to the process of cell division (see video). This will allow using ferroelectric cells to simulate biological processes and, in particular, to create so-called neuromorphic computers that mimic the human brain. The method proposed by scientists opens up unprecedented opportunities for fundamentally new information technologies, as well as for creating a new generation of tunable optoelectronic systems based on controlled chirality.

The research of the international team, led by Professor I. Lukyanchuk and consisting of Yu. Tikhonov, S. Kondovych, J. Mangeri, M. Pavlenko, L. Baudry, A. Sené, A. Galda, S. Nakhmanson, O. Heinonen, A. Razumnaya, and V. M. Vinokur are published in the Nature Group journal, Scientific Reports, 10, 8657 (2020)

Video: Switching between different skyrmion states resembles cell division and can be used in the so-called neuromorphic computers of the future.