April 24, 2024

Study reveals a spontaneous toroidal polar topology in the helielectric nematic state

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Toroidal polar topology swarm appearing in liquid background. Credit: Yang et al.

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Toroidal polar topology swarm appearing in liquid background. Credit: Yang et al.

Magnetic and electric dipoles, objects with two oppositely charged ends, have a similar symmetrical structure. It can therefore be assumed that they exhibit similar internal structures and physical states.

Researchers at the South China University of Technology in China recently showed that this is not always the case, examining the topology of an emergent state of ferroelectric liquid matter with polarized helices, known as the “helielectric nematic state.” Their findings, published in Nature Physicsshow that this state has a spontaneous toroidal polar topology generated through a flexoelectric effect that favors a specific form of spread deformation of the polarizations.

Although ferroelectricity in the nematic phase has been hypothesized for decades, it was only demonstrated experimentally in 2020, by a research group at the University of Colorado in Boulder. This team successfully observed this elusive liquid crystal phase in RM734, a chemical compound synthesized by a research group at the University of Leeds in 2017.

“In collaboration with a chemist, Prof. Huang, our group began to design highly polar and fluidic liquid crystal materials and understand their structure-property relationships in 2019, which had yet to be established at fundamental levels,” Satoshi Aya, the author corresponding to the current article in Nature Physics, told Phys.org. “We built on the pioneering work of Mandle and Goodby (RM734 molecule) and a Japanese group at Kyushu University led by Prof. Kikuchi (DIO molecule). Remarkably, both RM734 and DIO were found in 2017, at about the same time. “

Until recently, Aya and his collaborators compiled a molecular library containing several ferroelectric nematics and new polar liquid crystal materials. By analyzing materials in this library, which now includes approximately 300–400 materials, they were able to identify unexpected polar phases and phase transitions that lead to the formation of previously unknown polar topological structures.

“As a particular case, we found some ferroelectric nematic materials with relatively low shape anisotropy, but high polarity can go directly from the isotropic liquid to the ferroelectric nematic phase in 2020,” Aya explained. “This allowed us to spontaneously generate ferroelectric nematic droplets floating in the isotropic liquid background. Spatial confinement leads to several unique polar topological textures, some known as polar merons, whose formation has been attributed to being driven primarily by polar interactions in ferroelectric fluids.”

The phase previously discovered by Aya is driven by a conventional Frank elasticity as well as flexoelectricity and depolarization field effect. This interesting discovery inspired them to further explore the competition between polar interactions and liquid crystal elasticity in the phase.

Enlarged toroidals. Credit: Yang et al.

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Enlarged toroidals. Credit: Yang et al.

“In our recent study, we initially aimed to understand how chirality would be coupled to flexoelectricity and the depolarization field effect,” said Aya. “Therefore, we doped chiral dopants into the ferroelectric nematic molecule used in one of our previous papers published in Nature Communications. Of course, at first we didn’t expect such a lovely and unprecedented texture to appear.”

In their recent study, Aya and her colleagues employed two primary experimental techniques. First, they used second-harmonic generation interferometric microscopy, taking advantage of a nonlinear optical response that arises in systems where inversion symmetry is broken.

This first method allowed you to visualize the polar orientation field in your sample. The researchers later used a technique called polarized fluorescent microscopy to double-check the orientation field obtained by second-harmonic generation interferometric microscopy.

“Interferometric microscopy and polarized fluorescent microscopy are complementary methods,” explained Aya. “While the former probes the inequivalent (polar) head-to-tail orientation field, the latter captures the equivalent (nonpolar) head-to-tail orientation field.”

Overall, Aya and her collaborators have gathered very interesting observations. First, they showed that unlike crystal-based ferroelectric materials, in which only one or two strong polar interactions dominate and compete with lattice deformation, ferroelectric fluids balance interactions with much greater freedom.

“This delicate balance can lead to multiple influencers determining the topological details,” Aya said. “For example, in simple words, summarizing the current case, the competition between chirality and confinement judges whether an in-plane, untwisted field is favored; flexoelectricity determines where domain walls should be generated; and finally, the field of depolarization dictates what type of polar field orientation should be generated around the domain walls.”

The physical process observed by Aya and his colleagues has several steps, in which different interactions contribute to detailing the final topology of the materials. Their findings suggest that combinations of polar crystal and liquid interactions with different magnitudes can lead to a diverse range of unknown polar topologies. Building on this insight, researchers could soon begin to observe new polar topologies by designing molecules with different shapes and polar properties.

“The second main implication of our findings is that the depolarization field is a vital factor that affects the electric field-driven dynamics in confined ferroelectric fluids,” Aya said. “This message is very important. Imagine that you now have a uniform alignment of the polar orientation field to a specific direction in free space. If we apply a DC electric field antiparallel to the polarization, it is easy to expect that the polarization field will reorient toward the field direction, as verified by the UC Boulder group on ferroelectric nematics in 2020.

“We found that this scenario does not apply to confined nematodes. A similar work, but with a slightly different process, was also published a year before our publication.”

Another enlarged image of toridals. Credit: Yang et al.

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Another enlarged image of toridals. Credit: Yang et al.

Aya and his collaborators found that the topological structure observed by the group at UC Boulder does not apply to confined nematics, where nontrivial depolarization fields can develop through complex fields of spatial polar orientation. In the phase they observed, both the spatial charge due to the scattered deformation of the orientation field and the interracial charge created at interfaces or near orientational singularities act as the source of the depolarization fields.

“On the one hand, you need to realize this issue when you do experiments using ferroelectric fluids, especially when you want to judge which direction the polarization is oriented using the electric field (as the Boulder group did),” said Aya. “On the other hand, as a naive perspective, I think the non-trivial depolarization field can also be considered a tool for generating complex polarization patterns (hence topological engineering or topological switching) that would be impossible using complex electrodes.”

This recent work by Aya and his collaborators could soon pave the way for new studies investigating the polar interaction-driven toroidal polar topology they discovered. Furthermore, it could open new opportunities for the development of ferroelectric and liquid matter switchable optoelectronic devices.

“Of course, it is not easy to clarify the working mechanism behind the formation of unique topologies from the experimental side alone,” Aya said. “In this perspective, along with the development of new molecules with different interaction equilibria mentioned above, we will and have been working on developing a theoretical basis for polar nematic fluids and exploring new polar topologies, adjusting the balance between polar and polar interactions. liquid crystal. Furthermore, designing polar topological networks aimed at topological ferroelectrics is also very challenging.”

In some of their previous studies, the researchers showed that a complex polar orientation field is an advantageous feature for realizing systems that exhibit a nonlinear optical amplification known as phase matching. As part of their future research, they would like to leverage their findings to facilitate the potential development of these systems.

“Polarization engineering in crystal-based ferroelectrics is known to be very difficult,” Aya added. “Thus, developing polarization engineering previously impossible in polar fluids and therefore enabling the fabrication of highly efficient nonlinear optical devices will be one of our follow-up goals.”

More information:
Jidan Yang et al, Toroidal polar topology driven by flexoelectricity in liquid matter helielectrics, Nature Physics (2024). DOI: 10.1038/s41567-024-02439-7

Diary information:
Nature Communications

Nature Physics

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