Ferroelectric Nematic Liquid Crystals

 

We have identified a liquid crystal that has the long sought-after ferroelectric nematic phase [1]. Nematic liquid crystals have been a hot topic of materials research since the 1970s, when it was discovered that their curious mix of fluid-like and solid-like behaviors could be used to control light and make low-voltage, low-power, lightweight information displays (LCDs). Ferroelectric behavior has been observed previously in tilted chiral smectics and in bent-core smectics but a polar nematic phase has proven elusive for more than 100 years.

In 2019, group members began examining RM734, a new molecule created and studied over the past several years by a team of British and Slovenian scientists who reported it to have, in addition to a conventional nematic LC phase, an unusual, new phase at lower temperatures which they attributed to the wedge shape of the molecules. In our test cells, the higher temperature nematic phase looked quite typical under the microscope, but in certain sample geometries the second phase was astonishingly different, responding rapidly, in a palette of striking colors, to weak electric fields in regions far from the nominal active area of the cell. This observation led to the realization that the phase was 100 to 1000 times more responsive to field than the usual nematic, indicating strong polar order. A search for the tell-tale domains of opposite polarization showed them spontaneously appearing as the sample was cooled from the nematic, confirming that the second phase was indeed a ferroelectric nematic fluid. Computer simulations by the University of Utah group suggest that the phase has almost perfect polar order. The ferroelectric nematic responds readily to applied electric fields and displays a host of novel electrooptic and hydrodynamic properties. Combining ferroelectricity with liquid crystal behavior promises exciting new science and technology for the 21st century. (6/20)

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radiative metamaterial.

Figure 1. Macroscopic domains of opposite polarization appear spontaneously when the liquid crystal is cooled below the conventional nematic phase range. The domain boundaries are lens-shaped and only partially visible in this polarized light microscope image.

radiative metamaterial.

Figure 2. The response of ferroelectric domains to applied electric fields depends on their polarization direction: a field pointing opposite the polarization direction reorients the liquid crystal, causing the birefringence color to change from orange to green.

 

References

[1] X. Chen et al., Proc. Natl. Acad. Sci. USA 117, 14021 (2020). [download]
The article is accompanied by a Commentary by Oleg Lavrentovich.
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