By: Alenka Mertelj
From: Jozef Stefan Institute, Ljubljana, Slovenia
At: Online - Zoom (https://videoconf-colibri.zoom.us/j/82247724777)
Nematic liquid crystals have been known for more than a century, but it was not until the 60s-70s that, with the development of room temperature nematic liquid crystals, they becamewidely used in applications. Polar nematic phases have been long-time predicted, but have onlybeen experimentally realized recently. In this talk, I will focus on a recently discoveredferroelectric splay nematic phase. A few years ago, R. J. Mandle et al[1,2] designed polar liquid-crystalline materials which exhibits two distinct nematic mesophases, the usual uniaxialnematic phase and the lower temperature splay nematic phase. The weakly first order phasetransition between the phases is accompanied by pretransitional behavior, manifested asstrong splay orientational fluctuations. We showed that the transition between the uniaxial andthe new nematic phase is a ferroelectric-ferroelastic phase transition, in which flexoelectriccoupling causes simultaneous occurrence of diverging behavior of electric susceptibility and ofinstability towards splay deformation . The phase transition can be quantitatively describedby a Landau-de Gennes type of macroscopic theory. To understand the molecular origin ofpolar order, we compared two materials of similar chemical structure, one exhibiting the polarnematic phase and one not, by using experiments and simulations. We found out that just asubtle change in the molecular structure enables denser packing of the molecules when theyexhibit polar order which indicates that reduction of excluded volume is a driving mechanismof the polar nematic phase formation.
 R. J. Mandle, et al., Chem. Eur. J. 23, 14554 (2017), doi: 10.1039/C7CP00456G.
 R. J. Mandle, et al., Physical Chemistry Chemical Physics 19, 11429 (2017), doi:10.1002/chem.201702742.
 A. Mertelj, et al., Phys. Rev. X 8, 041025 (2018), doi: 10.1103/PhysRevX.8.041025.
 N. Sebastián, et al., Phys. Rev. Lett. 124, 037801 (2020), doi:10.1103/PhysRevLett.124.037801.
 R. J. Mandle, et al., arXiv:2011.02722 [cond-mat.soft] (2020).