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1. Phase-field model for a weakly compressible soft layered material: morphological transitions on smectic–isotropic interfaces

   https://doi.org/10.1039/D1SM00488C  

   Vitral, Eduardo; Leo, Perry H.; Viñals, Jorge (June 2021, Soft Matter)

   null (Ed.)

   A coupled phase-field and hydrodynamic model is introduced to describe a two-phase, weakly compressible smectic (layered phase) in contact with an isotropic fluid of different density. A non-conserved smectic order parameter is coupled to a conserved mass density in order to accommodate non-solenoidal flows near the smectic–isotropic boundary arising from density contrast between the two phases. The model aims to describe morphological transitions in smectic thin films under heat treatment, in which arrays of focal conic defects evolve into conical pyramids and concentric rings through curvature dependent evaporation of smectic layers. The model leads to an extended thermodynamic relation at a curved surface that includes its Gaussian curvature, non-classical stresses at the boundary and flows arising from density gradients. The temporal evolution given by the model conserves the overall mass of the liquid crystal while still allowing for the modulated smectic structure to grow or shrink. A numerical solution of the governing equations reveals that pyramidal domains are sculpted at the center of focal conics upon a temperature increase, which display tangential flows at their surface. Other cases investigated include the possible coalescence of two cylindrical stacks of smectic layers, formation of droplets, and the interactions between focal conic domains through flow. 

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2. Observation of a uniaxial ferroelectric smectic A phase

   https://doi.org/10.1073/pnas.2210062119  

   Chen, Xi; Martinez, Vikina; Nacke, Pierre; Korblova, Eva; Manabe, Atsutaka; Klasen-Memmer, Melanie; Freychet, Guillaume; Zhernenkov, Mikhail; Glaser, Matthew A.; Radzihovsky, Leo; et al (November 2022, Proceedings of the National Academy of Sciences)

   We report the observation of the smectic A F , a liquid crystal phase of the ferroelectric nematic realm. The smectic A F is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 10 5 V / m is observed. The SmA F phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)–smectic Z A (SmZ A )–ferroelectric nematic (N F )–SmA F phase sequence, and 7N/DIO, exhibiting an N–SmZ A –SmA F phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers. 

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3. The smectic Z _A phase: Antiferroelectric smectic order as a prelude to the ferroelectric nematic

   https://doi.org/10.1073/pnas.2217150120  

   Chen, Xi; Martinez, Vikina; Korblova, Eva; Freychet, Guillaume; Zhernenkov, Mikhail; Glaser, Matthew A.; Wang, Cheng; Zhu, Chenhui; Radzihovsky, Leo; Maclennan, Joseph E.; et al (February 2023, Proceedings of the National Academy of Sciences)

   We have structurally characterized the liquid crystal (LC) phase that can appear as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-y-old void in the taxonomy of smectic LCs and which we term the “smectic Z A ,” is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer with a 180 Å period. A Landau free energy, originally derived from the Ising model of ferromagnetic ordering of spins in the presence of dipole–dipole interactions, and applied to model incommensurate antiferroelectricity in crystals, describes the key features of the nematic–SmZ A –ferroelectric nematic phase sequence. 

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4. Observations of a streak texture in the hybrid- Q1 aligned smectic-C phase

   https://doi.org/10.1039/c7sm02129a  

   Nemitz, I.R.; Gryn, I.; Boudet, N.; Lemieux, R.P.; Goldmann, M.; Zappone, B.; Petschek, R.G.; Rosenblatt, C.; Lacaze, E. (December 2017, Soft matter)

   A novel structure was observed below the smectic-A–smectic-C phase transition in a very thin open cell having an air interface above and enforced planar anchoring at the substrate below. The structure appears as periodic dark and light streaks running perpendicular to the oily streaks, which are present in the smectic-A phase [D. Coursault et al., Soft Matter, 2016, 12, 678]. These new streaks, which we call ‘‘soapy streaks’’, form by extending from one oily streak to the next in discrete steps, eliminating optical evidence at visible wavelengths of the oily streaks. At lower temperatures the streaks can undulate and exhibit a sawtooth-like structure; such a structure is chiral in two dimensions. A possible scenario for the origin of these streaks is presented. 

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5. Interplay of Nanoscale Strain and Smectic Susceptibility in Kagome Superconductors

   https://doi.org/10.1103/PhysRevX.15.021074  

   Wang, Yidi; Li, Hong; Cheng, Siyu; Zhao, He; Ortiz, Brenden R; Salinas, Andrea Capa; Wilson, Stephen D; Wang, Ziqiang; Zeljkovic, Ilija (May 2025, Physical Review X)

   Exotic quantum solids can host electronic states that spontaneously break rotational symmetry of the electronic structure, such as electronic nematic phases and unidirectional charge density waves (CDWs). When electrons couple to the lattice, uniaxial strain can be used to anchor and control this electronic directionality. Here, we reveal an unusual impact of strain on unidirectional “smectic” CDW orders in kagome superconductors ${\mathrm{AV}}_3{\mathrm{Sb}}_5$using spectroscopic-imaging scanning tunneling microscopy. We discover local decoupling between the smectic electronic director axis and the direction of anisotropic strain. While the two can generally be aligned along the same direction in regions of a small CDW gap, the tendency for alignment decreases in regions where the CDW gap is the largest. This feature, in turn, suggests nanoscale variations in smectic susceptibility, which we attribute to a combination of local strain and electron correlation strength. Overall, we observe an unusually high decoupling rate between the smectic electronic director of the three-state Potts order and anisotropic strain, revealing weak smectoelastic coupling in the CDW phase of kagome superconductors. This finding is phenomenologically different from the extensively studied nematoelastic coupling in the Ising nematic phase of Ising nematic phase of Fe-based superconductor bulk single crystals, providing a contrasting picture of how strain can control electronic unidirectionality in different families of quantum materials. Published by the American Physical Society2025 

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