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1. Magnetic field-induced Freedericksz transition in a chiral liquid crystal

   Missaoui, Amine ; Susser, Adam L. ; Aharoni, Hillel ; Rosenblatt, Charles ( March 2023 , Applied Physics Letters)

   A wedge cell made of homeotropically treated glass plates is filled with a chirally doped nematic liquid crystal. When a sufficiently large magnetic field is applied in the cell plane, a bend-like distortion occurs above a Fréedericksz threshold field H th . H th is reduced from the achiral case because of a field-induced bend distortion that facilitates a chiral twist distortion. Measurements of H th vs sample thickness are reported and compared favorably with a theoretical model presented herein. A further theoretical comparison is made between H th and the electric-field-induced transition in a geometry, exhibiting a 2π azimuthal degeneracy. The results may have technological implications in, for example, in-plane switching devices. 

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2. Rods in a lyotropic chromonic liquid crystal: emergence of chirality, symmetry-breaking alignment, and caged angular diffusion

   https://doi.org/10.1039/D1SM01209F  

   Ettinger, Sophie ; Dietrich, Clarissa F. ; Mishra, Chandan K. ; Miksch, Cornelia ; Beller, Daniel A. ; Collings, Peter J. ; Yodh, A. G. ( January 2022 , Soft Matter)

   In lyotropic chromonic liquid crystals (LCLCs), twist distortion of the nematic director costs much less energy than splay or bend distortion. This feature leads to novel mirror-symmetry breaking director configurations when the LCLCs are confined by interfaces or contain suspended particles. Spherical colloids in an aligned LCLC nematic phase, for example, induce chiral director perturbations (“twisted tails”). The asymmetry of rod-like particles in an aligned LCLC offer a richer set of possibilities due to their aspect ratio ( α ) and mean orientation angle (〈 θ 〉) between their long axis and the uniform far-field director. Here we report on the director configuration, equilibrium orientation, and angular diffusion of rod-like particles with planar anchoring suspended in an aligned LCLC. Video microscopy reveals, counterintuitively, that two-thirds of the rods have an angled equilibrium orientation (〈 θ 〉 ≠ 0) that decreases with increasing α , while only one-third of the rods are aligned (〈 θ 〉 = 0). Polarized optical video-microscopy and Landau–de Gennes numerical modeling demonstrate that the angled and aligned rods are accompanied by distinct chiral director configurations. Angled rods have a longitudinal mirror plane (LMP) parallel to their long axis and approximately parallel to the substrate walls. Aligned rods have a transverse and longitudinal mirror plane (TLMP), where the transverse mirror plane is perpendicular to the rod's long axis. Effectively, the small twist elastic constant of LCLCs promotes chiral director configurations that modify the natural tendency of rods to orient along the far-field director. Additional diffusion experiments confirm that rods are angularly confined with strength that depends on α . 

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3. Electric Mode Excitation in the Atmosphere by Magnetospheric Impulses and ULF Waves

   https://doi.org/10.3389/feart.2020.619227  

   Pilipenko, V. A. ; Fedorov, E. N. ; Martines-Bedenko, V. A. ; Bering, E. A. ( January 2021 , Frontiers in Earth Science)

   null (Ed.)

   Variations of vertical atmospheric electric field E z have been attributed mainly to meteorological processes. On the other hand, the theory of electromagnetic waves in the atmosphere, between the bottom ionosphere and earth’s surface, predicts two modes, magnetic H (TE) and electric E (TH) modes, where the E-mode has a vertical electric field component, E z . Past attempts to find signatures of ULF (periods from fractions to tens of minutes) disturbances in E z gave contradictory results. Recently, study of ULF disturbances of atmospheric electric field became feasible thanks to project GLOCAEM, which united stations with 1 sec measurements of potential gradient. These data enable us to address the long-standing problem of the coupling between atmospheric electricity and space weather disturbances at ULF time scales. Also, we have reexamined results of earlier balloon-born electric field and ground magnetic field measurements in Antarctica. Transmission of storm sudden commencement (SSC) impulses to lower latitudes was often interpreted as excitation of the electric TH 0 mode, instantly propagating along the ionosphere–ground waveguide. According to this theoretical estimate, even a weak magnetic signature of the E-mode ∼1 nT must be accompanied by a burst of E z well exceeding the atmospheric potential gradient. We have examined simultaneous records of magnetometers and electric field-mills during >50 SSC events in 2007–2019 in search for signatures of E-mode. However, the observed E z disturbance never exceeded background fluctuations ∼10 V/m, much less than expected for the TH 0 mode. We constructed a model of the electromagnetic ULF response to an oscillating magnetospheric field-aligned current incident onto the realistic ionosphere and atmosphere. The model is based on numerical solution of the full-wave equations in the atmospheric-ionospheric collisional plasma, using parameters that were reconstructed using the IRI model. We have calculated the vertical and horizontal distributions of magnetic and electric fields of both H- and E-modes excited by magnetospheric field-aligned currents. The model predicts that the excitation rate of the E-mode by magnetospheric disturbances is low, so only a weak E z response with a magnitude of ∼several V/m will be produced by ∼100 nT geomagnetic disturbance. However, at balloon heights (∼30 km), electric field of the E-mode becomes dominating. Predicted amplitudes of horizontal electric field in the atmosphere induced by Pc5 pulsations and travelling convection vortices, about tens of mV/m, are in good agreement with balloon electric field and ground magnetometer observations. 

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4. Walking droplets interacting with single and double slits

   https://doi.org/10.1017/jfm.2017.790  

   Pucci, Giuseppe ; Harris, Daniel M. ; Faria, Luiz M. ; Bush, John W. ( January 2018 , Journal of Fluid Mechanics)

   Couder & Fort ( Phys. Rev. Lett. , vol. 97, 2006, 154101) demonstrated that when a droplet walking on the surface of a vibrating bath passes through a single or a double slit, it is deflected due to the distortion of its guiding wave field. Moreover, they suggested the build-up of statistical diffraction and interference patterns similar to those arising for quantum particles. Recently, these results have been revisited (Andersen et al. , Phys. Rev.  E, vol. 92 (1), 2015, 013006; Batelaan et al. , J. Phys.: Conf. Ser. , vol. 701 (1), 2016, 012007) and contested (Andersen et al.  2015; Bohr, Andersen & Lautrup, Recent Advances in Fluid Dynamics with Environmental Applications , 2016, Springer, pp. 335–349). We revisit these experiments with a refined experimental set-up that allows us to systematically characterize the dependence of the dynamical and statistical behaviour on the system parameters. The system behaviour is shown to depend strongly on the amplitude of the vibrational forcing: as this forcing increases, a transition from repeatable to unpredictable trajectories arises. In all cases considered, the system behaviour is dominated by a wall effect, specifically the tendency for a drop to walk along a path that makes a fixed angle relative to the plane of the slits. While the three dominant central peaks apparent in the histograms of the deflection angle reported by Couder & Fort (2006) are evident in some of the parameter regimes considered in our study, the Fraunhofer-like dependence of the number of peaks on the slit width is not recovered. In the double-slit geometry, the droplet is influenced by both slits by virtue of the spatial extent of its guiding wave field. The experimental behaviour is well captured by a recently developed theoretical model that allows for a robust treatment of walking droplets interacting with boundaries. Our study underscores the importance of experimental precision in obtaining reproducible data. 

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5. Strain induced anisotropy in liquid phase epitaxy grown nickel ferrite on magnesium gallate substrates

   https://doi.org/10.1038/s41598-022-10814-8  

   Liu, Ying ; Zhou, Peng ; Regmi, Sudhir ; Bidthanapally, Rao ; Popov, Maksym ; Zhang, Jitao ; Zhang, Wei ; Page, Michael R. ; Zhang, Tianjin ; Gupta, Arunava ; et al ( December 2022 , Scientific Reports)

   Abstract This work focuses on the nature of magnetic anisotropy in 2.5–16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850–875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field H σ , estimated from X-ray diffraction data, ranged from 2.8–7.7 kOe with the films on (100) MGO having a higher H σ value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H 4 and the uniaxial anisotropy field H a . Values of H 4 range from −0.24 to −0.86 kOe. The uniaxial anisotropy field H a was an order of magnitude smaller than H σ and it decreased with increasing film thickness for NFO films on (100) MGO, but H a increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices. 

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