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Topologically non-equivalent textures generated by the nematic electrohydrodynamics
摘要： Some classes of nematic liquid crystals can be driven through turbulent regimes when forced by an external electric field. In contrast to isotropic fluids, a turbulent nematic exhibits a transition to a stochastic regime that is characterised by a network of topological defects. We study the deformations arising after the electric field has been switched-off. In contrast to the turbulent regime, the relaxation of this topological-defect regime involves the annihilation of an interlacement of defect lines. We show that these defect lines separate regions of the nematic having topologically non-equivalent textures.
关键词： Nematic liquid crystals,electrohydrodynamics,twist deformation,topological defects
更新于2025-09-23 15:22:29
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Self-assembly of fractal liquid crystal colloids
摘要： Nematic liquid crystals are anisotropic fluids that self-assemble into vector fields, which are governed by geometrical and topological laws. Consequently, particulate or droplet inclusions self-assemble in nematic domains through a balance of topological defects. Here, we use double emulsions of water droplets inside radial nematic liquid crystal droplets to form various structures, ranging from linear chains to three-dimensional fractal structures. The system is modeled as a formation of satellite droplets, distributed around a larger, central core droplet and we extend the problem to explain the formation of fractal structures. We show that a distribution of droplet sizes plays a key role in determining the symmetry properties of the resulting geometric structures. The results are relevant to a variety of inclusions, ranging from colloids suspensions to multi-emulsion systems. Such systems have potential applications for novel switchable photonic structures as well as providing wider insights into the packing of self-assembled structures.
关键词： Self-assembly,Fractal structures,Topological defects,Colloids,Liquid crystals
更新于2025-09-23 15:22:29
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Mosaics of topological defects in micropatterned liquid crystal textures
摘要： Topological defects in the orientational order that appear in thin slabs of a nematic liquid crystal, as seen in the standard schlieren texture, behave as a random quasi–two-dimensional system with strong optical birefringence. We present an approach to creating and controlling the defects using air pillars, trapped by micropatterned holes in the silicon substrate. The defects are stabilized and positioned by the arrayed air pillars into regular two-dimensional lattices. We explore the effects of hole shape, lattice symmetry, and surface treatment on the resulting lattices of defects and explain their arrangements by application of topological rules. Last, we show the formation of detailed kaleidoscopic textures after the system is cooled down across the nematic–smectic A phase transition, frustrating the defects and surrounding structures with the equal-layer spacing condition of the smectic phase.
关键词： smectic A,nematic,birefringence,micropatterned,topological defects,liquid crystal
更新于2025-09-23 15:21:01
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Controlled creation of a singular spinor vortex by circumventing the Dirac belt trick
摘要： Persistent topological defects and textures are particularly dramatic consequences of superfluidity. Among the most fascinating examples are the singular vortices arising from the rotational symmetry group SO(3), with surprising topological properties illustrated by Dirac’s famous belt trick. Despite considerable interest, controlled preparation and detailed study of vortex lines with complex internal structure in fully three-dimensional spinor systems remains an outstanding experimental challenge. Here, we propose and implement a reproducible and controllable method for creating and detecting a singular SO(3) line vortex from the decay of a non-singular spin texture in a ferromagnetic spin-1 Bose–Einstein condensate. Our experiment explicitly demonstrates the SO(3) character and the unique spinor properties of the defect. Although the vortex is singular, its core fills with atoms in the topologically distinct polar magnetic phase. The resulting stable, coherent topological interface has analogues in systems ranging from condensed matter to cosmology and string theory.
关键词： Bose–Einstein condensate,spinor systems,superfluidity,topological defects,SO(3) vortices
更新于2025-09-19 17:13:59
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Topological defect enabled formation of nematic domains
摘要： We study numerically curvature and electric field driven domain formation and reorientations in two-dimensional nematic liquid crystals within square confinement. We use the Landau-de Gennes description in terms of the nematic tensor order parameter. We show that on increasing the amplitude of a sinusoidally shaped confining wall, pairs {defect, antidefect} are formed, which enable insertion of domains displaying significantly different average orientational order with respect to the surrounding nematic body. Furthermore, we study E-driven transformation between two competing degenerate domain-type structures. We show that transformations could be mediated via qualitatively different intermediate structures possessing topological defects.
关键词： Nematic liquid crystals,topological defects,order reconstruction,phenomenological modelling
更新于2025-09-11 14:15:04
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New bounds on dark matter coupling from a global network of optical atomic clocks
摘要： We report on the first Earth-scale quantum sensor network based on optical atomic clocks aimed at dark matter (DM) detection. Exploiting differences in the susceptibilities to the fine-structure constant of essential parts of an optical atomic clock, i.e., the cold atoms and the optical reference cavity, we can perform sensitive searches for DM signatures without the need for real-time comparisons of the clocks. We report a two orders of magnitude improvement in constraints on transient variations of the fine-structure constant, which considerably improves the detection limit for the standard model (SM)–DM coupling. We use Yb and Sr optical atomic clocks at four laboratories on three continents to search for both topological defect and massive scalar field candidates. No signal consistent with a DM coupling is identified, leading to considerably improved constraints on the DM-SM couplings.
关键词： massive scalar fields,quantum sensor network,dark matter,topological defects,optical atomic clocks,fine-structure constant
更新于2025-09-09 09:28:46
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Topological one-way fiber of second Chern number
摘要： One-way waveguides have been discovered as topological edge states in two-dimensional (2D) photonic crystals. Here, we design one-way fiber modes in a 3D magnetic Weyl photonic crystal realizable at microwave frequencies. We first obtain a 3D Chern crystal with a non-zero first Chern number by annihilating the Weyl points through supercell modulation. When the modulation becomes helixes, one-way modes develop along the winding axis, with the number of modes determined by the spatial frequency of the helix. These single-polarization single-mode and multi-mode one-way fibers, having nearly identical group and phase velocities, are topologically-protected by the second Chern number in the 4D parameter space of the 3D wavevectors plus the winding angle of the helix. This work suggests a unique way to utilize high-dimensional topological physics using topological defects.
关键词： topological defects,Chern number,Weyl photonic crystals,one-way waveguides,topological photonics
更新于2025-09-04 15:30:14
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Geometric stabilisation of topological defects on micro-helices and grooved rods in nematic liquid crystals
摘要： We demonstrate how the geometric shape of a rod in a nematic liquid crystal can stabilise a large number of oppositely charged topological defects. A rod is of the same shape as a sphere, both having genus g = 0, which means that the sum of all topological charges of defects on a rod has to be (cid:2)1 it usually shows only one hyperbolic hedgehog or a Saturn ring defect with negative unit charge. Multiple unit charges can be stabilised either by friction or large length, which screens the pair-interaction of unit charges. Here we show that the curved shape of helical colloids or the grooved surface of a straight rod create energy barriers between neighbouring defects and prevent their annihilation. The experiments also clearly support the Gauss–Bonnet theorem and show that topological defects on helices or grooved rods always appear in an odd number of unit topological charges with a total topological charge of (cid:2)1.
关键词： topological defects,geometric stabilisation,micro-helices,nematic liquid crystals,grooved rods
更新于2025-09-04 15:30:14