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Register a new userSupersonic dislocations observed in a plasma crystal
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Experimental results on the dislocation dynamics in a two-dimensional plasma crystal are presented. Edge dislocations were created in pairs in lattice locations where the internal shear stress exceeded a threshold and then moved apart in the glide plane at a speed higher than the sound speed of shear waves, $C_T$. The experimental system, a plasma crystal, allowed observation of this process at an atomistic (kinetic) level. The early stage of this process is identified as a stacking fault. At a later stage, supersonically moving dislocations generated shear-wave Mach cones.
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Solid state physics and quantum electrodynamics with its ultra-relativistic (massless) particles meet, to their mutual beneit, in the electronic properties of one-dimensional carbon nanotubes as well as two-dimensional graphene or surfaces of topological insulators. However, clear experimental evidence for electronic states with conical dispersion relations in all three dimensions, conceivable in certain bulk materials, is still missing. In the present work, we fabricate and study a zinc-blend crystal, HgCdTe, at the point of the semiconductor-to-semimetal topological transition. Three-dimensional massless electrons with a velocity of about 10$^6$ m/s are observed in this material, as testifed by: (i) the dynamical conductivity which increases linearly with the photon frequency, (ii) in a magnetic field $B$, by a $sqrt{B}$ dependence of dipole-active inter-Landau-level resonances and (iii) the spin splitting of Landau levels, which follows a $sqrt{B}$ dependence, typical of ultra-relativistic particles but not really seen in any other electronic system so far.
The asymptotic analysis of the radiation pattern of a classical dipole in a photonic crystal possessing an incomplete photonic bandgap is presented. The far-field radiation pattern demonstrates a strong modification with respect to the dipole radiation pattern in vacuum. Radiated power is suppressed in the direction of the spatial stopband and strongly enhanced in the direction of the group velocity, which is stationary with respect to a small variation of the wave vector. An effect of radiated power enhancement is explained in terms of emph{photon focusing}. Numerical example is given for a square-lattice two-dimensional photonic crystal. Predictions of asymptotic analysis are substantiated with finite-difference time-domain calculations, revealing a reasonable agreement.
Auxetic materials have the counter-intuitive property of expanding rather than contracting perpendicular to an applied stretch, formally they have negative Poissons Ratios (PRs).[1,2] This results in properties such as enhanced energy absorption and indentation resistance, which means that auxetics have potential for applications in areas from aerospace to biomedical industries.[3,4] Existing synthetic auxetics are all created by carefully structuring porous geometries from positive PR materials. Crucially, their geometry causes the auxeticity.[3,4] The necessary porosity weakens the material compared to the bulk and the structure must be engineered, for example, by using resource-intensive additive manufacturing processes.[1,5] A longstanding goal for researchers has been the development of a synthetic material that has intrinsic auxetic behaviour. Such molecular auxetics would avoid porosity-weakening and their very existence implies chemical tuneability.[1,4-9] However molecular auxeticity has never previously been proven for a synthetic material.[6,7] Here we present a synthetic molecular auxetic based on a monodomain liquid crystal elastomer (LCE). When stressed perpendicular to the alignment direction, the LCE becomes auxetic at strains greater than approximately 0.8 with a minimum PR of -0.8. The critical strain for auxeticity coincides with the occurrence of a negative liquid crystal order parameter (LCOP). We show the auxeticity agrees with theoretical predictions derived from the Warner and Terentjev theory of LCEs.[10] This demonstration of a synthetic molecular auxetic represents the origin of a new approach to producing molecular auxetics with a range of physical properties and functional behaviours. Further, it demonstrates a novel feature of LCEs and a route for realisation of the molecular auxetic technologies that have been proposed over the years.
Using computer simulations, we establish that the structure of a supercooled binary atomic liquid mixture consists of common neighbour structures similar to those found in the equilibrium crystal phase, a Laves structure. Despite the large accumulation of crystal-like structure, we establish that the supercooled liquid represents a true metastable liquid and that liquid can borrow crystal structure without being destabilized. We consider whether this feature might be the origin of all instances of liquids of a strongly favoured local structure.
Liquid crystal networks exploit the coupling between the responsivity of liquid-crystalline mesogens, e.g., to electric fields, and the (visco)elastic properties of a polymer network. Because of this, these materials have been put forward for a wide array of applications, including responsive surfaces such as artificial skins and membranes. For such applications, the desired functional response must generally be realized under strict geometrical constraints, such as provided by supported thin films. To model such settings, we present a dynamical, spatially-heterogeneous Landau-type theory for electrically-actuated liquid crystal network films. We find that the response of the liquid crystal network permeates the film from top to bottom, and illustrate how this affects the time scale associated with macroscopic deformation. Finally, by linking our model parameters to experimental quantities, we suggest that the permeation rate can be controlled by varying the aspect ratio of the mesogens and their degree of orientational order when cross-linked into the polymer network, for which we predict a single optimum. Our results contribute specifically to the rational design of future applications involving transport or on-demand release of molecular cargo in liquid crystal network films.
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