Defect Structures of Magnetic Nanoparticles in Smectic A Liquid Crystals

Topological defects in anisotropic fluids like liquid crystals serve as a playground for the research of various effects. In this study, we concentrated on a hybrid system of chiral rod-like molecules doped by magnetic nanoparticles. In textures of the smectic A phase, we observed linear defects and found that clusters of nanoparticles promote nucleation of smectic layer defects just at the phase transition from the isotropic to the smectic A (SmA) phase. In different geometries, we studied and analysed creation of defects which can be explained by attractive elastic forces between nanoparticles in the SmA phase. On cooling the studied hybrid system, clusters grow up to the critical dimension, and the smectic texture is stabilised. The presented effects are theoretically described and explained if we consider the elastic interaction of two point defects and stabilisation of prismatic dislocation loops due to the presence of nanoparticles.

Dislocation structure of deformed olivine single crystals from conventional EBSD maps

Dislocations, linear defects in a crystalline lattice characterized by their slip systems, can provide a record of grain internal deformation. Comprehensive examination of this record has been limited by intrinsic limitations of the observational methods. Transmission electron microscopy reveals individual dislocations, but images only a few square $$\upmu$$ μ m of sample. Oxidative decoration requires involved sample preparation and has uncertainties in detection of all dislocations and their types. The possibility of mapping dislocation density and slip systems by conventional (Hough-transform based) EBSD is investigated here with naturally and experimentally deformed San Carlos olivine single crystals. Geometry and dislocation structures of crystals deformed in orientations designed to activate particular slip systems were previously analyzed by TEM and oxidative decoration. A curvature tensor is calculated from changes in orientation of the crystal lattice, which is inverted to calculate density of geometrically necessary dislocations with the Matlab Toolbox MTEX. Densities of individual dislocation types along with misorientation axes are compared to orientation change measured on the deformed crystals. After filtering (denoising), noise floor and calculated dislocation densities are comparable to those reported from high resolution EBSD mapping. For samples deformed in [110]c and [011]c orientations EBSD mapping confirms [100](010) and [001](010), respectively, as the dominant slip systems. EBSD mapping thus enables relatively efficient observation of dislocation structures associated with intracrystalline deformation, both distributed, and localized at sub-boundaries, over substantially larger areas than has previously been possible. This will enable mapping of dislocation structures in both naturally and experimentally deformed polycrystals, with potentially new insights into deformation processes in Earth’s upper mantle.

Influence of Laser Radiation on Movement of Dislocations in Silicon Single Crystals

The behavior of linear defects in n-and p-type silicon, generated by laser radiation is studied for pulse energy density 417 – 1083 mJ/cm2. The features of the nondestructive and destructive effects of the laser pulse on the surface defects formation of the semiconductor crystals are revealed. The formation and movement of dislocations in the crater region and the development of micro cracks, accompanied by acoustic waves are revealed.

The Peculiarities of Mass Transfer in the Surface Layer of Materials under Extreme Heating

Possible causes of mass transfer acceleration of carbon atoms and alloying elements in the surface layers of steels and alloys under extreme heating, under pulsing laser irradiation in particular, are considered. The research shows that the anomaly accelerated mass transfer, including diffusion in particular, in steels and alloys under fast laser heating has a cooperative character and is a result of a simultaneous action of several processes of different physics. It is proved that the carbon atoms mass transfer parameters and alloying elements depend on the scale and the level of emerging tension, relaxation of which goes along with a local plastic deformation, and occurrence of increased number of linear defects in crystal structure.

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

Ultrafast manipulation of phases and phase domains in quantum materials is a key approach to unraveling and harnessing interwoven effects of charge and lattice degrees of freedom. In the intensely-studied charge-density-wave (CDW) material, 1<i>T</i>-TaS₂, static Rayleigh-phonon coupling to periodic lattice distortions (PLDs), as well as incommensurate (IC) domain growth and coarsening over the first 100 ps following femtosecond photoexcitation, suggests ultrafast, displacively-excited coherent acoustic phonons (CAPs) may strongly couple to PLDs. Here we find evidence for such coupling using 4D ultrafast electron microscopy (UEM). For ultrathin room-temperature crystals, photoinduced Bragg-peak dynamics spanning the first 75 ps are characterized by partial CAP coherence and localized low-amplitude <i>c</i>-axis dilations. These relatively weak, partially-coherent dynamics then give way to higher-amplitude, increasingly-coherent oscillations, the transition period of which is well-matched to timescales of photoinduced IC domain growth and stabilization from the nearly-commensurate (NC) phase. Diffraction experiments are correlated with nanoscale UEM imaging, where it is found that phonon wave trains emerge from nanoscale linear defects 100 ps after photoexcitation. The CAPs consist of coupled longitudinal and transverse character and propagate at an anomalously-high 4.6 nm/ps along wave vectors independent from NC-phase PLDs, instead being dictated by static defect orientation. Such behaviors illustrate a potential means to control phases in quantum materials using defect-engineered coherent-phonon seeding.<br>

Influence of DC Current on Movement of Dislocations in P-Silicon Single Crystals in Field of Internal Stresses

The behavior of linear defects in p-type silicon (111) carrying a direct current of density 0−15×105 A/m2 in the [110] direction are studied in the temperature range T=850–1000 K during isothermal annealing. The regularities of change in the linear density and maximum path of dislocations in slip lines are revealed. A model of linear defects displacement in silicon single crystals in the field of internal stresses under an electric current is proposed. Matching theory with experimental data has made it possible to reveal the dependence of this distribution on the internal stresses relaxation time.

Explosion as a phase transition due to the change of the gradient symmetry of the distortion tensor.

It is shown that the destruction of a continuous elastic medium is a phase transition associated with a change in the gradient symmetry of the equations of state for the distortion tensor. The equations of state for the distortion tensor, as a compensating field of the minimal fundamental interaction, are derived from the action minimum. In a continuous elastic medium, the distortion tensor is proportional to the conjugate stress tensor. The continuous medium is destroyed at critical stresses or pressures. At the same time, the vortex tension of the distortion tensor penetrates into it and the elasticity disappears. As a result of this phase transition, linear defects and cracks are formed in solids state, and high-temperature plasma occurs in the gas and an explosion occurs. It is shown that the explosion and lightning are the same phase transition, which is caused by the occurrence of a critical vortex tension of the distortion tensor.

Pattern Formation and Tropical Geometry

Sandpile models exhibit fascinating pattern structures: patches, characterized by quadratic functions, and line-shaped patterns (also called solitons, webs, or linear defects). It was predicted by Dhar and Sadhu that sandpile patterns with line-like features may be described in terms of tropical geometry. We explain the main ideas and technical tools—tropical geometry and discrete superharmonic functions—used to rigorously establish certain properties of these patterns. It seems that the aforementioned tools have great potential for generalization and application in a variety of situations.