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We revisit the interaction between a point charge and an inhomogeneous magnetic field that yields the magnetic quantum dot system. This magnetic field is defined by filling the whole space, except for a region of radius r 0 . Then, we assume that there is an impenetrable potential wall located at r 0 and discuss the quantum effects of screw dislocation topology and the missing magnetic flux. We first show that Landau levels can be achieved even though there is the presence of an impenetrable potential wall. We go further by discussing the confinement of a point charge to a cylindrical wire. In both cases, we show Aharonov–Bohm-type effects for bound states can be obtained from the influence of the screw dislocation topology and the missing magnetic flux. Later, we discuss the influence of the screw dislocation topology and the missing magnetic flux on the magnetization and the persistent currents. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the nonrelativistic limit of a quantum particle, subjected to a uniform magnetic field and in the presence of some confinement potentials, immersed in a background with a cosmic string and a screw dislocation. We separately examined four models, constructed the energy spectrum and cyclotron frequency for each of them, and checked the consistency of the results by considering the limit of some characteristic parameters of the potentials. [ABSTRACT FROM AUTHOR]

The synergy between Nb/Ti strengthening and precise isothermal transformation temperatures has resulted in the optimal microstructure and mechanical properties in Nb/Ti anti-seismic rebars. The microstructure, precipitates, and mechanical properties of experimental steels at different isothermal transformation temperatures were characterized using scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and universal tensile testing machine. As the isothermal transformation temperature decreased from 700 to 600°C, the ferrite grain size, pearlite interlamellar spacing, and carbon diffusion coefficient of the experimental steel decreased to 6.69 μm, 0.17 μm, and 0.14 cm2·s−1, respectively, while the yield strength and tensile strength increased to 584 and 714 MPa, respectively. At 600°C, the pearlite transformation rate in the experimental steel was the fastest, accompanied by the most rapid precipitation kinetics. The precipitates were (Nb, Ti)C of approximately 50 nm in size, with a mismatch of 14.24% at the matrix/precipitate interface and a screw dislocation angle of 2.07°. The presence of screw dislocation steps may facilitate nucleation of Nb/Ti precipitates, forming semi-coherent interfaces.

The state of stress arising within an elastic wedge generated by an antiplane singularity present within it is studied. An analytical solution is derived with a unified generic approach. The singularity may represent either an anti-plane concentrated force or a screw dislocation. To validate the general solution, two degenerate cases are presented. Further, the image force present on the screw dislocation due to the wedge free boundary is obtained. It is found that when the screw dislocations are placed in the vicinity of the wedge surface, the image force will drive dislocations to the free boundary where they will be annihilated. This implies that a dislocation-free zone may exist along the free surface of the wedge. To demonstrate the application of the fundamental solutions, a formulation for a slip band under anti-plane loading with Green's function method is provided. The solutions developed in this study may be used as building blocks to model the damage of material near a V-notch under versatile anti-plane load conditions or torsional loading. [ABSTRACT FROM AUTHOR]

We revisit the interaction between a point charge and an inhomogeneous magnetic field that yields the magnetic quantum dot system. This magnetic field is defined by filling the whole space, except for a region of radius r0. Then, we assume that there is an impenetrable potential wall located at r0 and discuss the quantum effects of screw dislocation topology and the missing magnetic flux. We first show that Landau levels can be achieved even though there is the presence of an impenetrable potential wall. We go further by discussing the confinement of a point charge to a cylindrical wire. In both cases, we show Aharonov–Bohm-type effects for bound states can be obtained from the influence of the screw dislocation topology and the missing magnetic flux. Later, we discuss the influence of the screw dislocation topology and the missing magnetic flux on the magnetization and the persistent currents.

The synthesis of chiral crystals such as helical crystals is important due to their chirality‐dependent applications in various fields. Since chiral crystals of achiral molecules are getting more attention, it is important to establish synthetic strategies for helical crystals of achiral molecules. Quite surprisingly, there are numerous examples of helical crystals of achiral molecules, although they are mostly obtained without specific synthetic strategies. Herein, it is proposed that Eshelby twist can be applied to noncovalent molecular crystals by introducing screw dislocations and demonstrates a successful synthesis of helical tube crystals of achiral titanyl phthalocyanine by vapor‐phase crystal growth. The dislocations form where interfaces are created via growth transition from wire growth to tube growth upon growth temperature change. The screw dislocation is confirmed by Burgers vector analysis using transmission electron microscopy. The screw dislocation induces crystal twisting along the growth direction to dissipate the strain applied around the dislocation. The diffraction peak broadening of the (200) and (002) planes confirms the nonuniform strain, i.e., twisting force. This work offers a new approach for the synthesis of helical crystals from achiral molecules. [ABSTRACT FROM AUTHOR]

We investigated the ferrite recrystallization behavior of pure iron with different dislocation characters. Two types of specimens with almost the same dislocation densities and different dislocation characters were prepared. The edge dislocation density in specimen A was larger than that in specimen B, whereas the screw dislocation density in specimen B was larger than that in specimen A. Ferrite recrystallization progressed faster in specimen A than in specimen B. Furthermore, cellular automaton simulation proved that changes in ferrite recrystallization behavior due to the dislocation character are attributed to the difference in the driving force for recrystallization and the activation energy for recovery in each specimen. Thus, we found that the difference in the dislocation character can affect the ferrite recrystallization behavior of pure iron.

The synthesis of chiral crystals such as helical crystals is important due to their chirality‐dependent applications in various fields. Since chiral crystals of achiral molecules are getting more attention, it is important to establish synthetic strategies for helical crystals of achiral molecules. Quite surprisingly, there are numerous examples of helical crystals of achiral molecules, although they are mostly obtained without specific synthetic strategies. Herein, it is proposed that Eshelby twist can be applied to noncovalent molecular crystals by introducing screw dislocations and demonstrates a successful synthesis of helical tube crystals of achiral titanyl phthalocyanine by vapor‐phase crystal growth. The dislocations form where interfaces are created via growth transition from wire growth to tube growth upon growth temperature change. The screw dislocation is confirmed by Burgers vector analysis using transmission electron microscopy. The screw dislocation induces crystal twisting along the growth direction to dissipate the strain applied around the dislocation. The diffraction peak broadening of the (200) and (002) planes confirms the nonuniform strain, i.e., twisting force. This work offers a new approach for the synthesis of helical crystals from achiral molecules.

Using complex variable techniques, we solve the problem in anti-plane elasticity associated with a screw dislocation in a three-phase composite which is composed of an anisotropic elastic half-plane perfectly bonded to an isotropic elastic half-plane containing a circular elastic inhomogeneity. Analytical solutions are obtained for three typical cases: a screw dislocation in the isotropic half-plane in the exterior of the circular inhomogeneity, a screw dislocation inside the circular elastic inhomogeneity, a screw dislocation within the anisotropic half-plane. Explicit expressions of the image force acting on the screw dislocation are derived for each of the three cases. The numerical results indicate that at least one equilibrium position for the screw dislocation co-exists in each phase under certain conditions on the elastic constants. [ABSTRACT FROM AUTHOR]

The discrete Peierls model is generalised to be able to study the nonplanar dislocations as well as the planar dislocations. In the discrete model, the six-fold 1 2 〈 111 〉 screw dislocation that is widely studied in numerical simulation and analytical theory splits into two different cores: the six-fold B core and the six-fold O core. The six-fold B core has the lowest energy and is the most stable while the six-fold O core has about 38 meV more energy in a period length. Because local dislocation hopping can only occur along the folds and hopping to sites out of the folds is not allowed, the dislocation movement is realised by transforming itself into a planar one in the model. The Peierls stress associated with this transition tunnel is σ p ∼ 0.05 μ , which is much higher than that obtained in simulations. Limitations of the model can be removed by using the two-dimensional energy landscape that is a natural consequence of the concept of the nonplanar dislocation. In the leading terms approximation of the two dimensional energy landscape, the non-Schmid behaviour of the 1 2 〈 111 〉 screw dislocation in Ta is quantitatively predicted based on the model. [ABSTRACT FROM AUTHOR]

We review our recent study on step-bunching instability (SBI) of ice melt growth, one of the interfacial instabilities driven by self-organization of elementary step flow associated with crystal growth dynamics. With the aid of an advanced optical microscope and its combination with a two-beam interferometer, we realized quantitative in situ observations of the spatiotemporal dynamics of the SBI. This enables us to examine the origin of the SBI at the level of the step-step interaction. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches. [ABSTRACT FROM AUTHOR]

In this paper, we investigate non-inertial effects induced by a rotating frame on a non-relativistic quantum harmonic oscillator as well as of the topology associated to a screw dislocation, which corresponds to a distortion of a vertical line into a vertical spiral. To do this, we obtain the analytical solutions of the time-independent Schrdinger equation for this harmonic oscillator potential in this background. The expressions for the energy spectrum are obtained and the solutions for four quantum states, namely, n = 0 , 1 , 2 and 3 , are analyzed. Our results show that the presence of the topological defect (screw dislocation) as well the fact that we are analyzing the system from the point of view of a rotating frame, changes the solutions of Schrdinger equation and the corresponding spectrum. Now these quantities depend on the angular velocity of the rotating frame, Ω , and also on the parameter β , which codifies the presence of the screw dislocation. Particularly, with respect to the energy spectrum of the system the changing is such that when Ω increases, the energy can increase or decrease depending on the values we assign to the eigenvalues of the angular and linear momenta. Additionally, we observe that the values of the parameter β that characterizes the screw dislocation cause a shift in the energy spectrum. [ABSTRACT FROM AUTHOR]

It is shown theoretically that an astigmatic transformation of an edge dislocation (straight line of zero intensity) of the ν-th order (ν=n+α is a real positive number, n is integer, 0

The purpose of this study to derive a theoretical solutions for two elliptical inclusions that are perfectly bonded to an elastic medium (matrix) of infinite extent under antiplane loading. These two ellipses have different shear moduli, long-axial radii, short-axial radii, inclining angles, and central points. The matrix is assumed to be subjected to arbitrary antiplane loading by, for example, uniform shear stresses, as well as to a concentrated force and screw dislocation at an arbitrary point. The solutions are obtained through iterations of the Möbius transformation as a series with an explicit general term involving the complex potential functions of the corresponding homogeneous problem. This procedure is referred to as heterogenization. Using these solutions, several numerical examples are presented graphically.

The interaction of 1/2 〈111〉 screw dislocation and point defects, including vacancy and self-interstitial atom (SIA), was investigated by molecular dynamics (MD) simulations. Molecular statics (MS) are used to analyze the factors affecting the binding energy of point defects around the dislocation. The results show that the point defects around the dislocation core have the highest binding energy. In addition, we use elasticity theory to find that the point defects binding is interrelated to strain distribution of dislocation and the residual stress of point defects. Furthermore, nudged elastic band (NEB) calculations have been applied to find that vacancy and SIA tend to migrate along the dislocation line around the core region. The results can clearly show the interaction behavior of point defects and screw dislocation, and provide a reasonable explanation for the diffusion mechanism of mono-vacancy and a single SIA around the dislocation line.

In this paper, the degenerate kernel in conjunction with the bipolar coordinates is employed to solve the anti-plane problems of interaction between a screw dislocation and circular holes or rigid inclusions. Once the degenerate kernel of the angular basis function (ABF) is provided in terms of the bipolar coordinates, the analytical solution for cases of one or two circular holes and rigid inclusions can be derived. Not only the radial basis function (RBF) but also the ABF is used. First, the observer objectivity of the degenerate kernel in terms of the bipolar coordinates is examined numerically. A special case, one circular hole or rigid inclusion, is considered to demonstrate the validity of the present approach. Finally, the cases containing two circular holes and two circular rigid inclusions were examined. The comparison between available results and ours is well done. Besides, for the solutions of two holes and rigid inclusions, it is interesting to find that the present method provides an analytical solution with a series form of explicitly determined coefficients, while the coefficients provided by the complex variable need to be determined using the recursive formulae. [ABSTRACT FROM AUTHOR]

In this paper, we present the quantum description that arose from the interaction of a moving magnetic quadrupole moment with electric field configurations in the background having a rotating frame of the presence of screw dislocation in the nonrelativistic regime. Interacting a moving particle involving a magnetic quadrupole moment with the chosen electric field configuration in a rotating frame can produce a kind of static scalar potential for the geometric background. We solve the relevant Schrödinger equation and acquire the exact solutions for that system by applying an analytical method. Besides, based on this interaction, owing to one of the choices for the electric field, we can see an Aharonov–Anandan quantum phase in the corresponding solutions. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to study the structure and removal of prismatic dislocation loops during graphitization. The carbon models contain a mixture of screw and edge dislocations, and are created by self-assembly at high temperature. Four mesh-based analysis tools are used to track the time-evolution of the dislocation loops, providing insight into loop structure and allowing quantification of kinetics. We find that the loop structure is complex, being dispersed in three dimensions with alternating screw and edge components in multiple slip planes. Loop removal involves edge glide through kink formation and propagation, followed by a slower screw glide mechanism. All analysis tools yield similar activation energies, in the range of 2.9 ± 0.3 eV, consistent with recent experimental work by ourselves. Literature values for the kinetics of graphitization fall into two brackets, a lower range of 2.8–3.9 eV, and a higher range of 7.8–11.6 eV. This work supports the lower range and suggests that prismatic dislocation loops are the key defect removed during graphitization. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Dislocation loop expansion was investigated using Phase Field Dislocation Dynamics in three refractory multi-principal element alloys at room temperature. • We present a rapid, highly efficient density function theory-based method for generating energetic landscapes for large 3D crystals for simulation. • In the low-stress regime, glide of the edge portions of the loop is smooth, and glide of the screw portions of the loop is jerky. • In the high-stress regime, the edge to screw mobility ratio is approximately two and the edge mobility doubles from that in the low-stress regime. In this work, we study loop dynamics at room temperature across three refractory multi-principal element alloys (RMPEAs) using a phase field dislocation dynamics simulation method with Langevin dynamics. The analyses reveal two regimes in stress for all RMPEAs studied. In the low-stress regime, glide of the edge portions is smooth, and glide of the screw portions is jerky. In the high-stress regime, the edge to screw mobility ratio is approximately two and the edge mobility doubles from that in the low-stress regime. We also test a rapid density function theory-based method for generating energetic landscapes for large 3D crystals for simulation. As another key result, we find that dislocation mechanisms, velocities, and mobilities predicted between the two methods agree over a wide range of effective stresses, where the effective stress is the difference between the athermal lattice friction stress and the applied stress at 300 K. The second method is highly efficient, offering a way for performing dislocation dynamics quickly over a broad composition space. [ABSTRACT FROM AUTHOR]

In this study, the interaction of a parabolic notch with a generalized antiplane singularity is studied, and its analytical solution is derived. The singularity may be either an antiplane concentrated force or a screw dislocation, and separate solutions for each of these are found. The driving force present on the screw dislocation due to the notch free boundary is obtained. It is found that a dislocation-free zone may exist beneath the notch root surface when the screw dislocation is placed on the notch geometric symmetry axis, as the driving force will pull dislocations to the free boundary where they will be annihilated. The solutions developed in this study may be used as building blocks to model the damage of material near a parabolic notch under antiplane load conditions and therefore serve as a step in quantifying crack nucleation conditions, which is the novelty of the current study. [ABSTRACT FROM AUTHOR]

The behavior of displacement field due to a screw dislocation is similar to the angular basis function (ABF) Arg(z). It is different from the radial basis function (RBF) ln(r) that is used to describe the velocity potential of a sink or source. Nevertheless, the complex-valued fundamental solution ln(z) contains the two parts of RBF ln(r) and ABF Arg(z). In this paper, not only the RBF in the null-field boundary integral equation (BIE) but also the ABF for the screw dislocation are employed to study the interaction between a screw dislocation and an elastic elliptical inhomogeneity. This problem is decomposed into a free field with a screw dislocation and a boundary value problem containing an elliptical inhomogeneity. The boundary value problem is solved by using the RBF and the null-field BIE. Since the geometric shape is an ellipse, the degenerate kernel is expanded to a series form under the elliptical coordinates, while the unknown boundary densities are expanded to eigenfunctions. By combining the degenerate kernel and the null-field BIE, the boundary value problem can be easily solved. The inconsistency between Sendeckyj (In: Simmons JA, et al (eds) Fundamental aspects of dislocation theory. US National Bureau of Standards, Gaithersburg, pp 57–69, 1970) and Gong and Meguid (Int J Eng Sci 32(8):1221–1228, 1994) for the problem was also found by using the present approach. The error in Gong and Meguid (Int J Eng Sci 32(8):1221–1228, 1994) was also printed out. Finally, some examples are demonstrated to verify the validity of the present approach. [ABSTRACT FROM AUTHOR]

The Griffith crack problem and the interaction between screw dislocation and semi-infinite crack in cubic quasicrystal piezoelectric materials are studied by using the complex variable function method. The stress intensity factors and electric displacement intensity factors are obtained. The effects of the linear force and coupling elastic coefficient on the stress intensity factor of phonon field and phason fields are discussed in detail. By numerical examples, it is found that the linear force and the coupling elastic constant have a significant effect on the stress intensity factor. [ABSTRACT FROM AUTHOR]

In this paper, we summarize a remarkable result obtained by Soskin et al. in Phys Rev A 56, 4064 (1997). We show that for an on-axis superposition of two different-waist Laguerre-Gauss beams with numbers (0, n) and (0, m), the topological charge equals TC=m up to a plane where the waist radii become the same, given that the beam (0, m) has a greater waist radius, changing to TC=n after this plane. This occurs because in the initial plane the superposition has an on-axis op-tical vortex with TC=m and on different axis-centered circles there are (n – m) vortices with TC= +1 and (n – m) vortices with TC= –1. On approaching the above-specified plane, the vortices with TC= -1 "depart" to infinity with a higher-than-light speed, with the TC of the total beam becoming equal to TC=n. If, on the contrary, the beam (0, m) has a smaller waist, then the total TC equals n on a path from the initial plane up to a plane where the waist radii become the same, changing to TC=m after the said plane. This occurs because after the said plane, n–m vortices with TC= –1 "arrive" from infinity with a higher-than-light speed.

Experimental results of the investigation of self-healing properties of terahertz Bessel beams with orbital angular momentum (OAM) with topological charges of l=3 and l=4 in free space after passing through a dispersive medium are presented.

The atomistic interactions between screw dislocations and coherent twin boundaries were investigated in face-centered metals of Al, Cu, Ni, Au, Ag, and Pd using molecular dynamics and nudged elastic band method. The interaction mechanism was affected by stable and unstable stacking fault energies predicted by various potentials. In Al, Cu, and Pd, screw dislocation would be absorbed by the twin boundary. However, transmissions were observed in Ni, Au, Ag, and Pd with another potential. It was found that both the critical interaction stress and energy barrier decreased as the reciprocal of the difference between unstable and stable stacking fault energies increased. Activation volumes and strain rate sensitivities were estimated around 14 b3 to 27 b3, where b is the Burgers vector, and 0.011 to 0.023, respectively. The results for Cu and Ni were further compared with experiments and showed good agreements. [ABSTRACT FROM AUTHOR]

In optical data transmission with using vortex laser beams, data can be encoded by the topological charge, which is theoretically unlimited. However, the topological charge of a single separate vortex (screw dislocation) is limited by possibilities of its generating. Therefore, we investigate here three examples of multivortex Gaussian light fields (two beams are form-invariant and one beam is astigmatic) with an unbounded (countable) set of screw dislocations. As a result, such fields have an infinite topological charge. The first beam has the complex amplitude of the Gaussian beam, but multiplied by the cosine function with a squared vortex argument. Phase singularity points of such a beam reside in the waist plane on the Cartesian axes and their density grows with increasing distance from the optical axis. The transverse intensity distribution of such a beam has a shape of a four-pointed star. All the optical vortices in this beam has the same topological charge of +1. The second beam also has the complex amplitude of the Gaussian beam, multiplied by the vortex-argument cosine function, but the cosine is raised to an arbitrary power. This beam has a countable number of the optical vortices, which reside in the waist plane uniformly on one Cartesian axis and the topological charge of each vortex equals to power, to which the cosine function is raised. The transverse intensity distribution of such beam consists of two light spots residing on a straight line, orthogonal to a straight line with the optical vortices. Finally, the third beam is similar to the first one in many properties, but it is generated with a tilted cylindrical lens from a 1D parabolic-argument cosine grating.

Ice-crystal growth in supercooled water is one of the most familiar examples of phase-transition dynamics, playing essential roles in various natural phenomena on Earth. Despite its fundamental importance, the microscopic view at the elementary step level remains elusive. Here, using an advanced optical microscope, we find self-organization of elementary steps during ice-crystal growth, called step-bunching instability (SBI), driven by the competition between step dynamics, interfacial stiffness, and latent heat diffusions. We also find that the SBI transiently induces screw dislocations and resulting spiral growth in the late stage of the growth process. Furthermore, quantitative observations with a two-beam interferometer allow us to obtain insights into the relative importance of the various mechanisms of the step–step interactions. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches. [ABSTRACT FROM AUTHOR]

The plasticity and hardness of metals are largely dependent on how dislocation interacts with solute atoms. Here, taking bcc molybdenum (Mo) as the example, the interaction of interstitial solutes carbon (C) and oxygen (O) with screw dislocation, and their influences on the dislocation motion, have been determined using first-principles calculations and thermodynamic models. Due to the incompact atomic structure and variation of electronic states in the dislocation core, C and O will segregate from the bulk system to the dislocation region. Notably, the presence of C/O at the dislocation induces the reconstruction of the core structure, from an easy-core to hard-core configuration. This originates from the fact that the hard-core structure provides a larger available volume at the interstitial site than the easy-core structure and, thus, facilitates the dissolution of C and O. More importantly, the addition of C/O in the dislocation significantly increases the Peierls stresses and double-kink formation enthalpies of screw dislocation in Mo, from 1.91 GPa and 1.18 eV for C/O-free dislocation to 5.63/4.69 GPa and 1.77/1.58 eV for C/O-saturated dislocation. Therefore, these interstitial solutes have a pinning effect on the dislocation motion, and this effect becomes stronger with higher segregating levels. This work reveals the profound effect of interstitial solutes on the properties of the dislocation core and provides a fundamental factor to account for the interstitial solutes-related phenomena in bcc metals. [ABSTRACT FROM AUTHOR]

Utilizing conformal transformation, a screw dislocation interacting with two unequal interface cracks emanating from an elliptical hole in a one-dimensional hexagonal piezoelectric quasicrystal bi-material was studied. The analytic expressions of the interface stresses, electric displacement and stress intensity factors were obtained. With the help of the generalization of the Peach–Koehler formula for quasicrystals, the image force acting on the dislocation due to the presence of the interface was then determined. Numerical examples are given to show the effects of the coupling elastic constants of the phonon field and phason field on the field intensity factors. [ABSTRACT FROM AUTHOR]

Development and safe application of nuclear energy depend on the performance of the structural materials. Mechanical property degradation of irradiated metals has been confirmed to be closely related with the interactions between dislocations and radiation defects. To understand the underlying interaction mechanism at atomic scale is significant for development of radiation resistant materials in future. In this work, three different kinetic mechanisms have been suggested by simulating the interactions between a 1/2[111] screw dislocation line and 1/2 〈111〉 interstitial dislocation loops with different orientations in the body-centered cubic (BCC) iron system. Molecular dynamics (MD) simulations indicate that formation of a helical turn configuration, cross-slip movement, and diffusion of screw segments on loop core during pinning and unpinning process are primary features related to three new interaction mechanisms. Detailed analysis suggests that for a given screw dislocation line, the specific Burgers vector of a 1/2 〈111〉 interstitial dislocation loop and its size determine the dominated mechanism for a line-loop interaction. It is also found that the critical stress for a screw dislocation to cross a 1/2 〈111〉 dislocation loop is varied due to the interactions involved in these new mechanisms. All these results provide new insights into the evolution of microstructure in irradiated BCC iron.

Polytypism is common in micas, and the frequency of polytype occurrence is believed to be related closely to the crystallization conditions and chemical compositions of the corresponding fluids and melts. Coexisting multiple standard and complex/disordered polytypes in igneous rocks generally reflect a complicated magma evolution history. The purpose of the current study was to clarify the origin of coexisting biotite polytypes and their growth mechanism. Micro-X-ray diffraction (μXRD) and transmission electron microscopy (TEM) were used to investigate Fe-rich biotite phenocrysts in rhyolite from the Long Valley Caldera, California, USA. The μXRD analyses characterized various polytypes, and TEM observations revealed that common polytypes (e.g. 1M, 2M1, and 3T) and rare polytypes (e.g. 4M2 and 4A8) coexist within biotite monocrystals. The two 4-layer polytypes of Fe-rich biotite, 4M2 and 4A8, were identified via selected-area electron diffraction (SAED) and high-resolution scanning transmission electron microscopy (HRSTEM) at the atomic resolution, with unique stacking sequences ([0222] for 4M2 and [002 2 ¯ ] for 4A8). Energy-dispersive X-ray spectroscopy (EDS) results showed differences in their chemical compositions, especially Fe and K. The 4A8 polytype is reported for the first time. The present study suggested that environmental changes, such as rapid cooling and inhomogeneous compositional distribution, led to chemical and structural oscillations and complex nucleation of the two 4-layer polytypes. Screw dislocations producing spiral growth enhance polytype stability and form ordered long-period/complex polytypes. These results are useful to understand the origin of long-period/complex polytypes and the intergrowths of diverse polytypes formed in non-equilibrium crystallization environments. [ABSTRACT FROM AUTHOR]

The mechanical behavior of fine-grained piezoelectric/substrate structure with screw dislocation and interface edge crack under the coupling action of heat, force and electricity are studied. Using the mapping function method, firstly, the finite area plane is transformed into the right semi-infinite plane, then the expression of the temperature field is given with the help of the complex function, and then the temperature field of the problem is achieved. By constructing the general solution of the governing equation with temperature function, the analytical expression of the image force is derived. Finally, the effects of material parameters, temperature gradient, coating thickness and crack size on image force are analyzed by numerical examples. The results show that the temperature gradient has a very significant effect on the image force, and thicker coating is conducive to the stability of dislocation and interface crack. [ABSTRACT FROM AUTHOR]

The Griffith crack problem and the interaction between screw dislocation and semi-infinite crack in cubic quasicrystal piezoelectric materials are studied by using the complex variable function method. The stress intensity factors and electric displacement intensity factors are obtained. The effects of the linear force and coupling elastic coefficient on the stress intensity factor of phonon field and phason fields are discussed in detail. By numerical examples, it is found that the linear force and the coupling elastic constant have a significant effect on the stress intensity factor.

Spontaneous deracemization has been used to separate homochiral domains from the racemic system. However, homochirality can only be referred to when the scales of these domains and systems are specified. To clarify this, we report self‐assembly of racemates of dissymmetric cages DC‐1 with a cone‐shape propeller geometry, forming a centrosymmetric columnar crystalline phase (racemic at crystallographic level). Owing to their anisotropic geometry, the two enantiomers are packed in a frustrated fashion in this crystalline phase; single‐handed double helices are observed (single‐handedness at supramolecular level). The frustrated packing (layer continuity break‐up) in turn facilitates screw dislocation during the crystal growth, forming left‐ or right‐handed spiral platelets (symmetry‐breaking at morphological level), although each platelet is composed of DC‐1 racemates. The symmetry correlation between DC‐1 molecules, the crystalline phase and spiral platelets, all exhibit C3 symmetry. [ABSTRACT FROM AUTHOR]

Magnesium (Mg), as the lightest structural metal, suffers from poor ductility and formability. Although in literature, the cross-slips in Mg have been identified as the key factors that are decisive for ductility, there are still many controversies and ambiguities in these cross-slip processes, e.g., energy, stability, etc. In this study, we propose a force correction method to eliminate the surface effects, which thus effectively enhances the efficiency/accuracy of the first-principles DFT calculations of screw dislocations. Based on this method, both a and c + a screw dislocations are calculated with convergence carefully examined. Our results show that a dislocation on the Basal plane has a lower energy than on the Prism plane, by 62 meV/ b. The Prism core is energetically unstable since no barrier appears in the Prism-Basal cross-slip path. However, the energy landscape around the Prism core is extremely flat, where the tiny configurational forces could be below the convergence criterion, making the Prism core obtainable in the atomistic simulations. Then for the c + a dislocation, the ground state is revealed to dissociate on the Pyramidal II plane. Since multiple metastable c + a core structures are found on the Pyramidal planes, the energy difference during the c + a cross-slip is thus within a range of 8 – 19 meV/ b. All these findings here provide the basis and guidance for understanding and enhancing the ductility in Mg and Mg alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

We present the study of hydrogen atom segregation on screw dislocation in Fe and W metals by means of rigid lattice Monte Carlo simulations using interaction energies obtained from ab initio calculations. These interaction energies are attractive or repulsive depending on direction and distance. This high interaction anisotropy leads to the formation of a 3D compound for both iron and tungsten. A second consequence is that, within a given temperature range, hydrogen enrichment is greater at the outer than at the innermost sites of the dislocation. This unusual behavior is only observed for iron. We propose a mean-field model that accounts for the ordering phenomenon to explain the results and the differences between the two metals. Hydrogen segregation profiles on a screw dislocation in iron (left) and tungsten (right) at different temperatures. For iron, over a given temperature range, hydrogen enrichment is greater at the first outer sites than at the core sites of the dislocation. This behavior is not observed for tungsten. [Display omitted] [ABSTRACT FROM AUTHOR]

The solute strengthening caused by the interaction between dislocations and solutes in random solid solution alloys has already been investigated widely provided that the solutes outside the dislocation core interact through nominally elastic interactions. However, the topological variation of dislocation core due to the solute segregation can influence the strengthening of system. We investigate the dislocation core reconstruction with solute C and Nb and their effects on dislocation glide in γ -TiAl by multiscale QM/MM scheme due to the long-range elastic field of a dislocation. The solutes doping in dislocation core region can influence the core energy and eventually cause a violation of Frank's rule. The contribution analysis of lattice distortion and chemical effect between dislocations and solutes is further investigated. The energy barriers of constriction and transition state in pure γ -TiAl and under solutes environment in the process of external stress reveal electronically the effects of alloying elements and heterogeneous mechanisms on cross-slip process. The carbon diffusion along and across dislocation core by the CI-NEB method are also investigated. [Display omitted] • Dislocation core reconstruction by C/Nb in the long-range elastic field of it. • Strain energy separated into lattice distortion and chemical effect contributions. • Dislocation mobility with C/Nb in its core under stress and the C diffusion in it. [ABSTRACT FROM AUTHOR]

Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.