Nucleation Of Pairs Research Articles

Micromechanics of plastic deformation through paired twinning in nanomaterials and polycrystals

A theoretical description of the plastic deformation of nanostructured and polycrystalline metals through paired nanotwinning on grain boundaries has been proposed. Within the micromechanical approach developed, deformation nanotwins are formed as a result of successive plastic shears. The energy characteristics and critical stresses of paired twin nucleation in titanium are calculated.

Vortex-antivortex proliferation from an obstacle in thin film ferromagnets

Magnetization dynamics in thin film ferromagnets can be studied using a dispersive hydrodynamic formulation. The equations describing the magnetodynamics map to a compressible fluid with broken Galilean invariance parametrized by the longitudinal spin density and a magnetic analog of the fluid velocity that define spin-density waves. A direct consequence of these equations is the determination of a magnetic Mach number. Micromagnetic simulations reveal nucleation of nonlinear structures from an impenetrable object realized by an applied magnetic field spot or a defect. In this work, micromagnetic simulations demonstrate vortex-antivortex pair nucleation from an obstacle. Their interaction establishes either ordered or irregular vortex-antivortex complexes. Furthermore, when the magnetic Mach number exceeds unity (supersonic flow), a Mach cone and periodic wavefronts are observed, which can be well-described by solutions of the steady, linearized equations. These results are reminiscent of theoretical and experimental observations in Bose-Einstein condensates, and further supports the analogy between the magnetodynamics of a thin film ferromagnet and compressible fluids. The nucleation of nonlinear structures and vortex-antivortex complexes using this approach enables the study of their interactions and effects on the stability of spin-density waves.

Superrotations and black hole pair creation

Recent work has shown that the symmetries of classical gravitational scattering in asymptotically flat spacetimes include, at the linearized level, infinitesimal superrotations. These act like Virasoro generators on the celestial sphere at null infinity. However, due to the singularities in these generators, the physical status of finite superrotations has remained unclear. Here we address this issue in the context of the breaking of a cosmic string via quantum black hole pair nucleation. This process is described by a gravitational instanton known as the C-metric. After pair production, the black holes are pulled by the string to null infinity with a constant acceleration. At late times the string decays and the spacetime settles into a vacuum state. We show that the early and late spacetimes before and after string decay differ by a finite superrotation. This provides a physical interpretation of superrotations. They act on spacetimes which are asymptotically flat everywhere except at isolated singularities with cosmic string defects.

Formation process of the incompatible martensite microstructure in a beta-titanium shape memory alloy

The formation modes of habit plane variant (HV) clusters (HVCs) in a Ti-23Nb-3Al (mol%) shape memory alloy were investigated by in situ optical microscopy with high-speed digital video camera. In addition to the kinematic compatibility condition, the nucleation mode and the interfacial energy between martensites were clearly shown to have critical roles. The VI cluster, which is formed by two HVs connected by a {111} type I twin, predominated in the early stage of the martensitic transformation, and had a lower energy barrier to nucleation and growth than did the VII cluster, which is formed by two HVs connected by a <211> type II twin. Eighty percent of the VI clusters were observed to form by paired nucleation or branching, in which the junction plane corresponds to the nucleation site of the relevant HV(s). In contrast, VII clusters formed as a result of collisions of HVs. Neither paired nucleation nor branching was observed for VII. VI nuclei rather than VII nuclei had a strong tendency to form, and this difference was not fully explained by the kinematic compatibility (KC) conditions. The junction planes formed by the collision of growing HVs should have larger deviation from the KC condition. The compatibility of the junction plane between HVs depends on the formation modes.

Temperature insensitivity of the flow stress in body-centered cubic micropillar crystals

Plasticity of body-centered cubic (bcc) crystals is known to have a strong dependence on temperature, as a direct consequence of the thermally-activated process of kink pair nucleation and migration with a high energy (Peierls) barrier. Here we demonstrate that, in the sub-micron size scale, such strong temperature dependence of the flow stress must disappear. We explore the flow stress and hardening behavior of micro-pillar sizes in the range 200–2000 nm at temperatures of 150–900 K. Discrete Dislocation Dynamics (DDD) simulations reveal that the weak temperature sensitivity can be rationalized in terms of the weak role of screw dislocations in controlling plasticity; unique to small crystals of finite size. It is shown that finite, sub-micron samples have limited ability to store screw dislocations. The necessity of applying high stress in sub-micron crystals is demonstrated to greatly enhance the mobility of screw dislocations, rendering it close to that of edge dislocations. This leads to a transition of the dislocation mobility mechanism from being thermal-activated kink dominated to being phonon-drag dominated. Thus, the flow stress gradually becomes not governed by the mobility of screw dislocations (as determined by the critical resolved shear stress), giving rise to the weak temperature sensitivity of the flow stress. A dislocation mechanism map in the temperature-size space is proposed to further illustrate this phenomenon in tungsten micropillars.

Vortex chain in a resonantly pumped polariton superfluid.

Exciton-polaritons are light-matter mixed states interacting via their exciton fraction. They can be excited, manipulated, and detected using all the versatile techniques of modern optics. An exciton-polariton gas is therefore a unique platform to study out-of-equilibrium interacting quantum fluids. In this work, we report the formation of a ring-shaped array of same sign vortices after injection of angular momentum in a polariton superfluid. The angular momentum is injected by a ℓ = 8 Laguerre-Gauss beam. In the linear regime, a spiral interference pattern containing phase defects is visible. In the nonlinear (superfluid) regime, the interference disappears and eight vortices appear, minimizing the energy while conserving the quantized angular momentum. The radial position of the vortices evolves in the region between the two pumps as a function of the density. Hydrodynamic instabilities resulting in the spontaneous nucleation of vortex-antivortex pairs when the system size is sufficiently large confirm that the vortices are not constrained by interference when nonlinearities dominate the system.

Propagation of optical vortex beams and nucleation of vortex-antivortex pairs in disordered nonlinear photonic lattices

The propagation of optical vortex beams through disordered nonlinear photonic lattices is numerically studied. The vortex beams are generated by using a superposition of several Gaussian laser beams arranged in a radially-symmetric manner. The paraxial nonlinear Schrodinger equation describing the longitudinal propagation of the beam array through nonlinear triangular photonic lattices with two-dimensional disorder is solved numerically by using the split-step Fourier method. We find that due to the spatial disorder, the vortex beam is destabilized after propagating a finite distance and new vortex-antivortex pairs are nucleated at the positions of perfect destructive interference. We also find that in the presence of a self-focusing nonlinearity, the vortex-antivortex pair nucleation is suppressed and the vortex beam becomes more stable, while a self-defocusing nonlinearity enhances the vortex-antivortex pair nucleation.

Topological singular chain reactions in dynamic speckle fields

Interactions of structured light and structured media belong to the topical trends of modern optics. Dynamic vector speckle fields induced by the feedback interaction with the dynamic “optical-damage” effect provide, for the first time, real possibilities to produce and investigate the topological regularities of structured photorefractive media and coherent structured light interactions [Proc. SPIE6905, 690505 (2008)]. The experiments provide evidence that dynamic topological speckle fields form ergodic systems, which develop through loop and chain trajectories in accordance with paired nucleation and annihilation of optical singularities. The loop/chain trajectories are realized for even/odd numbers of involved singular C points. Each loop reaction starts from a nucleated pair of C points and occurs in the same speckle up to their annihilation. A chain reaction starts from one C point of a nucleated C-point pair. It leaves the starting speckle and moves through the speckle field to other C-point pairs and annihilates with one of these. The second C point moves through the varying speckle field to the next pair of C points and so on. The conserved topological charge is equal to +(1/2) or –(1/2) for L-type and S-type chain reactions, respectively. Loop trajectories with zero topological charges are short-lived in contrast with the chain reactions that are theoretically not limited in space and time. The chain reactions consist of long links with a standard topological structure. The results obtained are important for fundamentals and applications. This paper is the first study of the optical dynamic topology.

Counter-rotating vortices in miscible two-component Bose-Einstein condensates

Counter-rotating vortices in miscible two-component Bose-Einstein condensates, in which superflows counter-rotate between the two components around the overlapped vortex cores, are studied theoretically in a pancake-shaped potential. In a linear stability analysis with the Bogoliubov--de Gennes model, we show that counter-rotating vortices are dynamically unstable against splitting into multiple vortices. The instability shows characteristic behaviors as a result of counter-superflow instability, which causes relaxation of relative flows between the two components in binary condensates. The characteristic behaviors are completely different from those of multiquantum vortices in single-component Bose-Einstein condensates; the number of vortices generated by the instability can become larger than the initial winding number of the counter-rotating vortex. We also investigate the nonlinear dynamics of the instability by numerically solving the Gross-Pitaevskii equations. The nonlinear dynamics drastically changes when the winding number of counter-rotating vortices becomes larger, which lead to nucleation of vortex pairs outside of the vortex core. The instability eventually develops into turbulence after the relaxation of the relative rotation between the two components.

Synchronized dynamics of Josephson vortices in artificial stacks of SNS Josephson junctions under both dc and ac bias currents

Nonlinear dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting Josephson junctions under simultaneously applied time-periodic ac and constant biasing dc currents is studied using the time dependent Ginzburg-Landau formalism with a Lawrence-Doniach extension. At zero external magnetic field and dc biasing current the resistive state of the system is characterized by periodic nucleation and annihilation of fluxon-antifluxon pairs, relative positions of which are determined by the state of neighboring junctions. Due to the mutual repulsive interaction, fluxons in different junctions move out of phase. Their collective motion can be synchronized by adding a small ac component to the biasing dc current. Coherent motion of fluxons is observed for a broad frequency range of the applied drive. In the coherent state the maximal output voltage, which is proportional to the number of junctions in the stack, is observed near the characteristic frequency of the system determined by the crossing of the fluxons across the sample. However, in this frequency range the dynamically synchronized state has an alternative---a less ordered state with smaller amplitude of the output voltage. Collective behavior of the junctions is strongly affected by the sloped sidewalls of the stack. Synchronization is observed only for weakly trapezoidal cross sections, whereas irregular motion of fluxons is observed for larger slopes of the sample edge.

Non-basal slip in Ni3(Ti, Nb) and Ni3(Ti, Al) single crystals with various long-period stacking ordered structures

Variations in operative non-basal slip systems in Ni3Ti-based long-period stacking ordered (LPSO) phases with the stacking sequence of the closely packed planes and the temperature dependence of the critical resolved shear stress (CRSS) were investigated. It was confirmed that the {1¯100} prism slip is operative in most of the LPSO phases with hexagonal unit cells, while the {1¯011} pyramidal slip is operative in 9R LPSO phase with rhombohedral unit cell. The mechanism controlling the deformation by non-basal slip at low temperatures is considered to be governed by the energy of the antiphase boundary (APB) formed between the a/6〈112¯0〉 superpartial dislocations in accordance with the kink pair nucleation mechanism proposed by Mitchell et al. The dependence of variations in CRSS for non-basal slip on the Nb content was found to be much smaller than that for basal slip. The present results suggest that the mobility of dislocations on the non-basal plane hardly affected the behavior of yield stress anomalies (YSAs) caused by basal slip in Ni-based LPSO phases, although the Kear–Wilsdorf locking process, which is responsible for the YSA by basal slip, accompanies microscopic cross-slip from the basal slip plane onto the non-basal planes.

The Nucleation and Propagation of Threading Dislocations with C-Component of Burgers Vector in PVT-Grown 4H-SiC

Studies of threading dislocations with Burgers vector of c+a have been carried out using synchrotron white beam X-ray topography. The nucleation and propagation of pairs of opposite sign threading c+a dislocations is observed. Overgrowth of inclusions by growth steps leads to lattice closure failure and the stresses associated with this can be relaxed by the nucleation of opposite sign pairs of dislocations with Burgers vector c+a. Once these dislocations are nucleated they propagate along the c-axis growth direction, or can be deflected onto the basal plane by overgrowth of macrosteps. For the c+a dislocations, partial deflection can occasionally occur, e.g. the a-component deflects onto basal plane while the c-component continuously propagates along the growth direction. One factor controlling the details of these deflection processes is suggested to be related to the ratio between the height of the overgrowing macrostep and that of the surface spiral hillock associated with the threading growth dislocations with c-component of Burgers vector.

Role of defects in the kink mechanism of state switching

State switching occurring via the kink mechanism in linear systems has been described considering the effect of defects forming centers of enhanced kink pair nucleation and obstacles for kink propagation. An equation describing the switching kinetics has been derived considering the stochastic nature of kink pair formation in time and the random distribution of defects in space. Using this equation, the average switching time has been calculated as a function of the defect density and delay times caused by defects, and the ranges of parameters with domination of this or that defect type have been determined. The theory has been applicable to magnetic nanowires, dislocations, biological macromolecules, and many other systems.

Dendritic Crystallization: A Single Process for all the Textures of Olivine in Basalts?

The olivine macrocrysts found in oceanites, picrites and magnesian basalts erupted at hotspot volcanoes are generally interpreted either as phenocrysts crystallized from the magma or as xenocrysts extracted from a deforming cumulate. To constrain the origin of these crystals we studied their texture and composition at Piton de la Fournaise volcano, La Reunion. We show that macrocrysts are organized and subdivided into parallel units; this suggests a crystallization by dendritic growth and ripening rather than by a complex combination of paired nucleation, crystal aggregation or synneusis. Dendritic growth is also evidenced by the occurrence of hollow faces, P-rich zones, melt and Cr-spinel inclusions formed from the accumulation of slow diffusing impurities (P, Cr, Al) in the liquid at the contact with rapid-growing olivine. We suggest that early dendritic crystallization may even cause branch misorientations and lattice mismatches, yielding subgrain boundaries, dislocation lamellae and to a certain extent undulose extinction, which have all been formerly interpreted in terms of plastic intracrystalline deformation. We interpret olivine macrocrysts as phenocrysts crystallized under a strong degree of undercooling (-ΔT > 60°C), and derived from a harrisitic mush formed on the cold walls of the magma reservoir. Given the growth shapes indicated by P zoning patterns and external faces, the olivine macrocrysts (which consist of groups of several subcrystals) have grown in suspension within the liquid and were neither aggregated into a dense cumulate nor corroded, shocked or deformed before or during their transport to the surface. The major consequence of our study is that most olivine macrocrysts are not xenocrysts, and very few of them, if any, have experienced intracrystalline deformation. The importance of deforming (creeping) cumulate bodies, thought to accommodate the spreading of basaltic volcanoes in La Reunion and Hawaii, may hence have been overestimated.

Static and dynamic friction in sliding colloidal monolayers

In a pioneer experiment, Bohlein et al. realized the controlled sliding of two-dimensional colloidal crystals over laser-generated periodic or quasi-periodic potentials. Here we present realistic simulations and arguments that besides reproducing the main experimentally observed features give a first theoretical demonstration of the potential impact of colloid sliding in nanotribology. The free motion of solitons and antisolitons in the sliding of hard incommensurate crystals is contrasted with the soliton–antisoliton pair nucleation at the large static friction threshold F s when the two lattices are commensurate and pinned. The frictional work directly extracted from particles’ velocities can be analyzed as a function of classic tribological parameters, including speed, spacing, and amplitude of the periodic potential (representing, respectively, the mismatch of the sliding interface and the corrugation, or “load”). These and other features suggestive of further experiments and insights promote colloid sliding to a unique friction study instrument.

Lifetime of gap discrete breathers in diatomic crystals at thermal equilibrium

Crystals having a gap in a phonon spectrum can support so-called gap discrete breathers (DBs), i.e., nonlinear localized vibrational modes existing in perfect crystals and having frequencies within the gap. In our recent work, for a two-dimensional crystal with stoichiometry A3B, we have demonstrated that if the atomic weight of the components is related as MA MB , then there is a wide gap in the phonon spectrum and the gap DBs can be easily excited. The situation is opposite if the above condition is not satisfied. In the present work the gap DBs are studied at thermal equilibrium, whereas in the previous study the temperature was zero. We demonstrate that in the crystal supporting DBs, in contrast to the opposite case, the lifetime of high-energy light atoms grows with temperature. The possible role of gap DBs in the thermally activated formation of vacancy and interstitial atom pairs (Frenkel pairs) was studied. We have measured the time needed for the appearance of a Frenkel pair as a function of temperature for the crystals supporting and not supporting DBs. A minor difference in waiting time of Frenkel pair nucleation was found in these two cases. We argue that, for the model and its parameters considered in the present work, the main mechanism of thermally activated Frenkel pair formation is the cooperative motion of atoms rather than the excitation of DBs.

Nucleation of a vortex-antivortex pair in the presence of an immobile magnetic vortex

It is shown that under the action of rotating magnetic field an immobile vortex, contrary to a general belief, can nucleate a vortex-antivortex pair and switch its polarity. Two different kinds of OOMMF micromagnetic modeling are used: (i) the original vortex is pinned by the highly--anisotropic easy-axes impurity at the disk center, (ii) the vortex is pinned by artificially fixing the magnetization inside the vortex core in the planar vortex distribution. In both types of simulations a dip creation with a consequent vortex-antivortex pair nucleation is observed. Polarity switching occurs in the former case only. Our analytical approach is based on the transformation to the rotating frame of reference, both in the real space and in the magnetization space, and on the observation that the physical reason for the dip creation is softening of the dipole magnon mode due to magnetic field rotation.

Vortex-antivortex pair generation by an in-plane magnetic dipole on a superconducting film

We apply the time-dependent Ginzburg-Landau approach to describe the nucleation of vortex-antivortex pairs in a hybrid system, formed by a thin superconducting film with a single in-plane magnetic dipole on top of it, and to simulate the vortex dynamics in the presence of a direct transport current. At current densities, sufficient to depin vortices and antivortices from a magnetic dipole, it periodically generates vortex-antivortex pairs, producing vortex and antivortex streams which eventually coalesce into phase-slip lines with further increasing current. We demonstrate the possibility to efficiently control the generation rate and trajectories of emitted vortices by the applied current density and an additional weak homogeneous magnetic field.

Antivortex domain walls observed in permalloy rings via magnetic force microscopy

Magnetic domain walls in patterned permalloy ring elements of different widths and thicknesses have been probed by magnetic force microscopy and micromagnetic simulations. We report the experimental observation of antivortex structures in single-layer magnetic rings, previously only theoretically predicted. Furthermore, vortex-antivortex chains are observed on occasion in wider rings that prefer a circulating flux closure state. It is possible that they are pinned transient states of the relaxation path from the magnetically saturated to the circulating flux closure state. Similar tendencies are seen in micromagnetic simulations implying that vortex-antivortex pair nucleation and annihilation are fundamental parts of the said relaxation. Finally we present a phase diagram of different domain-wall configurations as a function of ring width and thickness.

Threshold field in nucleation process for an inhomogeneous deformable nonlinear Klein-Gordon system

We investigate the effect of local inhomogeneity on the nucleation process of kink-antikink pairs in the driven nonlinear Klein-Gordon model with the Remoissenet-Peyrard substrate potential, whose shape can be varied as a function of the shape parameter and which has the sine-Gordon shape as a particular case. From numerical integrations of the model equations under defined conditions, the configuration of the critical nucleus in the presence of a localized inhomogeneity is determined and shown to be affected by the shape parameter of the substrate potential. The expression of the depinning threshold field of kink-antikink pairs, which is the value of the applied field at which the process of the nucleation of kink-antikink pairs takes place, is also obtained. The dependence of this depinning threshold field on the shape parameter r shows that it strongly increases, for small intensity of the impurity potential when the shape of the substrate potential deviates from the sinusoidal one.