Intermediate Pinning Research Articles

Interaction between basal edge/mixed dislocations and point defects in zirconium

Zirconium alloys are widely used in nuclear reactors. Due to frequent displacement cascade events under irradiation, a large number of point defects are generated in zirconium alloys. In this work, the interactions between edge/mixed dislocations and point defects (vacancies and SIAs (self-interstitial atoms)) are studied by molecular dynamics (MD) simulations. The simulation results indicate that vacancies are not absorbed by edge dislocations but by mixed dislocations. Compared with their weak pinning effect on edge dislocation motion, vacancies have an intermediate pinning effect on the motion of mixed dislocation. In contrast, SIAs are all absorbed by edge and mixed dislocations and have a strong pinning effect. Then, force equilibrium is established on point defects to explain the absorption of point defects. The attractive force between dislocations and point defects is calculated from the binding energy, and the friction force of point defects is calculated from the migration energy. We found that the friction force of vacancies on edge dislocations is larger than the attractive force, and thus the vacancies cannot be absorbed. For the other three cases, the attractive force is large enough to overcome the friction force, and the point defects are absorbed.

A new concept for column-to-column connections for multi-storey timber buildings—Numerical and experimental investigations

A new jointing concept for the vertical load transfer between glued laminated timber (GLT) columns in multi-storey timber buildings has been developed. The basic components of the connection are two stepped, pyramid-shaped inserts made from block-glued laminated veneer lumber (LVL), which are bonded oppositely into corresponding cavities of the jointed GLTs. The load transfer between the mating columns relies on an intermediate LVL pin, fitting into pyramid-type cavities. Beech LVL was chosen for both, insert and pin, due to its 2.2 times higher characteristic compressive strength as compared to the employed high-strength softwood GLT (GL32h). The form fit of the stepped components which are bonded together relies on high precision robotic milling and a gap-filling two-component polyurethane adhesive. Manufacturing and mechanical performance of the scalable connection was investigated with full-sized specimens, achieving up to 90% of the characteristic GLT compressive capacity of 1040kN for a cross-section of 180mm×180mm. A parametric model based on the finite element method (FEM) showed a good agreement with empiric strain distributions and provided a stress-spread angle of 35° for the concentrated vertical load for an analytical design approach. The location and magnitude of tensile stresses perpendicular to the grain causing splitting failure was identified by FEM. Reinforcement by self-tapping screws proved a significant load-capacity enhancement. The mean experimental axial compressive load capacity exceeded the analytical prediction by 19%. First investigations on the rotational stiffness delivered a value of 240kNm/rad. The column utilization due to additional directly supported floor slabs was assessed exclusively by FEM. The calculation results predict a significant reduction of the tensile stresses perpendicular to the grain to be verified experimentally.

Influence of the insulator geometry on the streamer propagation characteristics in polymeric insulators under positive polarity lightning impulse voltages

Polymer composite insulators acquire the position of their counterpart ceramic and glass insulators in high voltage overhead transmission systems due to their remarkable pollution performance. They have different potential and electric field distributions compared to that of ceramic insulators and the absence of intermediate pin and cap metal fittings makes them compact in size by reducing the distance between energised and grounded ends. This study presents a simulation-based approach to compute the streamer propagation characteristics in polymer suspension insulators under positive polarity lightning impulse voltages. The streamer inception, propagation, and sustenance criteria are evaluated using finite element method-based electric field computations. The significance of insulator geometries on streamer inception and propagation phenomena is well established. The insulator geometries such as weather sheds, location of the first shed, and shed radius have shown a strong influence on streamer inception and propagation path, which govern the flashover characteristics of the insulator. The lightning impulse flashover voltages of different insulators were determined experimentally to validate the computed results. A good agreement is found between simulation and experimental results.

Geometrical vortex lattice pinning and melting in YBaCuO submicron bridges.

Since the discovery of high-temperature superconductors (HTSs), most efforts of researchers have been focused on the fabrication of superconducting devices capable of immobilizing vortices, hence of operating at enhanced temperatures and magnetic fields. Recent findings that geometric restrictions may induce self-arresting hypervortices recovering the dissipation-free state at high fields and temperatures made superconducting strips a mainstream of superconductivity studies. Here we report on the geometrical melting of the vortex lattice in a wide YBCO submicron bridge preceded by magnetoresistance (MR) oscillations fingerprinting the underlying regular vortex structure. Combined magnetoresistance measurements and numerical simulations unambiguously relate the resistance oscillations to the penetration of vortex rows with intermediate geometrical pinning and uncover the details of geometrical melting. Our findings offer a reliable and reproducible pathway for controlling vortices in geometrically restricted nanodevices and introduce a novel technique of geometrical spectroscopy, inferring detailed information of the structure of the vortex system through a combined use of MR curves and large-scale simulations.

Interaction of vortices in anisotropic superconductors with isotropic defects

To assess a superconductor’s suitability for high-current applications, it is imperative to know how its critical current density () depends on magnetic field and its orientation. We present a comprehensive study of the anisotropy of the in-plane in iron-based superconducting single crystals carried out by an advanced magnetometry technique. As is governed by material defects capable of vortex pinning, we investigated three samples with three essentially different defect landscapes representing weak, strong and intermediate pinning. In the latter case, a second maximum in the field dependence of the critical current (fishtail effect) occured, allowing us to investigate the influence of this effect on the angular dependence of in an intermediate pinning regime for the first time in a quantitative way. For weak and strong pinning, the influence of the field orientation on the in-plane can be described by the anisotropic scaling theory. For weak pinning, we find the predicted scaling for the field () and for the in-plane current (unity). For strong pinning, we find the same scaling for the field, but show for the first time that an additional scaling factor () emerges for the in-plane critical current density. We attribute this new scaling rule (double scaling) to the presence of defects which are larger than the vortex core.

Dynamic Reorganization of Vortex Matter into Partially Disordered Lattices.

We report structural evidence of dynamic reorganization in vortex matter in clean NbSe(2) by joint small-angle neutron scattering and ac susceptibility measurements. The application of oscillatory forces in a transitional region near the order-disorder transition results in robust bulk vortex lattice configurations with an intermediate degree of disorder. These dynamically originated configurations correlate with intermediate pinning responses previously observed, resolving a long-standing debate regarding the origin of such responses.

Quenched pinning and collective dislocation dynamics.

Several experiments show that crystalline solids deform in a bursty and intermittent fashion. Power-law distributed strain bursts in compression experiments of micron-sized samples, and acoustic emission energies from larger-scale specimens, are the key signatures of the underlying critical-like collective dislocation dynamics - a phenomenon that has also been seen in discrete dislocation dynamics (DDD) simulations. Here we show, by performing large-scale two-dimensional DDD simulations, that the character of the dislocation avalanche dynamics changes upon addition of sufficiently strong randomly distributed quenched pinning centres, present e.g. in many alloys as immobile solute atoms. For intermediate pinning strength, our results adhere to the scaling picture of depinning transitions, in contrast to pure systems where dislocation jamming dominates the avalanche dynamics. Still stronger disorder quenches the critical behaviour entirely.

Controlling flux flow dissipation by changing flux pinning in superconducting films

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state.

Effect of Pin Length and Rotation Rate on the Tensile Strength of a Friction Stir Spot-Welded Al Alloy: A Contribution to Automated Production

Friction stir spot welding (FSSW) is performed on thin plates of an aluminum alloy in a lap joint configuration with tools of different pin lengths and various rotation rates. The effects these process parameters have on the joint properties of the welds are investigated. The tensile strength of the welds decreased when the rotation rate was increased. The tensile strength of welds made with a pinless tool is on average 90% the strength of the full penetration spot welds. Intermediate pin lengths were tested between these two extremes. It was found that the tensile strength decreases as the pin length increases from pinless to 10% bottom plate penetration. Three distinct failure modes were identified when the welds were placed under tensile loading: shear mode, mixed mode, and nugget-pullout mode. The dependence of static joint strength on these process parameters is discussed.

Current-induced domain wall motion: Domain wall velocity fluctuations

The spin transfer torque generated by a spin polarized current can generate the domain wall (DW) motion in a magnetic nanowire. We examine fluctuations of a DW velocity induced by uncontrolled edge roughness and defects. In the weak pinning regime (regime I), the fluctuation is small. In the intermediate pinning regime (regime II), the DW velocity fluctuates significantly as the density of pinning centers changes. Moreover the threshold current density for the finite density of pinning centers is found to be noticeably larger than the threshold current density for the escape from a single pinning center. Finally, in the strong pinning regime (regime III), the DW velocity fluctuates as the density of pinning centers changes. But the enhancement of the threshold current density does not occur. Thus the fluctuations of the DW velocity are different in the three pinning regimes.

Vortex-lattice pinning in two-component Bose-Einstein condensates

We investigate the vortex-lattice structure for single- and two-component Bose-Einstein condensates in the presence of an optical lattice, which acts as a pinning potential for the vortices. The problem is considered in the mean-field quantum Hall regime, which is reached when the rotation frequency $\ensuremath{\Omega}$ of the condensate in a radially symmetric trap approaches the (radial) trapping frequency $\ensuremath{\omega}$ and the interactions between the atoms are weak. We determine the vortex-lattice phase diagram as a function of optical-lattice strength and geometry. In the limit of strong pinning the vortices are always pinned at the maxima of the optical-lattice potential, similar to the slow-rotation case. At intermediate pinning strength, however, due to the competition between interactions and pinning energy, a structure arises for the two-component case where the vortices are pinned on lines of minimal potential.

Statics and Dynamics of 2D Colloidal Crystals in a Random Pinning Potential

We report the first experimental study of a model system of a two-dimensional colloidal crystal in a random pinning potential. The colloidal crystal consists of monodispersed charged polystyrene microspheres suspended in deionized aqueous media and confined near a rough charged surface. It is found that the static orientational correlation function g6(r) decays exponentially for intermediate and strong pinning, in agreement with theories. The driven depinning is dominated by thermally activated creep motion along 1D-like channels between regions with short-range order. A coexistence model is proposed for describing the observations.

Vibration of Tensioned Beams with Intermediate Damper. I: Formulation, Influence of Damper Location

Exact analytical solutions are formulated for free vibrations of tensioned beams with an intermediate viscous damper. The dynamic stiffness method is used in the problem formulation, and characteristic equations are obtained for both clamped and pinned supports. The complex eigenfrequencies form loci in the complex plane that originate at the undamped eigenfrequencies and terminate at the eigenfrequencies of the fully locked system, in which the damper acts as an intermediate pin support. The fully locked eigenfrequencies exhibit “curve veering,” in which adjacent eigenfrequencies approach and then veer apart as the damper passes a node of an undamped mode shape. Consideration of the evolution of the eigenfrequency loci with varying damper location reveals three distinct regimes of behavior, which prevail from the taut-string limit to the case of a beam without tension. The second regime corresponds to damper locations near the first antinode of a given undamped mode shape; in this regime, the loci bend b...

Experimental Evidence for New Pinning Conditions in Strongly Pinned Permalloy Film

Experimental evidence suggests that spin pinning conditions for standing spin mode in polycrystalline <TEX>$Ni_{84}Fe_{16}$</TEX> alloy film, is confirmed using the four categories of Camley et al. and the value of q<TEX>$\bot$</TEX> for the pinning conditions, this was achieved by performing Brillouin light scattering measurements. The value of q<TEX>$\bot$</TEX> was observed, in order to increase at a rate of <TEX>$\pi$</TEX>/2 with an increasing mode number. With this condition, the coexistence of standing spin wave modes was shown, with intermediate pinning, in addition to free and strong pinning. The values for spin-wave-stiffness constant (<TEX>$D_B$</TEX>), g-factor (g), and surface magnetization (4<TEX>$\pi$</TEX>M) of the Permalloy film were obtained from the results of BLS, and determined to be <TEX>$D_B=1.85{\pm}0.05Oecm^2$</TEX>, <TEX>$g=2.11{\pm}0.02$</TEX>, and 4<TEX>$\pi$</TEX>M=8.7 kG, respectively.

Localization and coercivity of random granular nanomagnets

We study the spin configurations of the random granular nanomagnets in the strong to intermediate pinning limit by Monte Carlo simulations. The difference in spin configurations at zero and finite field is localized. A delocalization transition takes place at a critical field. We develop an analytic method to understand the localization pattern of the difference in spin configurations. We also analytically calculate the finite temperature coercive field and find good agreement with our Monte Carlo results.

Forced Harmonic Response of a Continuous System Displaying Eigenvalue Veering Phenomena

Prior studies of self-adjoint linear vibratory systems have extensively explored the free vibration phenomena associated with veering of eigenvalue loci that depict the dependence of natural frequencies on a system parameter. The present work is an exploration of the effect of such phenomena on the response of a nearly-periodic continuum to harmonic excitation. The focus of the analysis is the prototypical system of a two-span beam with a strong torsional spring at the intermediate pin support. The results of an exact eigenvalue analysis, not previously disclosed, are used to perform a modal analysis of the steady-state response to a harmonic concentrated force applied to the middle of one span. The modal equations are used to identify situations in which the force response is localized to one span, as well as the degree to which the location and magnitude of the peak displacement display parameter sensitivity. The effect of hysteretic loss on forced localization is discussed.

A perturbation solution for forced response of systems displaying eigenvalue veering and mode localization

Various aspects associated with eigenvalue veering, such as extreme parameter sensitivity and localization of response, are of paramount importance for predicting the dynamic response of lightly coupled systems having nearly periodic components. This paper presents a perturbation solution that simplifies analysis of forced response of continuous systems whose free vibration eigensolutions display eigenvalue veering and mode localization. In this investigation, the classic two-span beam with a strong torsional restraint at the intermediate pin support provides a prototype for developing the perturbation solution. Offset of the pin from the center position destroys periodicity of the two-span system, whereas the strength of the torsional spring relates the extent of interspan decoupling. A study by Chen and Ginsberg [J. Vib. Acoust. 114, 141 (1992)] established a relationship between the eigenfunctions and the eigenvalues for different values of the pin offset. The previous findings are merged with a Ritz expansion to formulate the forced response. The adequacy of the solution is determined by comparisons to results of baseline ‘‘Sturm–Liouville-type’’ analyses. Flexibility of the perturbation solution is assessed in various case studies of lightly coupled two-span systems.

Halogen-induced surface states acceptors on GaAs(110) surfaces

The adsorption of iodine was followed by using X-ray photoemission spectroscopy. The electronic properties of the system I 2:GaAs(110) were investigated by measuring the change in work function by using a Kelvin probe and of the surface band-bending as well as of the ionization energy by using ultraviolet photoemission spectroscopy. As a function of exposure to I 2 molecules, provided by a solid-electrolyte cell, the bands were initially found to remain flat but to bend upward on samples doped p-type and n-type, respectively. This behavior immediately indicates that iodine induces surface states of acceptor character within the bulk band gap. From an intermediate pinning position of the Fermi level as a function of exposure to I 2, the energy position of these surface acceptors results as (0.3±0.03) eV above the valence-band maximum. The formation of adatom-induced surface states may be described in a tight-binding approach which considers nearest-neighbor interactions only.

Theory of the threshold field for the depinning transition of a charge density wave

The Hamiltonian of an elastic string pinned by random potentials is often used to describe the depinning transition of a charge density wave in the presence of impurity pinning. The properties of the pinned states show close analogies to those of glassy systems, while the depinning transition resembles a dynamical critical phenomenon. Here we develop an approximate analytical approach based on the quasi-harmonic treatment of the nonlinear pinning potentials together with an expression of the correlation functions to lowest order. This method allows a nonperturbative treatment of a problem with many nonlinear degrees of freedom and quenched disorder. In this way it is possible to characterize the pinned configurations and to compute the threshold field as a function of the coupling constant of the model. The results are in very good agreement with the computer simulations for strong and intermediate pinning and provide an approximate but quantitative method for the determination of the pinned-unpinned phase diagram.

Mode localization in a fluid-loaded plate.

The effects of fluid loading on the response of nearly periodic structures is a topic of current concern. Several dynamic features of such systems are closely related to the vibrational properties of a general self-adjoint system in which two eigenvalues are close. This often leads to mode localization, in which eigensolutions are confined to subregions. A two-span beam with a torsional spring at its intermediate pin support displays such behavior when that support is slightly off-center. This paper examines the degree to which such phenomena are important for fluid-loaded structures by considering a two-dimensional plate in an infinite baffle with a similar type of intermediate line support. In addition to the fluid loading, the plate is subjected to point forces. Acoustic field pressure and the plate displacement are determined by a coupled formulation of the surface variational principle and Hamilton’s principle based on the assumed modes technique. Results for surface displacements are examined for evidence of mode localization in the presence of fluid loading. Radiated power as a function of frequency is also examined for the significance of the center support offset. [Work supported by ONR.]