Double Kink Research Articles

Nonglassy relaxation by double kinks inYBa2Cu3O7−δwith columnar defects

We present long-term relaxation measurements of the irreversible magnetization in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals with columnar defects. Within the single vortex pinning regime we find a nonglassy response at intermediate currents, in between the half loops and superkinks glassy regimes. We demonstrate that such behavior is due to double kink excitations whose critical nucleus has a size independent of current, and occur even in the presence of pinning energy dispersion.

Molecular dynamics simulation of crystal dissolution from calcite steps

Molecular-dynamics simulations were used to model two stepped ${101\ifmmode\bar\else\textasciimacron\fi{}4}$ surfaces of the calcium carbonate polymorph calcite. The acute monatomic steps were found to be more stable than the obtuse monatomic steps. The initial stages of dissolution from the steps were considered in vacuo and in water. In vacuo ${\mathrm{CaCO}}_{3}$ was shown to dissolve preferentially from the obtuse step. In aqueous environment both stepped surfaces are stabilized by the presence of the water molecules although the relative stabilities remain similar. Using potential parameters that reproduce experimental enthalpies of the dissolution of calcite crystal, the formation of the double kinks on the obtuse step is shown to cost less energy than dissolution from the acute step, probably due to the lower stability of the obtuse surface. The simulations suggest that formation of the kink sites on the dissolving edge of the obtuse step of calcite is the rate determining step and this edge is predicted to dissolve preferentially, which is in agreement with experimental findings of calcite dissolution under aqueous conditions.

Effect of rotation on ideal and resistive MHD modes

Experimental observations suggest that toroidal plasma rotation can play a crucial role in determining the stability of a tokamak. Conventional MHD stability analysis ignores rotation. Results from analytic and numerical analysis of a number of tokamak stability issues including toroidal plasma rotation are reported. Depending on the plasma and rotation profiles, the net effect of rotation can be either stabilizing or destabilizing; it also has a substantial effect on the observed mode structure. Localized interchange, the double kink mode, low-n resistive MHD modes and the effect of sheared rotation on magnetic island structure are discussed.

Deformation by a kink mechanism in high temperature materials

A new double kink dislocation model has been developed to explain the temperature dependence of the yield stress in materials such as oxides and intermetallics that require high temperatures for plastic flow. The major variation in the free energy for the formation of a double kink nucleus with stress is the kink–kink interaction energy. However, there is also a stress dependence of the pre-exponential factor in the strain rate constitutive equation arising from kink diffusion. Numerical solution of the resulting equations shows that there are temperature regimes where the stress varies logarithmically with temperature. The model explains quantitatively the temperature dependence of the critical resolved shear stress on different slip systems for sapphire and spinel in terms of different activation energies for kink diffusion. The model can be modified to explain compositional softening in spinel by incorporating enhanced kink nucleation at cation vacancies. This changes both the pre-exponential term and the activation energy and explains why the critical resolved shear stress (CRSS) decreases as the inverse of the square of the vacancy concentration, as is observed.

A new model for dislocation kink-pair activation at low temperatures based on the Peierls-Nabarro concept

Abstract The generation of double kinks in sessile screw dislocation cores is a highly nonlinear process but is the rate-controlling step for plasticity at low temperatures. In this work, double-kink generation in threefold screw dislocations in bcc crystals is studied within the framework of the Peierls-Nabarro (P-N) model in which the partial dislocations are described by non-singular displacement fields. The kink geometry is modelled by dividing the dislocation into elemental slabs, each of which is assumed to have its own displacement field calculated using the P-N model under the plane-strain condition. Explicit expressions for the apparent activation energy and volume have been obtained as a function of applied stress.

Vortex pinning in YBa 2Cu 3O 7 single crystals with crossed columnar defects

We have studied the vortex pinning properties in YBa 2Cu 3O 7 single crystals with crossed columnar defects of various defect angles (0–±40°). The critical current density, the normalized creep rates, the vortex activation energies, and the irreversibility lines (IRL) were found to be a sensitive function of defect angle. In a temperature range from 50 to 70 K, a crystal with 20° showed the highest enhancement in J c as well as the lowest relaxation rate near the matching field of the defect density. We found a change in vortex dynamics from the half-loop excitation to the double kink creep with increasing magnetic field in the cross-defect crystals. The temperature dependence of IRL as a function of defect angle suggested that the crossed defects may promote the breaking of vortex lines into vortex segments near T c.

Energy considerations regarding yield points during indentation

Two experiments that probe the nature of the rapid transition from elastic to plastic deformation are described. The load, and therefore stress, at which this yield point occurs is shown to be relatively independent of temperature in an iron alloy. When stresses lower than those required to generate a yield point during loading are applied for times between seconds and minutes, yielding occurs while the sample is under an applied stress. The time to generate a yield point increases as the applied stress is decreased. The possibilities of dislocation glide loop nucleation, double kink nucleation, and dislocation breakaway from pinning points are examined. Only glide loop nucleation appears to match the experimental observations. Criteria based on the stress-volume requirements of glide loop nucleation and the stress field underneath an indenter are presented which qualitatively describe the experimental data.

Effect of toroidal rotation on the localized modes in low beta circular tokamaks

Effects of finite toroidal rotation on the stability of radially localized ideal magnetohydrodynamic (MHD) modes are investigated analytically for low-beta, large aspect-ratio circular tokamaks. Specifically, it is found that, for the Mercier interchange mode, a finite rotation shear is generally stabilizing. For the double internal kink mode, however, finite toroidal rotation shear further enhances the ballooning destabilizing effect.

Molecular dynamics study of edge dislocation motion in a bcc metal

The motion of an edge dislocation gliding in a bcc lattice under shear loading is simulated by molecular dynamics. The time evolution of the dislocation profile, derived by analyzing the spatial disregistry between two adjacent atomic rows, reveals frequent nucleation of double kinks and little activity in kink migration in association with the dislocation motion. An essentially linear stress variation is obtained, while the temperature dependence suggests the mechanism of phonon drag.

Angular variation of pinning near the irreversibility temperature in single crystal Y Ba 2Cu 3O 7− δ with splayed columnar defects

We have studied vortex pinning through ac susceptibility measurements in single crystals of YBCO with columnar defects (CDs). The CDs have 0°, 10° and 20° splay angle and average direction 10° off the c-axis. By studying the angular variation we can compensate for the anisotropy and effects of twins, etc. Using a simple expression we can obtain the angular spread when the field direction is outside the splay angle. An increase of pinning when the field direction is inside the angle defined by the CDs can be attributed to vortex entanglement due to the splay and the suppression of the sliding of double kinks.

Dislocation mobility and the mechanical response of b.c.c. single crystals: A mesoscopic approach

A mesoscopic simulation of dislocations and plasticity in b.c.c. crystals at low temperatures is developed and applied to the case of Ta. The thermally activated nucleation of double kinks is taken as controlling the mobility of screw dislocations. The resulting temperature and strain rate dependence of the yield stress are investigated and a detailed thermal activation analysis is performed on the output of the simulations. It is shown that within the simplifications made to the input parameters and slip geometry, one is able to simulate realistic dislocation structures and mechanical response. By checking the conformity between the input and output, it is shown that the essential dislocation property (activation energy for screw dislocation motion) can effectively be deduced from the mechanical tests. The mesoscopic simulation provides a potential tool to link atomistic studies at the microscopic scale to the macroscopic modeling of mechanical properties of b.c.c. metals at low temperatures.

Electronically enhanced kink motion on 30° partial dislocations in Ge directly observed by plan-view high resolution electron microscopy

In the high resolution electron microscopic (HREM) images of dislocations in Ge obtained with the electron beam incident normal to the slip plane, 30° partial dislocation lines were extracted by image processing, and kinks in them were identified at atomic resolution. Analysis of sequential images taken from the same dislocation lines revealed that (1) geometrical kinks of different signs, 30° (left) and 90° (right) kinks, are both mobile even at room temperature (2) generation of kink pairs occurs, albeit less frequently, in the course of HREM observations. These results are reasonably understood when considering that the migration of kinks, at least one of the two types, and the formation of the smallest double kinks are enhanced by the electronic excitation that is caused by the electron beam used for the HREM observations.

The influence of fast ions on the magnetohydrodynamic stability of negative shear profiles

The influence of energetic ions on the stability of ideal double kink modes in a tokamak plasma with negative magnetic shear is investigated. It is found that the fast ions play a similar role as for the ordinary m=n=1 internal kink. In particular, phenomena analogous to sawtooth stabilization and fishbone excitation are possible.

A scanning force microscope study of a tribochemicalsystem: stress-enhanced dissolution

Scanning force microscopy (SFM) is employed to study a modeltribochemical system: single-crystal calcite in saturated(≥ 60 μ M) CaCO3solution. The tribological stimulus is provided by the SFM tipitself. Local dissolution along monolayer steps is readilyincreased by an order of magnitude by scanning at high contactforces (e.g., 270 nN). Indentations near pre-existing steps alsolocally enhance the step velocity. At very high contact forcesand point-like scans (10 x 10 nm2), etch pits can benucleated in flat terraces. The wear produced by drawing the SFMtip back and forth in a linear fashion across monolayer steps canbe quantitatively described in terms of the volume-activatednucleation of double kinks. The resulting activation volumes (4x 10-29 m3 and 6 x 10-29m3 for the two crystallographically distinct steps inthis study) are similar to the average volume per ion in thecalcite lattice. This technique is readily adapted to morecorrosive environments, and may allow for a more fundamentalunderstanding of tribochemical effects.

An analytic study of the magnetohydrodynamic stability of inverse shear profiles

This paper reports on the ideal magnetohydrodynamic (MHD) stability of tokamak field profiles that have a non-monotonic safety factor q(r). An analytic criterion is obtained for these ‘‘inverse shear’’ profiles by expanding in inverse aspect ratio and assuming that the minimum in q is slightly less than the m/n value of the mode under examination (m and n being the principal poloidal and toroidal mode numbers of the instability). Three terms are identified as controlling the stability of this ‘‘double kink’’; two of them are stabilizing and due, respectively, to field line bending and the interaction of average favorable curvature with the pressure gradient. The possibility of instability comes from the third term which is due to toroidal coupling and is ballooning in character. The analytic results are compared with those from a fully toroidal stability code.

Ion scattering and scanning tunneling microscopy studies of stepped Cu surfaces

The structure and dynamics of stepped Cu(115) surfaces and their behaviour under oxygen adsorption was studied by ion scattering spectroscopy (ISS) and scanning tunneling microscopy (STM). Azimuthal ion intensity distributions show pronounced minima along the first layer atom rows which can be reproduced by MARLOWE simulations. Detailed analysis in the backscattering mode indicates relaxation of step edge atoms. Increasing the surface temperature leads to scattering into otherwise blocked regions of the azimuthal and polar intensity distributions that can be explained by the production of adatoms or kinks along the [1 1 0] step edges. This thermal roughening is observed above an onset temperature of about 390 K and is completely reversible. These results are supported by STM studies that show that regular monoatomic steps are rather stable at room temperature, whereas for double steps pronounced kink motion is observed with an activation energy of about 0.1 eV. Oxygen adsorption causes pinning of the steps and above temperatures of about 500 K it leads to strong faceting of the surface with (014), (104) and (113) faces.

Atomic layer wear of single-crystal calcite in aqueous solution using scanning force microscopy

Scanning force microscopy (SFM) is employed to study nm-scale wear of single-crystal calcite in an aqueous solution. When the SFM tip is drawn back and forth in a linear fashion across the edge of a preexisting single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The wear rate as a function of contact force is consistent with a thermally activated wear process, where the activation energy is locally reduced in the strain field of the SFM tip. The activation volume for the strain dependence is on the order of the average volume per ion in the CaCO3 lattice. This study provides further support for strain enhanced nucleation of double kinks along preexisting steps.

Formation, microstructure and development of the localized shear deformation in low-carbon steels

An investigation has been made into the effect of microstructural parameters on the propensity for forming shear localization produced during high speed torsional testing by split Hopkinson bar with different average rates of 610, 650 and 1500 s −1 in low carbon steels. These steels received the quenched, quenched and tempered as well as normalized treatments that provide wide microstructural parameters and mechanical properties. The results indicate that the occurrence of the shear localization is susceptible to the strength of the steels. In other words, the tendency of the quenched steel to form a shear band is higher than that of the other two steels. It is also found that there is a critical strain at which the shear localization occurs in the steels. The critical strain value is strongly dependent on the strength of the steels. Before arriving at this point, the material undergoes a slow work-hardening. After this point, the material suffers work-softening, corresponding to a process during which the deformation is gradually localized and eventually becomes spatially correlated to form a macroscopic shear band. Examinations by SEM reveal that the shear localization within the band involves a series of sequential crystallographic and non-crystallographic events including the change in crystal orientation, misorientation, generation and even perhaps damage in microstructures such as the initiation, growth and coalescence of the microcracks. It is expected that the sharp drop in the load-carrying capacity is associated with the growth and coalescence of the microcracks rather than the occurrence of the shear localization, but the shear localization is seen to accelerate the growth and coalescence of the microcracks. The thin foil observations by TEM reveal that the density of dislocations in the band is extremely high and the tangled arrangement and cell structure of dislocations tends to align along the shear direction. The multiplication and interaction of dislocations seems to be responsible for work-hardening of the steels. The avalanche of the dislocation cells corresponds to the sharp drop in shear stress at which the deformed specimen is broken. Double shear bands and kink bands are also observed in the present study. The principal band develops first and its width is narrower than that of the secondary band.

Stellar Models and Luminosity Functions for the Population II Main Sequence Down to Its Lower End

view Abstract Citations (47) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Models and Luminosity Functions for the Population II Main Sequence Down to Its Lower End D'Antona, Francesca ; Mazzitelli, Italo Abstract We have computed new sets of stellar models, for masses from ∼0.8 Msun down to the lower termination of the main sequence, for metallicities and helium contents adequate to represent the halo and globular cluster populations, with the most updated opacity tables. By means of theoretical color Teff transformations by Kurucz and by Allard, we have derived MI versus V-I data to be compared with globular clusters and nearby subdwarf data. We show that the characteristic "double kink" shape of the MI versus V-I diagram is well reproduced by the models, although better agreement would have been obtained if the V-I theoretical colors were redder by ∼0.1 mag. We then discuss the mass-luminosity relation, particularly in correspondence with the kinks, since they turn out to be critical in establishing the luminosity functions (LFs) of the observed sets of stars. We present tables of the mass-MI-V-I relations, which can be used for comparisons with observed LFs. We also discuss the general features of the luminosity functions obtained from these models assuming simple analytic mass functions of different power-law slope. We find that all the LFs peak in the range 8 ≤ MI < 10, the magnitude of the maximum increasing both with Z and with the power-law exponent in the mass function. As an example, we compare our results with the recent Hubble Space Telescope observational results by Paresce et al. for the globular cluster NGC 6397. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176654 Bibcode: 1996ApJ...456..329D Keywords: STARS: INTERIORS; STARS: LOW-MASS; BROWN DWARFS; STARS: LUMINOSITY FUNCTION; MASS FUNCTION; STARS: POPULATION II; STARS: SUBDWARFS full text sources ADS | data products SIMBAD (2)

Photo-Hall studies of modulation-doped field-effect trasistor heterostructures using (InAs) m(GaAs) n superlattice channels

We studied the electrical properties, the Hall mobility and the free carrier concentration of modulation-doped field-effect transitor heterostructures using M(InAs) m N(GaAs) m short-period superlattice (SPS) channels. We changed the number of monolayers m for InAs, n for GaAs and the numbers M and N of interfaces InAs m GaAs n in the channel. In the SPS, the thickness of the InAs layer was varied from 0.5 ± 0.1 to 1.6 ± 0.1 monolayers (ML) and that of GaAs from 5 to 15 ML. The number M of interfaces varied from 3 to 7 while the number N varied from 2 to 6. The total SPS channel thickness was varied, but always kept below the critical layer thickness calculated from the double kink model derived by Matthews and Blakeslee. The persistent photoconductivity effect, at 77 K, was employed to study the variation of the Hall mobility and free carrier density in the SPS channels. A red light-emitting diode was used as the illumination source. Our results indicate that the Hall mobility increases for (a) a decreasing number of InAsGaAs and GaAsInAs interfaces, (b) on employing an integral number of InAs monolayers, and (c) on employing an increasing number of GaAs monolayers. We believe that they are effective in producing surface smoothing.