Dipole Loops Research Articles

Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H ℓ = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10 9 cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization ( ϱ c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot − penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10 9 cm, maximum electron temperatures T e(max) = 3 × 10 6 K, emission measures ∫N e 2d l = 10 28 cm −5, and electron densities N e = 10 9 cm −3 (or pressures p = 1 dyn cm −2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10 7 K and 2 × 10 8 K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.

Defects, Dislocations and Degradation of Compound Semiconductors

ABSTRACTDiode lasers and/or LEDs fabricated from the Ga1 − xAlxAs/GaAs or In1 −xGaxAsyP1−y alloy system provide the basis for rapidly developing optical communications systems. These devices are operated at very high optical and electrical power densities, and the inevitable less-than-perfect efficiency results in intense local thermal and athermal lattice excitation. This in turn can lead to generation, motion and growth of extended defect structures including dislocation networks, precipitation of impurities, or phase separation.For the AlxGa1−xAs material, a predominant degradation effect is the development of so-called <100> “dark line defects” (DLDs). These have been shown to arise from recombination enhanced climb of threading dislocations giving rise to dipole loops of a primarily interstitial character. A less common form, the <110> DLDs, is associated with slip and typically arises from the strain associated with mechanical damage or careless handling or device processing.The In1−xGaxAsyP 1−y device material affords less energy per recombina ion event by virtue of its smaller band-gaps, but is likely to have more lattice mismatch strain than Ga1−xAlxAs for typical compositions of device interest. Thus slip, <110> DLDs, and precipitates are the commonly observed features of In1−xGaxAsyP1−y degradation.

X-ray topographic examination of the generation, glide and locking of basal dislocations during the microplastic deformation of zinc and cadmium crystals

Abstract Nearly perfect zinc and cadmium single crystals were subjected to increasing tensile stresses and simultaneously characterized by transmission X-ray topography. Fresh dislocations were emitted in basal slip planes at resolved shear stresses as small as a few 104 Pa; in some crystals, an emission mechanism could be identified as the elongation and/or the expansion of original dipolar loops; in most eases however, the nucleation took place mainly from stress-enhanced parts of the specimens. Once emitted, the dislocations were observed to interact with sub-grain boundaries, prismatic loops of Burgers vector c or c + a, and with the crystal surface. A model is developed for the latter case, based on the dragging force felt by a dislocation at its point of emergence while it develops a slip step on the surface. In the rather thin crystals (80 pm) used for transmission X-ray topography, this surface effect, could be proved to be responsible for the locking of some fresh dislocations in their glide plane.

Molecular dynamics studies of melting: III. Spontaneous dislocation generation and the dynamics of melting

Abstract The dislocation theory of melting has been further investigated by means of dynamical computer experiments. An f.c.c. Lennard-Jones solid was taken through the melting transition, and it was found that the process is initiated by thermal generation of tiny dislocation loops of the Shockley type. The dislocation density was found to increase steadily during the transition, producing ultimately a system consisting of dislocation cores preferentially arranged as dipole loops, and predominantly in the screw orientation. A simple geometrical interpretation of Lindemann's rule emerges from the model.

Formation of dislocation substructure and hardening mechanism in cyclic strain of FCC metals

1. A cellular dislocation substructure is formed at the moment of hardening saturation in cyclic strain of fcc metals having a high stacking fault energy. 2. The influence of strain amplitude is manifested in a different structure of the cell boundaries; after low-amplitude stressing they represent principally pile-ups of dipolar prismatic loops, after high-amplitude stressing, dislocation networks. 3. The influence of a reduction in stacking fault energy has been followed; it is shown by the example of nickel that it leads to an increase in the stress ensuring the formation of a given dislocation configuration. 4. An examination is made of the possible mechanisms of hardening in cyclic cycles, and also a model of the formation of intrusions and extrusions, taking into account the specific character of the dislocation structure in cyclically strained fcc metals.

Dislocation substructures in deformed uranium dioxide single crystals

Single crystals of uranium dioxide were deformed in compression to nominal strains of 1 and i %. The crystals were oriented to promote slip on only one slip system of the family {100} 〈110〉. The strain rate and temperature ranges studied were 10 −3 to 10 −1 min and 750 to 1400°C, respectively. The dislocation density of the as-grown crystals was 2 × 10 6/ cm 2. Sections of the deformed specimens were examined by transmission electron microscopy, indicating that the dislocation substructures at a strain of 1% consist of numerous dipoles and dipole loops, the edge components of the dipoles lying along 〈110〉 directions. At 5% strain, extensive dislocation tangles are present in addition to the dipole configurations. Particular features are noted in the dislocation arrangements which can be related to several of the theories of dipole formation. The critical resolved shear stress on the {100} 〈110〉 slip system is reported as a function of temperature, as is the variation in the appearance of the loaddeflection curve with temperature level. Of particular interest is the markedly serrated load-deflection behavior observed at 1150 °C, and the minimum in the shear stress curve at 950 °C. The dislocation density as a function of strain has been evaluated, and dislocation velocities are estimated.

Emission et absorption de défauts ponctuels par les dislocations dissociées

Climbing of dislocations is dependent upon the variation in the number of point defects in the crystal; atomic transport cannot, in general, take place except by thermally activated means. Such a process can be described by the diffusion of jogs along the dislocation when the latter is perfect. In the case of an extended dislocation, such jogs, being more difficult to form, become too rare to explain climbing. Two mechanisms of emission or absorption of point defects by extended dislocations as a function of temperature, T, of stress, σ, and of the supersaturation of point defects, c c 0 , are proposed for this last case. One or the other cannot become active unless the values of c c 0 are high or low. The physical conditions for their operation are not greatly dependent upon the nature of the metal considered and are influenced only through the tension of the stacking fault. While the conditions given below pertain to copper, they can be adopted to other metals with changes of only a few hundredths. Climbing is easiest for the edge of a band of stacking faults formed from extended dislocations. This places a portion of the fault in the nearest neighbouring plane the limits of which form a dipole in the fault plane. The easiest to climb are probably the extended edge dislocations which emit or absorb beginning at values of U = kT ln c c 0 + σb 3 ⩾ 0.40 eV. Beginning at 0.65 eV, partial screw dislocations can become active as well as the nuclei of such dipole loops at even the interior of the fault. The second mechanism consists in the nucleation and growth of imperfect dislocation loops at the interior of the bands in which these form “holes”, i.e. regions where the tension of the stacking fault is lower or even zero. Geometrically, this is realized when a vacancy cluster in the plane of the band, for example, collapses. Energetically, such a process is not possible unless U > 0.50 eV in the rare cases of nuclei containing three or four vacancies only. Finally, these loops may serve as stable anchors at high temperatures for extended dislocations.