Effect Of Planar Defects Research Articles

Channeling effects in gold nanoclusters under He ion irradiation: insights from molecular dynamics simulations

The interpretation of helium ion microscopy (HIM) images of crystalline metal clusters requires microscopic understanding of the effects of He ion irradiation on the system, including energy deposition and associated heating, as well as channeling patterns. While channeling in bulk metals has been studied at length, there is no quantitative data for small clusters. We carry out molecular dynamics simulations to investigate the behavior of gold nanoparticles with diameters of 5–15 nm under 30 keV He ion irradiation. We show that impacts of the ions can give rise to substantial heating of the clusters through deposition of energy into electronic degrees of freedom, but it does not affect channeling, as clusters cool down between consecutive impact of the ions under typical imaging conditions. At the same time, high temperatures and small cluster sizes should give rise to fast annealing of defects so that the system remains crystalline. Our results show that ion-channeling occurs not only in the principal low-index, but also in the intermediate directions. The strengths of different channels are specified, and their correlations with sputtering-yield and damage production is discussed, along with size-dependence of these properties. The effects of planar defects, such as stacking faults on channeling were also investigated. Finally, we discuss the implications of our results for the analysis of HIM images of metal clusters.

Mechanical behaviors of nanowires

The mechanical behaviors of nanowires (NWs) are significantly different from those of their bulk materials because of their small dimensions. Determining the mechanical performance of NWs and understanding their deformation behavior are crucial for designing and manufacturing NW-based devices with predictable and reproducible operation. Owing to the difficulties to manipulate these nanoscale materials, nanomechanical testing of NWs is always challenging, and errors can be readily introduced in the measured mechanical data. Here, we survey the techniques that have been developed to quantify the mechanical properties and to understand the deformation mechanisms of NWs. We also provide a general review of the mechanical properties and deformation behaviors of NWs and discuss possible sources responsible for the discrepancy of measured mechanical properties. The effects of planar defects on the mechanical behavior of NWs are also reviewed.

Anisotropic Magnus Force in Type-II Superconductors with Planar Defects

The effect of planar defects on the Magnus force in type-II superconductors is studied. It is shown that the deformation of the vortex due to the presence of a pla- nar defect leads to a local decrease in the mean free path of electrons in the vortex. This effect reduces the effec- tive Magnus coefficient in normal direction to the planar defect, leading to an anisotropic regime of the Hall effect. The presented developments here can qualitatively explain experimental observations of the anisotropic Hall effect in high-Tc superconductors in the mixed state.

Planar defects as Ar traps in trioctahedral micas: A mechanism for increased Ar retentivity in phlogopite

The effects of planar defects and composition on Ar mobility in trioctahedral micas have been investigated in samples from a small marble outcrop (∼500m2) in the Frontenac Terrane, Grenville Province, Ontario. These micas crystallized during amphibolite-facies metamorphism at ∼1170Ma and experienced a thermal pulse ∼100Ma later at shallow crustal levels associated with the emplacement of plutons. 87Rb/86Sr ages of the phlogopites range from ∼950 to ∼1050Ma, consistent with resetting during the later thermal event. The same phlogopites however, give 40Ar/39Ar ages between ∼950 and 1160Ma, spanning the age range of the two thermal events. This result is intriguing because these micas have undergone the same thermal history and were not deformed after peak metamorphic conditions. In order to understand this phenomenon, the chemical, crystallographical, and microstructural nature of four mica samples has been characterized in detail using a wide range of analytical techniques. The scanning electron microscope (SEM), electron microprobe (EMP), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data show that the micas are chemically homogeneous (with the exception of Ba) and similar in composition. The Fourier transform infrared spectroscopy and Mossbauer results show that the M sites for three of the micas are dominated by divalent cations and the Fe3+/(Fe2++Fe3+) ratio for all four phlogopites ranges from 0.10 to 0.25. The stable-isotopic data for calcite indicate that this outcrop was not affected by hydrothermal fluids after peak metamorphism. No correlation between chemical composition and 87Rb/86Sr and 40Ar/39Ar age or between crystal size and 40Ar/39Ar age is observed. The only major difference among all of the micas was revealed through transmitted electron microscope (TEM), which shows that the older 1M micas contain significantly more layer stacking defects, associated with crystallization, than the younger micas. We propose that these defect structures, which are enclosed entirely within the mineral grain may serve as Ar traps and effectively increase the Ar retentivity of the mineral. As this phenomenon has not been previously documented in micas, this may have significant implications for the interpretation of 40Ar/39Ar ages of minerals which have similar defect structures.

Effect of Planar Defects on the Stability of the Bragg Glass Phase of Type-II Superconductors

It is shown that the Bragg glass phase can become unstable with respect to planar crystal defects as twin or grain boundaries. A single defect plane that is oriented parallel to the magnetic field as well as to one of the main axis of the Abrikosov flux line lattice is always relevant, whereas we argue that a plane with higher Miller index is irrelevant, even at large defect potentials. A finite density of parallel defects with random separations can be relevant even for larger Miller indices. Defects that are aligned with the applied field restore locally the flux density oscillations which decay algebraically with distance from the defect. The current-voltage relation is changed to lnV(J) approximately -J(-1). The theory exhibits striking similarities to the physics of Luttinger liquids with impurities.

Energetics and effects of planar defects in CdTe

The formation energies of planar defects such as lamellar twins and intrinsic and extrinsic stacking faults in CdTe are calculated using first-principles total-energy calculations. We find that the formation energies, 16 erg/cm2 for lamellar twins and 34 and 31 erg/cm2 for intrinsic and extrinsic stacking faults, are very small. This explains why high densities of planar defects are always present in the fast-grown CdTe thin films. The effects of the planar defects on the formations of important point defects in p-type CdTe are also investigated. We find that the planar defects have negligible effects on Cd vacancies and substitutional Cu, whereas they lower the formation energy of Te antisites by about 0.1 eV compared to the perfect regions.

Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder

The effects of planar‐defect density in a β‐SiC starting powder and the addition of α‐SiC seeds to that powder on microstructural evolution in liquid‐phase‐sintered (LPS) SiC have been studied separately. Planar‐defect density is altered by appropriate heat treatment of an as‐received β‐SiC starting powder. It was found that a decrease in the planar‐defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α‐SiC occurs on the planar defects present in the β‐SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar‐defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β‐SiC grains coarsen in an equiaxed manner. In contrast, addition of external α‐SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α‐SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.

Planar defects in itinerant ferromagnets

The effect of planar defects, particularly antiphase boundaries, upon the electronic structure and magnetic ordering of the ground states of certain binary and Heusler alloys is investigated theoretically. Because the magnetisation in question arises from bands of itinerant d electrons, the systems are taken to be described by a variant of Hubbard's Hamiltonian for a narrow s band. The planar defects are represented as a pair of adjacent planes between which anomalous values exist for the nearest-neighbour transfer matrix elements which constitute the kinetic part of the Hamiltonian. A tractable if crude model is generated by decoupling the equations of motion of the corresponding Green functions in the Hartree-Fock approximation, and self-consistent solutions are then obtained which manifest the site density of states and the distribution of magnetic moments in the vicinity of the defect. Comparison of the total energies of the pertinent ground states shows that, when the anomalous transfer matrix integrals are sufficiently larger than those characteristic of the bulk, the defect may support a narrow domain wall. Such a phenomenon has been observed to be associated with the periodically recurring stacking faults in Pd3Mn and the random antiphase boundaries found in the CuMnAl Heusler alloys and in MnAl.

Some effects of planar defects near the Si/SiO2 interface on electrical properties of silicon-on-sapphire/MOS devices

Data are presented which show some primary effects of widely different planar defect densities (at the top surfaces of the films) on the electrical properties of silicon-on-sapphire/MOS devices. These electrical properties are directly related to the microstructure of the films using transmission electron microscope methods. Both surface mobility and effective surface threshold potential are found to depend strongly on the surface defect density. Specimens having surface densities of 3×104 and 4×105 cm−1 were examined.

The effect of planar defects on exchange interactions in ferromagnetic metals

A Ruderman-Kittel interaction between magnetic ions is assumed and the defect is represented by a change in the hopping integral of the conduction electrons. If the hopping integral across the defect is substantially larger than the perfect crystal value, the direction of magnetization reverses across the defect. This is a possible explanation of the observed pinning of domain walls to antiphase boundaries in Cu-Mn-Al Heusler alloys and in MnAl.