Molecular dynamics simulations of the interaction between 60° dislocation and self-interstitial cluster in silicon

Molecular dynamics simulations are performed to investigate the interaction between 60° shuffle dislocation and tetrainterstitial (I₄) cluster in silicon, using Stillinger-Weber (SW) potential to calculate the interatomic forces. Based on Parrinello-Rahman method, shear stress is exerted on the model to move the dislocation. Simulation results show that the I₄ cluster can bend the dislocation line and delay the dislocation movement. During the course of intersection the dislocation line sections relatively far away from the I₄ cluster accelerate first, and then decelerate. The critical shear stress unpinning the 60° dislocation from the I₄ cluster decreases as the temperature increases in the models.     

Stability and diffusion properties of self-interstitial atoms in tungsten:a first-principles investigation

Employing a first-principles method based on the density function theory,we systematically investigate the structures,stability and diffusion of self-interstitial atoms(SIAs) in tungsten(W).The <111> dumbbell is shown to be the most stable SIA defect configuration with the formation energy of ~9.43 eV.The on-site rotation modes can be described by a quite soft floating mechanism and a down-hill "drift" diffusion process from <110> dumbbell to <111> dumbbell and from <001> dumbbell to <110> dumbbell,respectively.Among different SIA configurations jumping to near neighboring site,the <111> dumbbell is more preferable to migrate directly to first-nearest-neighboring site with a much lower energy barrier of 0.004 eV.These results provide a useful reference for W as a candidate plasma facing material in fusion Tokamak.     

Clustering of helium atoms at a 12<111>{110} edge dislocation in α-iron

Atomistic calculations on a $\text{1}\text{2}\text{<111>{110}}$ edge dislocation show a restricted tendency of clustering of helium atom along this dislocation. Clusters with up to 4 helium atoms have been studied. A cluster with 3 helium proved to be most stable.     

Laser annealing of Bi implanted <111> and <100> silicon

The influence of surface orientation in Bi implanted silicon, annealed by Q-switched ruby laser pulse irradiation was investigated. Depth distributions and lattice location of the Bi atoms were obtained using⁴He backscattering and channeling techniques and the electrical behavior studied by sheet resistance measurements. The dopant profiles show partial surface accumulation and in-depth broadening without influence of the substrate orientation. These profiles can be fitted by a numerical calculation based on the normal freezing model with an interfacial segregation coefficient much higher than the equilibrium one. The impurity atoms located in the in-depth profile are shown to be electrically active when their maximum concentration does not exceed 10²⁰ atoms/cm³.     

Pairing interaction in 1<Emphasis Type="Italic">f</Emphasis> <Subscript>7/2</Subscript> shell nuclei

The pairing interaction of neutrons and protons in calcium isotopes and N = 28 isotones is considered. Ground state multiplets formed by neutrons and protons on the 1f_7/2 subshell with seniority values ν = 2, 3, 4 are investigated. Good agreement between the multiplet splitting value and nucleon pairing energies is shown not only for multiplets with seniorities ν = 2, but for higher seniority values as well.     

Frequency shift of the magnetic resonance in 5<Emphasis Type="Italic">s</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">S</Emphasis><Subscript>1/2</Subscript> rubidium atoms due to spin-exchange collisions with optically oriented cesium atoms

For the case of cesium atoms optically oriented in a mixture of cesium and rubidium vapors, the temperature dependence of the frequency shift of a magnetic resonance excited in a set of Zeeman sublevels for two hyperfine states of ⁸⁷Rb 5s ² S _1/2 atoms. It is shown that, in a weak magnetic field of about 2 × 10^?6 T, this shift is determined by the spin-exchange interaction of rubidium atoms with optically oriented ¹³³Cs atoms.     

A comparison of proton channeling in the <111> direction for BaF2 and CaF2

Protons in the region of 400 KeV have been used to study the channeling process in BaF₂ and CaF₂. The rate of loss of protons from the 111 linear channel, stopping power, and critical angle for the channel were found. Two techniques were employed, namely observationof elastic scattering by the lattice ions, and alpha particles produced by the reaction ¹⁹F(p,αy)¹⁶O, from the sharp resonance at 340 keV. Both techniques gave results in good agreement. The channel parameters were much closer together for the two crystals than had been anticipated on the basis of the large difference in charge between Ba and Ca.     

On the interaction between a vacancy and self-interstitial atom clusters in metals

Atomic-scale computer simulation is used to study the interaction between a vacancy and a cluster of self-interstitial atoms in metals with hcp, fcc and bcc crystal structure: α-zirconium, copper and α-iron. Effects of cluster size, atomic structure, dislocation nature of the cluster side and temperature are investigated. A vacancy can recombine with any interstitial in small clusters and this does not affect cluster mobility. With increasing sizes clusters develop dislocation character and their interaction with vacancies depends on whether the cluster sides dissociate into partial dislocations. A vacancy recombines only on undissociated sides and corners created with undissociated segments. Vacancies inside the cluster perimeter do not recombine but restrict cluster mobility. Temperature enhances recombination by either increasing the number of recombination sites or assisting vacancy diffusion towards such sites. The results are discussed with relevance to differences in irradiation microstructure evolution of bcc, fcc and hcp metals and higher level theoretical modelling techniques.     

Anisotropy of g-factor of Cu<Subscript>2+</Subscript> ion and Anisotropic exchange interaction between Spins in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

To study the anisotropy of g-factor of Cu²⁺ ion in the antiferromagnetic Bi₂CuO₄ having an interesting crystal structure, we calculate the energy splitting of the orbital state of Cu²⁺ ion due to the electric crystalline field arising from the surrounding O²⁺, Bi³⁺ and Cu²⁺ ions. On the basis of Pryce's spin Hamiltonian we evaluate g-tensor of each Cu²⁺ ion in the unit cell. We also discuss the anisotropic exchange interaction which is important as the origin of the anisotropy energy for Cu²⁺ ion with S=1/2. The results are discussed in connection with observations.     

表面效应对铁<100>间隙型位错环的影响

在材料辐照损伤过程中,间隙型位错环的形成及动力学行为严重影响材料在辐照条件下的服役行为.在常用的以体心立方铁为基的合金材料中,1/2<111>和<100>是两种主要的位错环,其对辐照损伤的影响一直都是核材料领域研究的热点之一.在之前的研究中,人们对{111}面与单个1/2<111>位错环的相互作用进行了深入研究,发现表面对位错环性质确实有重要的影响.采用分子动力学方法,在原子尺度详细研究了另一个重要的表面铁{100}面对<100>间隙型位错环动力学过程的影响.模拟发现位错环伯格斯矢量与表面法线方向的关系、距表面的深度、位错环之间的相互作用以及温度等,都对位错环与表面的相互作用产生重要影响,其中,表面作用下的伯格斯矢量的演化以及<100>位错环在此过程中的一维运动首次被发现.基于这些模拟结果,就<100>位错环对表面辐照损伤结构的影响进行详细地研究,给出<100>位错环对表面凹凸结构的贡献,这些结果为理解辐照过程中材料表面的演化提供一种可能的解释.     

Mössbauer studies of57Co implanted along <100> and <111> axes of Al

A significant difference in the quadrupole splittings of the weak doublet components, in the Mössbauer Spectra of⁵⁷Co implanted along <100> and <111> axes of A1, is observed. This is qualitatively explained as due to differences in defect production and recovery for implantations along these two directions.     

Fabrication of <1 1 0> oriented tungsten nano-tips by field-assisted water etching

We report the field-assisted H₂O etching that enabled us to fabricate nano-tips from polycrystalline <1 1 0> oriented tungsten wires at room temperature. We optimized the sharpening procedure in order to obtain field emissions (FEs) with high collimation. The typical tip apex was composed of a large base and a nano-protrusion with a radius of curvature less than 3.5 Å. The narrowest opening angle (full width at half maximum) of the FE was 4.3° at 150 pA. We prepared two types of tips using two different applied bias voltages during the H₂O etching. The electron microscope images and the analysis of Fowler-Nordheim (FN) plots revealed that the sizes of the individual bases depended on the fixed bias voltages during the H₂O etching and affected their FE properties. In addition, we could confirm that the FE current from the nano-tip was more stable than that of the normal tip.     

Influence of an external electric field on the probabilities of two-photon transitions between 2<Emphasis Type="Italic">s</Emphasis>, 2<Emphasis Type="Italic">p</Emphasis> and 1<Emphasis Type="Italic">s</Emphasis> levels for hydrogen and antihydrogen atoms

The probabilities of two-photon decay for hydrogen (H) and antihydrogen (\(\bar H\)) atoms in the presence and absence of an external electric field are analytically calculated. In particular, the probabilities of the E1E2 and E1M1 transitions between the 2p and 1s levels are calculated for the case when emitted photons are characterized by polarization vectors and wavevectors. It is shown that, in an external electric field, the decay probabilities for 2s and 2p levels differ for H and \(\bar H\) atoms because of interference terms linear in field. Coulomb Green’s function method is used for summing over intermediate states.     

Electronic surface structure of <Emphasis Type="Italic">n</Emphasis>-ML Ag/Cu(111) and Cs/<Emphasis Type="Italic">n</Emphasis>-ML Ag/Cu(111) as investigated by 2PPE and STS

We investigated the electronic structure of epitaxially grown silver films on Cu(111) with and without adsorption of cesium by means of scanning tunneling spectroscopy and two-photon photoemission. This system has been chosen as a model system to engineer and measure the dynamics of charge-transfer processes between an adsorbate and a heterogeneous substrate. Special emphasis has been laid on the investigation of the energy shift of the Shockley-type surface state and an excited cesium resonance as a function of Ag film thickness. For the cesium resonance we observe an increase in line width with increasing layer thickness. PACS 68.37.Ef; 68.43.-h; 68.55.Jk; 73.20.-r     

Decay of the excited state 3<Superscript>2</Superscript><Emphasis Type="Italic">P</Emphasis><Subscript>3/2</Subscript> of sodium atoms taking into account radiation trapping

The effective lifetime of the excited state 3² P _3/2 of sodium atoms corresponding to the transition with λ = 589 nm has been studied numerically. It is shown that the nonmonotonic behavior of the dependence of the Biberman-Holstein escape factor on the optical thickness of sodium vapor measured in the experiment (A. Romberg and H.-J. Kunze, J. Quant. Spectrosc. Radiat. Transfer 39 (2), 99 (1988)) should not be attributed to the manifestation of the effects of partial frequency redistribution.     

Activation energy for elastic interaction between Frank dislocation loops and glide dislocations in f.c.c. Metals

The activation energyG for the thermally activated passing of glide dislocations close to Frank dislocation loops which do not intersect the slip planes is calculated by means of the linear theory of elasticity as a function of the applied stress, of the distances loop centre-slip plane, and of the orientations and radii of the loops. The influence of anisotropy onG is investigated for some f.c.c. metals, especially for copper. For sufficiently high stresses the dependence ofG on stress is shown to approach theG-stress expression suggested by Seeger in his theory of radiation hardening.On leave fromFaculty of Mathematics and Physics, Charles University, Praha, Ke Karlovu 5, Czechoslovakia.     

Heat transfer between tungsten surface and glow discharge plasmas in argon and CO<Subscript>2</Subscript>

A hot-filament method is used to study the heat transfer between tungsten surface and hollow-cathode glow discharge plasmas in argon and CO₂. The dependence of the electric power supplied to a tungsten wire on the discharge current is determined for argon and carbon dioxide in the temperature range between 1000 and 1700 K. A difference in heat transfer at the tungsten wire surface is found between experiments on argon and carbon dioxide. The difference is attributed to heterogeneous recombination in CO₂ plasma.     

Statistical mechanics of cation ordering in PbSc<Subscript>1/2</Subscript>Ta<Subscript>1/2</Subscript>O<Subscript>3</Subscript> and PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> solid solutions

A model Hamiltonian for B cation ordering (Sc-Nb(Ta)) in PbSc_1/2Nb_1/2O₃ and PbSc_1/2Ta_1/2O₃ solid solutions is constructed. The parameters of the model Hamiltonian are determined from the ab initio calculation within the ionic crystal model with allowance made for the deformability and the dipole and quadrupole polarizabilities of the ions. The temperatures of the phase transition due to the ordering of the B cations are calculated by the Monte Carlo method in the mean-field and cluster approximations. The phase transition temperatures calculated by the Monte Carlo method (1920 K for PbSc_1/2Ta_1/2O₃ and 1810 K for PbSc_1/2Nb_1/2O₃) are consistent with the experimental data (1770 and 1450 K, respectively). The thermodynamic properties of the cation ordering are investigated using the Monte Carlo method.     

Nonadiabatic interaction of acoustic phonons with spins <Emphasis Type="Italic">S</Emphasis>=1/2 in the two-dimensional Heisenberg model

The ground state of a two-dimensional antiferromagnet having spins S=1/2 and interacting with acoustic phonons is investigated in the nonadiabatic approximation using the quantum-mechanical Monte Carlo method. The critical parameters of the spin-phonon coupling, corresponding to the formation of bound spin-phonon excitations, crystal symmetry lowering, and the emergence of a gap in the spin excitation spectrum, as well as the antiferromagnet-quantum spin liquid transition, are determined. The orthorhombicity parameter, the sublattice magnetization, the violation of the spherical symmetry of spin-spin correlation functions, and the magnetic moment in Gd₂CuO₄ and Eu₂CuO₄ are calculated.