Excitonic instabilities,vacancies, and reconstruction of covalent surfaces

Two possibilities for explaining the superlattices of (111) surfaces of Si(7 × 7) and Ge(2 × 8) are considered. The equilibrium concentration of surface vacancies is related to the ratio of metallic and directional contributions to vacancy energies. The macroscopic thermo-dynamic explanation based on vacancy concentrations is preferred to an alternate microscopic one based on excitonic instabilities. A relation between anisotropy of surface energies and vacancy concentrations is proposed that predicts new superlattices for Si (100) 2 × 1, (110) 2 × 1, 5 × 1 and 4 × 5.     

Quantum tunnelling of vacancies in light metals

Summary The usefulness of the WKB method is illustrated by calculating characteristic tunnelling rates in several elemental solids and establishing the temperatures at which the tunnelling becomes of comparable importance to the thermally activated processes. In this way we have verified the importance of tunnelling which dominates the low-temperature region, while it becomes negligible above a temperature value depending on the mass and the barrier thickness. In studying vacancy migration in lithium metal we have considered various pathways in the configuration space of Li atoms. A (1, 1, 1)-jump of the vacancy, accompanied by the simultaneous outward movement of between 4 to 12 lithium atoms, is shown to be most efficient. In this model the lithium motion achieves a 10⁴-fold increase in the jump probability and a reduction of the activation energy from ∼0.25 to ∼0.16 eV.

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Riassunto L’utilità del metodo WKB è illustrata calcolando la probabilità di transizione per tunnelling in diversi solidi elementari e stabilendo la temperature alla quale il contributo del tunnelling diviene confrontabile con quello dovuto ai processi termicamente attivati. In tal modo si stabilisce l’importanza del tunnelling che domina la regione delle basse temperature, mentre divience trascurabile a temperature piú alte, dipendenti dalla massa e dallo spessore della barriera. Studiando il moto di vancaze nel litio, sono stati considerati diversi cammini nello spazio delle configurazioni degli atomi Li. Un salto della vacanza nella direzione (1, 1, 1), accompagnato dal simultaneo spostamento di 4÷12 atomi di litio, è dimostrato essere il piú efficiente. In questo modello la probabilità di moto delle vacanze aumenta di un fattore 10⁴ con una riduzione dell’energia di attivazione da ∼0.25 a ∼0.16 eV.

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Резюме Метод ВКБ применяется для вычисления характеристических вероятностей туннельных переходов в некоторых злементарных твердых телах. Определяются температуры, при которых туннелирование дает сравнимый вклад в термически активированные процессы. Мы показываем важность туннелирования, которое доминирует в области низких температур, но становится пренебрежимо малым при более высоких температурах, которые зависят от массы и толщины барьера. Исследуя движение вакансий в литии, мы рассматриваем различные траектории в конфигурационном пространстве атомов лития. Показывается, что скачок вакансии в направлении (1, 1, 1), который сопровождается одновременно смещением от 4 до 12 атомов лития, является наиболее эффективным. В этой модели вероятность движения вакансии увеличивается в 10⁴ раз при уменьшении энергии активации от ∼0.25 эВ до ∼0.16 эВ.

     

Relaxation round vacancies in the alkali metals

The long-range ionic displacements round a vacancy are shown to come from both the elastic, long-wavelength, limit and from the Kohn anomaly. The effect of the topology of the Fermi surface is stressed.Attention is then focussed on the alkali metals with almost spherical Fermi surfaces, where it can reasonably be assumed that the elastic displacements dominate. Complete relaxation around a vacancy relates formation energy E_v, bulk modulus B and atomic volume Ω through E_v = αBΩ where α is a constant. The formation volume follows as a balance between a negative harmonic contribution and a positive and larger anharmonic term.     

Atomic simulation of the vacancies in BCC metals with MAEAM

The formation energy of the mono-vacancy and both the formation energy and binding energy of the di-and tri-vacancy in BCC alkali metals and transition metals have been calculated by using the modified analytical embedded-atom method (MAEAM). The formation energy of each type of configuration of the vacancies in the alkali metals is much lower than that in the transition metals. From minimum of the formation energy or maximum of the binding energy, the favorable configuration of the di-vacancy and tri-vacancy respectively is the first-nearest-neighbor (FN) or second-nearest-neighbor (SN) di-vacancy and the [112] tri-vacancy constructed by two first-and one second-nearest-neighbor vacancies. It is indicated that there is a concentration tendency for vacancies in BCC metals.     

Residual resistivity of vacancies in simple metals

The residual resistivities of vacancies in simple metals are estimated by using the Kohn-Sham method to calculate a self-consistent scattering potential for free electrons. The results are compared to other calculations and experimental resistivities.     

Radiation-induced diffusion of quenched-in vacancies in metals

It is shown that the plateau observed by Lucasson and coworkers in the resistivity versus fluence curve during electron irradiation of pre-quenched metals can be explained in terms of radiation-induced diffusion of vacancies.     

Positron annihilation in vacancies at grain boundaries in metals

The behavior of vacancies in selected coherent grain boundaries (GBs) in Fe and Ni is studied by means of molecular dynamics simulations. Corresponding positron lifetimes are calculated using the atomic superposition method. There is a difference between the vacancy behavior in Fe and Ni in dependence on temperature. In Ni, vacancies at GBs appear to diminish substantially their free volume (and lifetime) with the increasing temperature, which can be attributed to ‘vacancy delocalization’. Contrary, GB vacancies remain stable up to apparently higher temperatures in Fe.     

The loss of single vacancies in metals during quenching

The Lomer equation, describing the rate of change of single vacancy concentration in a metal specimen during exponential quenching, was numerically solved for different quenching temperatures and different quenching rates. A relation has been found between the constantb, which characterizes the initial quenching rate, and the quenching temperature leading to the same percentual loss of vacancies. This relation enables us to determine the conditions of quenching, yielding the predetermined percentage of retained vacancies.The possibility is discussed of applying the above results in measurements of the formation energyEf^F, to estimate the migration energyE ^M and to determine the change of the mean number of jumps during quenching.The author would like to express his gratitude to Dr. K. Míek, head of the Metal Physics Department, for his interest and useful comments.     

The surface plasmon modes of spherical voids in irradiated metals

The surface plasmon modes of spherical voids in metals modeled by a free-electron gas are calculated within a hydrodynamic formulation. Spatial dispersion, retardation, and surface electron density variation are included. A sharp-surface model is considered first and it is shown that the modes are non-radiative. A novel matrix method is used to discuss the sensitivity of the results to the use of different surface density profiles. It is shown that the interplay between surface inhomogeneity and spatial dispersion always results in a mode in the near neighbourhood of the sharp-surface mode. There are, however, other structure-induced modes which are well-separated from the sharp-surface results. The l = 0 breathing mode is affected by retardation and surface structure but is unaffected by spatial dispersion.     

Electric quadrupole interaction due to vacancies and interstitials in cubic host metals

A procedure has been developed to evaluate from first principle the electric field gradient (efg) due to vacancies and interstitials in cubic host metals. For this purpose the screening charge distribution in the valence-effect has been constructed in scattering theory using appropriate muffin-tin potential. In the size-effect, the efg has been evaluated in a more realistic model of the lattice than the commonly used elastic continuum. Unlike other calculations, no adjustable parameter has been used in the present procedure.     

Pseudopotential calculations of electrical resistivities due to vacancies in noble metals

Electrical resistivities due to vacancies in noble metals are calculated using the pseudopotential form factors given by Moriarty and Animalu. On the whole the values using the Moriarty's potentials are in good agreement with those calculated from phase shift except for Cu, but the Animalu's potentials do not produce reasonable values for these metals.     

Effect of vacancies in the lattice of an oxide on interaction with films of metals

It is shown that oxygen vacancies in the lattice of an oxide, being acceptor centers, facilitate ionic interaction in contact with films of metallic condensates and increase adhesion at the interface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 45–49, October, 1986.     

Upon the magnitude of the recombination volume between vacancies and self-interstitials in metals

Previously obtained results of measurements of the radiation damage rate in metals as a function of the irradiation temperature are analysed. It is found that the effective migration energy of self-interstitials is linearly decreasing with decreasing irradiation temperature i.e. with increasing concentration of defects. It results from the analysis that i) the recombination volume must be as large as 10⁵ atomic volumes and that ii) the migration energy of defects is slightly decreased during an irradiation with high energy particles.     

Oxygen vacancies and peroxo groups on regular and low-coordinated sites of MgO, CaO, SrO, and BaO surfaces

The formation of an oxygen vacancy and the simultaneous re-adsorption of an oxygen atom on regular and low-coordinated (LC) sites (edges and corners) of the surface of alkaline-earth oxides with cubic rock salt structure, MgO, CaO, SrO, and BaO, has been investigated using DFT cluster model calculations. The process corresponds to the formation of a surface Frenkel defect when the vacancy formation energy is partially compensated by the energy gained in the formation of a peroxo group. The structural and electronic properties of vacancies and peroxo groups along the series of alkaline-earth oxides have been analyzed. We found that the role of low-coordinated sites on the surface chemistry of alkaline-earth oxides is of crucial importance for MgO, but decreases for the heavier members. For instance, on BaO the formation of a peroxo group is practically site-insensitive. This is not the case of the vacancy formation, which is always favored on the low-coordinated sites.     

Effects of simple vacancies and divacancies on the physical properties of metals

The effects of simple vacancies and divacancies on the electrical and thermal conductivities of metals are calculated. It is shown that, although the concentration of divacancies is usually less than that of simple vacancies, the changes caused in the kinetic coefficients by the two types of vacancies can be comparable.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 88–93, October, 1969.     

Atoms with vacancies

Methods of orthogonalization of the wave function for an atom with vacancies in the s shell and the energetically lower state for ns(n + 1)s²-type configurations are considered. Non-orthogonal radial orbitals are used. An equation is obtained for the overlap integral for atomic wave functions in terms of single-electron radial orbitals. The parameters of the generalized hydrogenic analytic variational radial orbitals and the energies of F, Ne⁺, and Na²⁺ in the configuration 1s2s²2p⁶ are determined. Results are compared to calculations from other authors and experimental values, demonstrating that the approximation is good for calculating atoms with vacancies.     

Electron Cooling of Positrons in LEPTA

Hyperfine Interactions - Electron cooling of positrons is the essential peculiarity of the method of antihydrogen and positronium fluxes generation, which is based on low-energy positron storage...     

Migration of vacancies in tantalum and stage III recovery in refractory bcc metals observed by PAC

Annealing of radiation-induced defects in high-purity tantalum was studied by perturbed angular correlation (PAC) between 77 and 380 K. Vacancy migration around 270 K is deduced. From comparison with PAC studies on niobium, molybdenum, and tungsten a highly systematic picture of stage III defects in ¹¹¹In traps in the refractory bcc metals evolves.     

ESCA studies of fracture surfaces in welded metals

A fracture device for ESCA studies of internal surfaces in metals is described. By fracturing a specimen under UHV, an uncontaminated surface is obtain The specimen is cooled to produce cleavage or intergranular fracture, depending upon the type of information sought.The technique has been applied to weldments in steels of composition C 0.020, Mn 1.15. In the as-welded state, fracture surfaces yield signals of C, N, and O as well as iron in oxidized state beside metallic iron. Antimony is found on the fracture surface of specimens embrittled by long annealing in ox A strong chemical shift indicates that it is present in the oxidized state.