Temperature dependence of hyperfine fields at impurities in metals

A model is developed to account for the spin-densities at dilute non- magnetic impurities in ferromagnetic metals which exhibits a non-linear dependence of the hyperfine field on the host magnetization. The theory is used to calculate the hyperfine field at an interstitial positive muon in gadolinium.     

Progress in pion-decay channelling: Refractory bcc metals at high and low temperatures

By means of π⁺/μ⁺ channelling, positive pions (π⁺) implanted intoTa, Mo, andW are investigated up to high temperatures. A striking observation is that the channelling effect disappears in a rather narrow temperature interval centred at 0.26 (Ta) to 0.51 (W) of the melting temperature. From studies of π⁺ trapping by oxygen atoms inTa estimates for the low-temperature π⁺ diffusivity inTa [D _π(23K)=1.4·10^−10±0.3 m²s⁻¹,D _π(47K)=5.7·10^−10±0.3 m²s⁻¹] as well as for the binding enthalpy of π⁺ to 0 atoms (H _B=7·10⁻² eV) have been obtained. The diffusion data are in reasonable agreement with the theory of phonon-assisted tunnelling.     

Radiation damage in bcc metals studied by TDPAC

The time-differential perturbed angular correlation (TDPAC) method is an excellent tool for the study of radiation damage in metals and it has been applied successfully to many fcc metals. Recently, some investigations have been performed with bcc metals, where it is more difficult to obtain reliable results because the open lattice easily absorbs many impurities. After a short general introduction to the field, the power of the TDPAC method is demonstrated by showing in detail recent results obtained by our group in Bonn for the bcc metal tungsten. Finally, the present situation for all non-ferromagnetic bcc metals is summarized.     

Superconductivity of amorphous indium with silver impurities

Amorphous, vapour quenched films of In_0.88Ag_0.12 have been investigated. Depressions of the superconducting transition temperatures and energy gaps were proportional to inverse film thickness. The phonon induced tunneling structure displayed one pronounced dip in analogy with amorphous tin. The resistivity changed in a step-like manner upon annealing.     

Peierls potential for crowdions in the bcc transition metals

We present the first derivation of the analytic expression for the Peierls-Nabarro potential for crowdion migration using the double sine-Gordon model. The analysis is guided by the group-specific trend in the shapes of the periodic lattice potentials calculated for the body-centered-cubic transition metals in groups 5B and 6B of the periodic table. We combine density-functional calculations of the crowdion's profile and environment with an extended version of the analytical Frenkel-Kontorova model, and determine the effective potential experienced by the defect's center of mass. This reveals important underlying differences between the metals in these groups, which are inaccessible to either the numerical or analytical approaches alone, and accounts for the previously unexplained significantly higher crowdion migration temperatures observed in the metals of group 6B relative to those of group 5B.     

Ground-states of light impurities in metals

A straight forward expression to calculate the ground-state energies of light impurties in metals is derived on the basis of the Thomas-Fermi theory of screening. It is tested for position and a good agreement is found with earlier results.     

Interphase interaction at indium phosphide contacts with metals of group I

The interphase interaction at indium phosphide contacts with layers of metals of group I (Cu, Ag, Au) annealed in an atmosphere of oxygen is investigated using the method of Rutherford backscattering, x-ray structure analysis, and electrophysical measurements. The relation between the character of the interphase process and the behavior of the electrophysical parameters of the Schottky barriers is traced. The mechanism of degradation in the contacts mentioned is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 8–13, January, 1986.     

Electronic selection rules controlling dislocation glide in bcc metals

The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/<2111> screw dislocations.     

Pressure-volume calculation in bcc metals using Born stability criteria

A simple analytical two-body potential φ(r) = −Ar ⁻ⁿ + B exp(−pr ^m) is considered for P-V calculations in bcc metals using Born stability criteria. It is shown that the stability of bcc metals can be expressed uniquely as a function of a parameter q (discussed in the text). The P-V calculations are done in ten bcc metals. The calculations are compared with the experimental data of shock-wave measurements and also with other potential available. It is found out that the present potential is better than the other two-body potentials in case of bcc metals. Further, the calculations done for TOEC and the first pressure derivative of SOEC are found in good agreement with the reported results.     

Theory of magnetic impurities in metals

The theory of magnetic impurities in metals is considered starting from the strong-correlation or ionic limit as a zero-order approximation. An appropriate model Hamiltonian is derived which includes collective intra-ionic structure and new kinds of mixing interactions between local and conduction electrons. The relation of the present approach to the Friedel-Anderson theory is discussed in some detail.     

Charge perturbations around impurities in metals

The calculated properties of the charge perturbations around impurities in metals are summarized and contrasted with the experimental situation in this field.

The effects of coupling between the field gradient excited by the charge perturbation and the quadrupole moments of (host) nuclei on the nuclear magnetic resonance (N.M.R.) signal are summarized and various kinds of experiment based on this quadrupole effect are discussed.

The gross overall features of the charge perturbation are established by analysing N.M.R. data for copper and aluminium based alloys. This analysis leads to the conclusion that simple theories can account for the main experimental findings, but details, in particular the anisotropy of the charge perturbation, cannot be fully explained because band structure effects play an important role.

The relevance of phase-shift analysis is also assessed, and its applicability in cases of complicated Fermi surfaces is discussed. Effects connected with the energy dependence of the scattering potentials for three-dimensional transition metal impurities are reviewed and evidence for the depression of the charge perturbation near to the impurities is summarized for copper and aluminium based alloys. Temperature dependence, caused by the thermal smearing of the host Fermi surface, by the reduction of the mean free path and by the temperature dependence of the scattering potential (connected to the Kondo effect) is also discussed.

Other relevant experimental techniques are briefly summarized together with effects determined by the charge perturbation around the impurities.     

Precipitation and re-solution of impurities at the surface of indium on traversing the melting-point

In the course of preparing a clean indium surface for solid-versus-liquid studies, changes in the surface concentrations of sulphur and oxygen were observed by AES and XPS while the metal was heated and cooled through its melting-point. Both impurities disappeared on melting, and reappeared on solidification, over a very narrow temperature range; the disappearance and reappearance were to a certain extent reproducible. The effect was found to be similar in characteristics to that observed for the behaviour of carbon on a nickel surface by Shelton et al., and the same Bragg-Williams model is invoked to explain the sharpness of the impurity concentration changes with temperature. Although the maximum temperature reached by the indium was only 200°C, traces of platinum were also observed on the indium surface after melting, in both the AES and XPS spectra, probably as a result of solution from the platinum boat.     

Dislocation structure and deformation hardening of bcc metals

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 23–34, March, 1991.     

The non-Arrhenius migration of interstitial defects in bcc transition metals

Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).     

Screening of light impurities in simple metals

The Kohn-Sham density functional method is used to calculate self-consistent electron densities around H, He and Li impurities in jellium host corresponding to Li, Al and Mg metals. The differences in the density profiles for interstitial and substitutional impurities are investigated. Residual resistivities, relaxation energies and charge transfer in dilute alloys are evaluated.     

A model for light impurities in metals

The feasibility of studying the energetics of light impurities in metallic hosts by treating the impurity ion plus its screening cloud as a unit, or “quasi-atom”, is explored. In the simplest approximation the energy of the impurity is given in terms of the local host electron density. This approach is tested against results taken from the literature for the energy of light atoms in inhomogeneous electronic environments. Corrections to the simplest approximation are discussed.     

Nontransition element impurities in ferromagnetic transition metals

Effects of nontransition element impurities on the ferromagnetism of transition metals are discussed in the light of our theoretical analyses of the hyperfine interaction data of implanted nuclei which have been carried out in recent years. A detailed discussion of the effective filling of the d band by impurity valence electrons is given after presenting a brief summary of our theory of the hyperfine interaction. The change of the saturation magnetization is discussed in particular. It is pointed out that an sp valence impurity at an interstitial site in iron can increase the magnetic moments of surrounding host atoms.     

A short range many-body potential for modelling bcc metals

The problem considered is the fitting of a many-body interaction potential to bulk crystal data. A parameterisation of the potential is assumed which is based on physical considerations. The free parameters are determined by using global optimization to perform a least squares fit, to a large number of crystal properties. This has been achieved for body centered cubic (bcc) materials. The approach adopted here fits the bcc crystal structure, as the preferred minimum energy configuration for tungsten, and also fits the dimer energetics and the elastic properties of crystalline tungsten.     

Phonon dispersion for the bcc metals Mo and Cr

In this paper the phonon dispersion for the bcc metals Mo and Cr is calculated based on the pair potentials obtained from cohesive energies and the Slater-Kirkwood- type three body interaction. In the calculation of the pair potentials the Möbius transform in the number theory is used and the cohesive energy is evaluated by the LMTO method. The results show a good agreement with inelastic neutron scattering data and indicate that the three-body interaction is necessary to account for the phonon dispersion.