Positron diffusion in metals

Calculations are presented of the positron diffusion constant, mobility and other diffusion related quantities in simple metals. The mobility is found to be largely limited by the positron-phonon interaction which is treated in the deformation potential model. The effect of positron conduction electron scattering is also evaluated and found to be small. The effect of positron impurity scattering is discussed and detailed estimates given for Li in Al. In dilute alloys at moderate temperatures the positron phonon interaction will dominate also over this effect.     

Muon diffusion in metals

The hopping rate W of the positive muon in metals is studied theoretecally. We consider four interactions: muon-electron interaction, linear as well as quadratic muon-lattice interactions and inhomogeneous broadening of the muon levels due to imperfections. The W-vs-T curve consists of four regions. The highest-T region, where the small polaron theory is applicable, is followed by a minimum, below which the Kagan-Klinger damping may be dominant. Then, the damping due to metallic electrons dominates and the hopping rate obeys a power law of a negative power. When kT is smaller than the inhomogeneous broadening, the hopping rate obeys another power law of a positive power, as Sugimoto has shown. We analyze experimental data on muon diffusion in Cu, Al and Fe.     

Test of the universal scaling law for the diffusion coefficient in liquid metals

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of a liquid [M. Dzugutov, Nature (London) 381, 137 (1996)] is tested for several metals using molecular dynamics simulations. Interatomic potentials derived from the embedded atom method are used to study Ag, Au, Cu, Ni, Pd, Pt, Ni(3)Al, and AuPt and the angular dependent Stillinger-Weber form is used to investigate Si.     

Positron diffusion and restoration of local symmetry breaking induced by positrons in liquid metals

We present the effective Lagrangian with spontaneously broken density (the hedgehog-like fluctuation) and the massive internal gauge fields, and propose one explanation for the increase of the positron diffusion length in the liquid metal phase with temperature. From the view point of the restoration of the spontaneously broken density around the positron, the positron diffusion length data in the liquid phase of Bi metal are analyzed.     

Interstitial solute diffusion in metals

The diffusion of interstitial solute atoms, which were subject to nearest neighbour solute-solute interactions, was simulated by means of a modified Monte Carlo method. Solute correlation factors, nearest neighbour vacancy availability factors and effective jump frequency factors were calculated as functions of composition and temperature. Compared with non-interacting atoms attraction between solute atoms always retarded diffusion whereas repulsion always enhanced diffusion except within the ordered region.     

Self-diffusion in liquid metals

A survey is made of all published data on self-diffusion in liquid metals and an examination is carried out on the temperature-dependence of D for studies covering a wide range of temperatures. Log D versus 1/T and D versus T represent the best data equally well, while D versus T ^1/2 and D versus T ² do not. The proper functional form is taken to be D=CT experimentally, and a simple free diffusion theory is developed from the standpoint of vibrational atom motion. An equation having no adjustable parameters results (D=k ² ? _D T/hk), in which κ is the vibrational force constant. Values calculated for D at the melting point are in good agreement with experimental values for potassium, sodium, copper and silver, but are below the measured values for lead, indium, zinc and tin.     

Quantum diffusion of muons in metals

We consider the influence of electrons on the quantum diffusion of muons in metals. The temperature dependence of the muon-diffusion coefficient is found.     

Ultrasound absorption in liquid metals

The excess longitudinal ultrasonic absorption over and above the classical value has been attributed to the solid-like mechanisms of the interaction of sound waves with the conduction electrons and the thermal phonons in liquid metals, both exhibiting a quadratic dependence on the sound frequency.     

Electronic states in liquid metals

An approximation scheme has been developed for calculating the density of states and autocorrelation of independent electrons in a dense array of weak stationary scatterers. This method, which avoids the customary perturbation expansions in powers of the scattering potential, requires a knowledge only of the autocorrelation of the potential. Results are presented of calculations for liquid Na at various temperatures. Comparison is made with the experimentally observed resistivity and angular correlation in positron annihilation experiments.     

Creep and diffusion parameters in submicrocrystalline metals

We have studied the creep of nickel and copper in a submicrocrystalline (SMC) state in a vacuum and in the presence of a diffusion contact with an impurity (Cu and Al, respectively). It is shown that a reduction of the resistance in the presence of a diffusion contact with an impurity is observed in the SMC materials in the temperature range 398 to 473 K. This range is 200 to 400 K lower than the corresponding range for coarse-grained material. It is shown that in this temperature interval the coefficients of grain boundary diffusion for copper in SMC nickel are 5 to 6 orders of magnitude larger than in the coarse-grained material. We propose that the reduction in the temperature for the manifestation of a creep activation effect in the presence of a diffusion contact with an impurity in SMC materials is caused by the increase in the diffusion permeability of the submicrocrystalline grain boundaries. Institute of Materials and Strength Physics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 77–82, March, 1998.     

Theory of incoherent hydrogen diffusion in metals

Hydrogen migration in a one-dimensional model of the bcc lattice is examined using the double-time Green functions, for temperatures at which the migration mechanism is mainly due to the thermally activated tunnelling of a proton. In deriving the coefficient of incoherent hydrogen diffusion, allowance is made for the lattice distortion around the hydrogen and for the two-phonon scattering processes. It is shown that the observed temperature break in the activation energy of incoherent hydrogen diffusion in bcc metals is the result of the change in the mechanism responsible for an elementary diffusion process near the Debye temperature, anmely, it is the result of the transition from “passive” to “active” transport.     

Quantum diffusion of atomic species in metals

The coefficient of quantum diffusion D(T) is calculated using exact states of the localized impurity in the deformed vibrating lattice. Without resorting to the Born-Oppenheimer approximation an explicit general expression for D(T), valid for impurities of arbitrary mass and holding in the whole range of temperature, is obtained in terms of the crystal properties. Interesting new features arise from retaining some terms of the Hamiltonian which were left aside in previous theories.     

Quantum diffusion of light interstitials in metals

A quantum theory of diffusion of self-trapped light interstitials in metals is presented. The theory encompasses both coherent and incoherent tunneling, but the approximation used neglects the dependence of the interstitial transfer matrix element on the vibrational state of the crystal. The coherent tunneling contribution is estimated by fitting the incoherent diffusion rate to experimental data for hydrogen and muon diffusion. It is predicted that coherent diffusion should be dominant below ～ 80 K for H in Nb and below ～ 190 K for μ⁺ in Cu. Experimental verifications of these predictions would require high purity strain free samples and low concentrations of the diffusing species.     

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.     

New results on diffusion in fcc metals

The Kondo model for the diffusion of light particles in metals has provided a satisfactory explanation for the low temperature diffusion rates for muons in the fcc metals Cu and Al. Explicit experiments which show the strong dependence of muon behaviour on the presence of conduction electrons have now been performed belowT=1 K in Al. Challenging new diffusion studies on fcc metals such as Pt are also presented.