Theory for superconductivity in amorphous transition metals

A theory is presented for superconductivity in amorphous transition metals. It is shown that in contrast to simple metals for transition metals the changes in the phonon spectrum, in the electronic density of states and in the electronic matrix elements which result from strong lattice disorder can enhance as well as decreaseT _c. The numerical results for the superconducting transition temperatureT _c of amorphous 4d-and 5d-transition metals agree well with the experimental results.     

Twinning propensity in nanocrystalline face-centered cubic,body-centered cubic,and hexagonal close-packed metals

The nanotwinned structures in metals exhibit the unique combination of physical properties. The unifying approach is developed that can be applied to nanocrystalline (nc) materials with different crystal structures. It is used to make a bridge between microscopical mechanisms of twin nucleation and macroscopic characteristics of the twinning and calculate them. The grain size range of the nanotwinning propensity, the grain size of its peak, and the requisite external twinning stress are calculated for the nc face-centered cubic metals Al, Cu, Ni, Pd, Au, Ag, for nc body-centered cubic metals Ta, Fe, Nb, Mo, and for hexagonal close-packed nc metals Co, Zr, Mg, Ti.     

The contribution of 5d electrons to the saturation magnetic moments and magnetic hyperfine fields in the heavy rare earth metals

The main results of a model for 5d electrons in the heavy rare earth metals are presented. The model involves the use of wave functions based on published analyses for 4fⁿ5d6s² atomic configurations, and the spreading of each of these energy levels uniformly over a band of width W in the metals. Excess saturation magnetic moments above those of the tripositive ions can be explained by the model with W in the range 0.84±0.16eV in the five metals Gd, Tb, Dy, Er and Tm. The magnetic hyperfine fields in the metals include negative contributions from the 5d electrons which have been shown to amount to about ?250koe in Gd, Er and Tm.     

Pressure effect on the melting point of alkali metals

The mean-square displacement of alkali metals is studied theoretically using our local Heine-Abarenkov-type model potential in the perturbational scheme. The temperature-dependent mean-square displacement of alkali metals decreases as function of the compressed volume. Lindemann's criterion for melting x_m, which is defined as the ratio of two times the root-mean-square displacement to the nearest-neighbour distance, is found to be nearly constant for five alkali metals. The volume effect on the melting temperature of alkali metals is studied by keeping x_m constant. The obtained melting curve increases as function of the compressed volume and are qualitatively in good agreement with the observed tendency for alkali metals.     

On the mechanical stability of bcc transition elements and alloys

A volume independent and a volume dependent lattice energy function involving short-range interatomic potentials were able to be fitted to the elastic constants, cohesive energy, lattice parameter and for the latter function to the vacancy formation energy and bcc-fcc lattice stability energy, as well, for fcc metals and bcc alkali metals, but not to the cohesive energy and C' elastic constant of bcc transition metals. The assumption that directional, but partial, covalent bonds exist between nearest-neighbors in the bcc transition elements provides an explanation for the latter results and in addition explains the identical dependence of C' and the bcc-fcc lattice stability upon N_d, where N_d is the average number of d electrons, for the bcc transition metals and alloys. Both the mechanical and thermodynamic stability of the bcc structure for transition metals and all transition metal alloys disappears for 5 ? N_d > 2 and <?1.     

Tc map and superconductivity of simple metals at high pressure

We calculate T_c map in region of weak electron–phonon coupling based on simple phonon spectrum. By using linear-response method and density functional theory, we calculate phonon spectra and Eliashberg functions of simple metals under pressure. Based on the evolutions of superconducting parameters of simple metals on the T_c map with increasing pressure, we find that there are two different responses to pressure for simple metals: (1) enhancing electron–phonon interaction λ such as for La and Li, (2) increasing phonon frequency such as for Pb, Pt. The λ threshold effect is found, which origins from the competition between electron–phonon interaction and electron–electron Coulomb interaction and is the reason why T_c of most superconductors of simple metals are higher than 0.1 K.     

Hall-Effekt und spezifischer elektrischer Widerstand flüssiger Übergangsmetalle

The Hall-coefficient and the electrical resistivity of liquid transition metals and their alloys with mono- and polyvalent simple metals have been measured with a sensitiveac current-ac magnetic field method. The transport properties of liquid transition metals are quite different from the well known behavior of liquid simple metals. The pure liquid metals La, Ce, Pr, Nd and U and also a great number of alloys of transition metals with simple metals show a positive sign of the Hall-coefficient. For alloys of transition metals with polyvalent simple metals we observed negative temperature coefficients of the electrical resistivity over large concentration ranges. This behavior can be understood by a modified form of the Faber-Ziman formula for the electrical resistivity of liquid metals.     

Anomalous antiferromagnetic structures in metals

The many-sublattice antiferromagnetic NNSS and NNNSSS structures may be stable in metals with isotropic indirect exchange provided it is strongly non-Heisenbergian (e.g. metals with a quasi-one-dimensional electron energy spectrum). This makes it possible to explain other anomalies of metals of the CeSb type by the non-Heisenberg exchange. Under certain conditions it may cause the canted two-sublattice structure to be most energetically favoured in isotropic metals of the GdMg type.     

Surface energies of metals in both liquid and solid states

Although during the last years one has seen a number of systematic studies of the surface energies of metals, the aim and the scientific meaning of this research is to establish a simple and a straightforward theoretical model to calculate accurately the mechanical and the thermodynamic properties of metal surfaces due to their important application in materials processes and in the understanding of a wide range of surface phenomena. Through extensive theoretical calculations of the surface tension of most of the liquid metals, we found that the fraction of broken bonds in liquid metals (f) is constant which is equal to 0.287. Using our estimated f value, the surface tension (γ_m), surface energy (γ_SV), surface excess entropy (−dγ/dT), surface excess enthalpy (H_s), coefficient of thermal expansion (α_m and α_b), sound velocity (c_m) and its temperature coefficient (−dc/dT) have been calculated for more than sixty metals. The results of the calculated quantities agree well with available experimental data.     

On the relationship between the ionization potential and the work function: Metals

A relationship is analyzed between the ionization energy of atoms I and the work function φ of a metal composed of these atoms. The transition energy ∑=I?φ is assumed to be the sum of kinetic K and coulombic C contributions. Contribution K is calculated in the framework of a model of uniform gas of quasi-free electrons, and C is determined from experimental values of Σ. Calculations performed for a wide range of metals have shown that the dimensionless coefficients determining the Coulombic contribution C differ insignificantly between various groups of metals. The relationships obtained have been used for determining the work function of binary alloys.     

Investigation of the formation of the electron structure of metallocarbonic nanoforms

Model samples of nanostructures are synthesized with the use of transition metals, with a different number of d electrons and sp elements of groups II and III with a different number of valence p electrons, as modifiers. The activity of nanostructure synthesis is investigated depending on the addition of sp elements. A procedure is developed for determining the atomic magnetic moment of the d metals and the change in the distances between atoms of the metal and the sp element via the parameters of the X-ray photoelectron spectrum of transition metals, which allows the degree of hybridization of valence electrons in the chemical bond of adjacent atoms (Me-X) to be determined. As a result of the investigation the laws of nanoform growth are found which facilitate the development of new directions in the synthesis of nanostructures with unique properties.     

d Electron momentum densities for position annihilation in transition metals

The relative contribution of 3d electrons to the momentum densities for positron annihilation in the iron series transition metals are calculated, using the atomic Hartree-Fock-Slater orbitals. A discussion is given of the observed systematics. The per electron contribution to the angular correlations is found to decrease with the filling up of the d shell. The high momentum components are found to be relatively enhanced in case of higher Z metals. The built in spin dependence of the electron HFS wavefunction is reflected in the calculated curves.     

Role of hydrogen in the absorption of ionizing radiation energy by a metal-hydrogen system

Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdH_x and TiH_x (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation.     

Theoretical study of surface plasmons coupling in transition metallic alloy 2D binary grating

The excitation of a surface plasmon polariton (SPP) wave on a metal–air interface by a 2D diffraction grating is numerically investigated. The grating consists of homogeneous alloys of two metals of a formula A_xB_1−x, or three metals of a formula A_xB_yC_z, where A, B and C could be silver (Ag), copper (Cu), gold (Au) or aluminum (Al).It is observed that all the alloys of two metals present a very small change of surface plasmon resonance (SPR) irrespective of composition x. Moreover, the addition of 25% of Al to two metals alloy is insufficient to change the SPR curves. The influence of the different grating parameters is discussed in details using rigorous coupled-wave analysis (RCWA) method. Furthermore, the SPR is highly dependent on grating periods (d_x and d_y) and the height of the grating h. The results reveal that d_x= d_y= 700 nm, h=40 nm and duty cycle w=0.5 are the optimal parameters for exciting SPP.     

Laser synthesis of nitrides on the surface of refractory metals immersed in liquid nitrogen

In this paper we describe an approach for the formation of composite layers on the surface of refractory metals. We show that laser radiation on refractory metals (Ti, V, Zr, Mo, Hf, Ta, and W) immersed in liquid nitrogen can provide a chemical synthesis of nitride phases on the surface of metals. The metals were subjected to pulsed laser radiation with a wavelength of 1.06 μm. The power density ranged from 10⁴ to 10⁹ W cm⁻². The synthesis of nitrides began with the formation of Me_xN_y (x > y) phases with low contents of nitrogen. When the melting point was reached at the metal surface, the quantity of MeN phases increased sharply. Study of the melting zone showed that it contained a non-uniform distribution of nitride phases. The quantity of nitrides was a maximum on the surface and decreased with the increase of the depth of melting zone. Due to the high-cooling rates, titanium nitride crystallized in the form of columns. Maximum microhardness in the Ti surface layer was up to 20,000 MPa.     

Current-voltage behaviour of Schottky diodes fabricated on p-type silicon for radiation hard detectors

Current-voltage (I-V) measurements were carried out on Schottky diodes fabricated on undoped and on metal-doped p-type silicon. The metals used are gold, platinum, erbium and niobium. The I-V data were used to extract the saturation current, the ideality factor and the Schottky barrier height for each of the five diodes. These parameters were correlated to the defect levels generated by the metals in silicon. The results show that in all cases the silicon has become relaxation-like after doping since the device current is Ohmic. This is in agreement with the existence of the midgap defect in all the doped devices as compiled from the literature. Such metal doped (or relaxation) devices have been found to perform better as radiation-hard particle detectors.     

Coexistence of ferro- and antiferromagnetism — A spin fluctuation theory

The coexistence of and transition between ferro- and antiferromagnetism in itinerant electron system are investigated by extending the spin fluctuation theory of ferromagnetic metals by Usami and Moriya. Calculation is made on a model density of states, which simulates the one for d metals with a body centered cubic crystal structure. The result shows that the wavevector-dependent susceptibility χ_q has a two peaks at q = 0 and q = Q for the suitable choice of parameters and the coexistence is realized when the amplitude of spin fluctuation takes a proper value. Paramagnetic susceptibility of this system is also discussed.     

Temperature dependence of the electrical resistivity in amorphous metals

In amorphous metals the electrical resistivity increases linearly in the temperature range from 2 to 40 K. This result differs fundamentally from the nonlinear behaviour known for crystalline metals and it suggests the conduction electrons not to be scattered by the vibrations of the amorphous point lattice. The temperature dependent part of the resistivity in amorphous metals is explained with scattering of conduction electrons by fluctuations ofp-electrons.     

Temperature dependence of the surface free energies of solid chemical elements

The “surface free energy” of metals is αΔH′, where ΔH′ is the internal enthalpy (heat) of atomization and α is a constant (0<α<1). In this Letter an expression (−R ln m/ΔH′) for the temperature dependence of α has been derived and compared to the measured values. It has been found theoretically that this expression is independent of coordination number and that the m=4−10 upper layers of the surface contribute to the constitution of the properties of the metals (in good agreement with experiments).