Fermi Surface Of Alloys Research Articles

Fermi surface and electron dispersion of PbTe doped with resonant Tl impurity from KKR-CPA calculations

We present results of detailed study on the electron dispersions and Fermi surface of lead telluride doped with $2\%$ of thallium, which is resonant impurity in PbTe. Using the KKR--CPA method, Bloch spectral functions (BSFs), which replace the dispersion relations in alloys, are calculated, and BSFs intensity maps over the Brillouin zone (alloy Fermi surface cross-sections) are presented. It is shown, that close to the valence band edge, Tl does not create an isolated impurity band, but due to its resonant character, strongly disturbs the host electronic bands, leading to disappearance of sharp and well defined electronic energy bands. Consequences of this effect on the transport properties are discussed and new qualitative explanation of the improvement in thermoelectric properties of PbTe:Tl is suggested.

First-principles theory of the temperature and compositional dependence of atomic short-range order in disordered Cu-Pd alloys

We combine the first-principles, Korringa-Kohn-Rostoker coherent potential approximation based calculations of compositional fluctuations with a statistical mechanical ring approximation to study the temperature (T) and composition (c) dependence of the atomic short-range order (SRO) in disordered, face-centred cubic, Cu-Pd alloys. The fourfold splitting of SRO peaks around the equivalent $X(0,1,0)$ points in reciprocal space is obtained in a wide $T\ensuremath{-}c$ region. Such splitting is shown to be an ``energy'' effect caused by the absolute minima of the Fourier transform of the effective atomic interactions and related previously to the existence of nested sheets of the disordered alloy's Fermi surface. However, we find that the T dependence of the SRO peak position is mostly an ``entropy'' effect. Both the calculated T and c dependences of the SRO peaks position are in good correspondence with the experimental data. The real-space effective atomic interactions and SRO parameters indicate the tendency for longer-period structures with increasing Pd concentration, as observed.

Oscillatory exchange coupling acrossCu1−xNixspacers: A first-principles calculation of the amplitudes and phases using asymptotic analysis

We apply a recently developed first-principles asymptotic approach to the problem of the oscillatory exchange coupling ~OEC! in the Co/Cu(12x)Nix /Co trilayer system for all ~100!, ~110!, and ~111! directions of growth. We compare results of the asymptotic analysis with full calculations. Our results are consistent with the available experimental data and give strong evidence for the existence of the exponential decay in the asymptotic form of the OEC for x.0.04. Moreover, we discover a caliper vector of the alloy Fermi surface Q(100) (3) which did not exist in the case of pure Cu, but contributes to the OEC significantly for x>0.11.

Fermi surfaces and electronic topological transitions in metallic random alloys. II. AgcPd1-c.

We have calculated the Fermi surfaces of ${\mathrm{Ag}}_{\mathit{c}}$${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathit{c}}$ vs Ag atomic concentration, within the fully relativistic Korringa-Kohn-Rostoker--coherent-potential-approximation framework. The Fermi surfaces of these alloys change, on varying the concentration, from the Pd- to the Ag-like topology. This implies that a number of electronic topological transitions (ETT's) occur on changing the e/a ratio, as a consequence of the fact that the Fermi level moves away from the d states towards the sp states. It is well known that such changes in connectivity of the Fermi surfaces affect transport coefficients; however, as we have shown in Paper I, equilibrium properties such as equilibrium volume, specific heat, enthalpy of mixing, etc., also display anomalies at the same concentrations where such topological changes occur. In particular we have found 5 ETT's in correspondence to the deviations from Vegard's law of the lattice parameter shown in Paper I. Another unexpected result of this study is that, in the range of atomic concentration beteen 0.55 and 0.69, this alloy Fermi surface is simply connected, although nonspherical, as for a simple metal. Moreover, we show that a fully relativistic study is necessary for AgPd Fermi surfaces, because the spin-orbit interaction is responsible for the fact that the two hole pockets at X close at different concentrations.

Electronic structure of disordered CuPd alloys: A two-dimensional positron-annihilation study.

Two-dimensional-angular-correlation experiments using positron-annihilation spectroscopy were performed on a series of disordered Cu-rich CuPd-alloy single crystals. The results are compared with theoretical calculations based on the Korringa-Kohn-Rostoker coherent-potential approximation. Our experiments confirm the theoretically predicted flattening of the alloy Fermi surface near [110] with increasing Pd concentration. The momentum densities and the two-dimensional-angular-correlation spectra around zero momentum exhibit a characteristic signature of the electronic states near the valence-band edge in the alloy.

Compton profiles for polycrystalline CuxNi1-x alloys: Theory and experiment.

We present experimental and theoretical Compton profiles, J(q), for a series of ${\mathrm{Cu}}_{\mathrm{x}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys. This is the first comprehensive study of Compton profiles in an alloy system. The experiments were carried out using 60-keV \ensuremath{\gamma} rays on five polycrystalline alloy samples with Cu concentrations x=0.40,0.50,0.55,0.60,0.65 and on pure Cu and pure Ni. Our computations of J(q) are based on the Korringa-Kohn-Rostoker coherent-potential approximation scheme for treating the average electronic spectrum of the alloy. The theory reproduces the main features of the observed Compton profiles. The experimental Compton profiles show a nearly linear dependence on concentration while the theoretically predicted Compton profiles are more nonlinear at low q. We discuss possible explanations for this discrepancy. We also discuss the relation between J(q) and the alloy Fermi surface.

Numerical data and functional relationships in science and technology, group iii: crystal and solid state physics, Vol. 13: Metals: phonon states, electron states and fermi surfaces, subvolume a, phonon states of elements, electron states and fermi surfaces of alloys

Numerical data and functional relationships in science and technology, group iii: crystal and solid state physics, Vol. 13: Metals: phonon states, electron states and fermi surfaces, subvolume a, phonon states of elements, electron states and fermi surfaces of alloys

Fermi-surface properties of alpha-phase alloys of copper with zinc

%'e consider the Fermi-surface properties of the a-Oui 0 Zn, system over the composition range 0&c &0.3. Our calculations are based on the application of the self-consistent coherent potential approximation to the muffin-tin Hamiltonian. The predictions of the theory with respect to the radii k„„„,k lpo and k „0, and the size and shape of the alloy Fermi surface in several planes in the Brillouin zone are in good agreement with the available positron annihilation measurements and with the predictions of the rigid-band model. %e discuss how the hitherto unmeasured disorder smearing of the alloy Fermi surface may be amenable to a direct experimental determination in concentrated alloys. The dilute impurity limit is considered in detail. The computed changes in areas for several orbits are in good accord with the corresponding results of de Haas-van Alphen experiments. The predicted Dingle temperatures, however, agree with measurements only to within a factor of 2. It is hoped that the present work will encourage further measurements, especially of the disorder smearing of the Fermi surface in concentrated alloys.

A new high-resolution, high-intensity apparatus for the study of Fermi surfaces of alloys by positron annihilation

A new method for the investigation of Fermi surfaces by positron annihilation is introduced. In spite of its high resolution of 1.6 × 1.1 mrad2 the counting rates are sufficient. The “rotating specimen method” is applied. Instead of crossed slits thin grooves which are engraved in steel-plates, are used. Since external 64Cu-sources produce the positrons, all solids can be studied. The results of test measurements of copper single crystals are presented. Es wird ein neues Verfahren zur Untersuchung von Fermi-Flachen mittels Positronenzerstrahlung vorgestellt. Trotz seines hohen Auflosungsvermogens von 1,6 × 1,1 mrad2 liefert es ausreichende Zahlraten. Das Verfahren basiert auf der „rotating specimen method”. Die Blenden fur die γ-Strahlen werden nicht wie ublich durch gekreuzte Spalte gebildet, sondern durch in Stahlplatten gravierte Nuten. Da es ausere 64Cu-Positronenquellen benutzt, ist es auf alle Festkorper anwendbar. Die Ergebnisse von Probemessungen an Kupfereinkristallen werden mitgeteilt.

Phase stability and the fermi surface of alloys

The Ising formulation was applied by Clapp and Moss [ Phys. Rev. 171, 754 (1968)] to determine the relative values of pair potentials for which various ordered phases are stable. In particular, the Fourier transform of the pair potential, V(k), was examined, but the extrema of the transform were not taken into account. This is done in this study. Regions of stability are given for ordered phases which are at absolute minima of V(k), for structures based on both f.c.c. and b.c.c. lattices. A new method for obtaining the Fermi surface of alloys is proposed, based on a symmetry relationship between V( k) and the Fermi surface. The surface calculated in this way for disordered Cu 3Au is similar to that of Cu, except for shrinkage along the 〈100〉 directions.

The Fermi surface of the HCP Cu-Ge alloys containing 12-15 at.% Ge studied by positron annihilation

The Fermi surface of the Cu-Ge alloy in the HCP zeta phase field is studied by measurements of momentum distributions of annihilation photons from two single crystals at e/a=1.38 and 1.45. The Fermi surface of the alloy at e/a=1.38 lies inside the Jones zone, and has a high degree of contact with the zone faces. A normal to (10.0) and a small degree of contact with the zone faces B normal to (00.1). The overlap of the Fermi surface across the A faces is observed for the alloy at e/a=1.45. Remarkable bulges of the Fermi surface are detected along the directions towards the faces C normal to (10.1) in both the compositions. The present results reveal the correlation between the overlapping of the Fermi surface across the zone faces and the change of lattice constant. The relative stability of the HCP zeta phase is discussed in the light of the experimental results.

A point detector positron annihilation study of Cu85Zn15

A high resolution point detector geometry has been applied to the study of the angular correlation spectra from positron annihilation in Cu85Zn15 single crystals. Strong anisotropies are observed and some features are interpreted in terms of the alloy Fermi surface. A value for the [111] neck diameter is obtained which is 50% greater than that for pure copper. Considerable smearing of the expected slope discontinuity in the neighbourhood of the Fermi momentum is interpreted as evidence for Stern's theory of charging. A large anisotropy in the higher momentum contribution to the annihilation is also observed.

Alloy Fermi-Surface Topology Information from Superconductivity Measurements under Pressure

A modified BCS gap equation is used to obtain an approximate solution for the pressure derivative of the superconducting transition temperature, $\frac{d{T}_{c}}{\mathrm{dp}}$, at values of the Fermi energy ${E}_{F}$ near critical points in the density of states. $\frac{d{T}_{c}}{\mathrm{dp}}$ is a functional of the energy derivative of the density of states, and therefore reflects strong structure at values of ${E}_{F}$ near the van Hove singularities in the density of states associated with a Fermi-surface topology change. The model is extended to include the case of a dilute random alloy system in which impurity scattering broadens the singularities. A Lorentzian spectral distribution is assumed, and an analytic equation is obtained for the broadened van Hove-singularity contributions to the density of states. The model is applied to the case of indium doped with cadmium, the rate of change of the Fermi energy with concentration being estimated from a free-electron formula, and the rate of change of the life-time estimated from the residual resistivity. A qualitative fit between the model calculation and the structure of available $\frac{d{T}_{c}}{\mathrm{dp}}$ data requires the existence of two singularities: an electron saddle point disconnecting at 0.9 at.% Cd, and an electron sphere vanishing at about 1.6 at.% Cd. A quantitative calculation is made of the variation of $\frac{d{T}_{c}}{\mathrm{dp}}$ with alloy concentration, assuming only the empirically determined singular points, and deriving all other quantities from available indium Fermi-surface information on the third-band ring. The model calculation gives surprising quantitative agreement with the data, suggesting that valuable information on the Fermi-surface topology of dilute alloys can be obtained from measurements of $\frac{d{T}_{c}}{\mathrm{dp}}$ as a function of concentration.

Fermi Surface of Alloys

Received 12 February 1962DOI:https://doi.org/10.1103/PhysRevLett.8.239©1962 American Physical Society