Density Of States In Alloys Research Articles

Space-charge-limited current investigation of the density of states in a-SiGe:H alloys

Space-charge-limited current measurements have been carried out on undoped amorphous silicon-germanium alloys as a function of the Ge content in the range 0%–36%. The scaling law is checked for different series of samples with varying thickness, and the J-V data consequently analyzed by using the Weisfield method. The position of the Fermi level EF is obtained from the activation energy of the ohmic conductivity Ea. The deduced value of the density of states (DOS) near EF increases as a function of the Ge content in the range 1016 to 4×1017 cm−3 eV−1. A factor of 7 improvement of the Si0.74Ge0.26:H alloy DOS is clearly evidenced when the material is prepared using high H2 dilution of GeH4–SiH4 mixtures, leading to a DOS value of 1×1016 cm−3 eV−1.

Partial densities of states of alloys measured with x-ray-photoelectron diffraction: AuCu3(001).

Using a single crystal of AuCu 3 (001) as a test case, it is shown that x-ray-excited core-level-photoelectron-diffraction patterns of an ordered alloy can be employed to decompose the total alloy density of states of the valence band into the partial densities of states of the constituents. The results of this new method are in good agreement with earlier experiments and recent band-structure calculations. The ratios of core-level to valence-band cross sections of each constituent of the alloy are also determined and compared to those of the pure elements

Modified CPA theory as the method for calculation of the density of states for materials having complex crystal structures

In the theory presented the structure of crystalline or amorphous materials is described by the number and position of nearest neighbours and by an average medium. CPA theory is modified to the first four moments which makes possible the calculation of the magnetic moments of constituents in different sites. Extension of CPA theory, using the energy dependent potential in CPA equations, provides the possibility of extending previous calculations for any arbitrary shape of the density of states of the average medium. The method presented for calculation of the density of states in alloys can be used for any crystalline and amorphous material.

Simple theory for alloy density of states and conductivity including local atomic configurations

A numerical Monte Carlo technique is used to calculate the density of states and the conductivity of model binary alloys. The strength of the method is its ability to accurately account for local atomic configurations while avoiding explicit many-atom-cluster calculations. Another important feature of the method is its ability to describe Anderson localisation. Results for densities of states and conductivities are presented for various model systems and compared with the corresponding results of an effective-medium theory. A new result of the theory is the occurrence of multiple mobility edges separating regions of Anderson localised states inside the minority band.

Tight-binding theory of alloy heats of formation: a moments approach

The heats of formation of transition metal alloys are related to moments of the density of states within a tight-binding model for the d band which takes account of diagonal disorder. The contribution of charge transfer to alloy stability is argued to be small. The dominant effect comes from deformation of the bands as manifested by differences in the moments of order four and higher between the alloy density of states and a reference distribution obtained by superposing the pure metal constituents so that their Fermi levels coincide. The difference in fourth moment is trivial to calculate directly from the Hamiltonian and a simple fit to it reproduces to within 20% the results of some recent model calculations by Pettifor (see ibid., vol.7, p.613 (1977)).

Temperature variation of spin susceptibility for nickel-platinum alloys

Temperature variation of spin susceptibility for Ni-Pt alloys is calculated in the band model. Densities of states for alloys are calculated by the coherent potential approximation with off-diagonal randomness.

On the density of states of disordered alloys and their moments

Compares two methods to calculate the density of states n(E) of disordered alloys, both of which use the continued fraction expansion of the Green functions and have the same number of exact moments. In one of the procedures the authors approximate n(E) by the average of local densities of states, each calculated by the continued fraction expansion. In the other, the moments of the alloy are calculated first and from these n(E) is obtained from a single continued fraction. It is shown that, for a tight-binding one-dimensional alloy, the first method is definitely better due to the pronounced structure of the alloy density of states. For more realistic Hamiltonians and for amorphous and liquid systems, both methods may be more comparable.

Calculation of Ferromagnetic Properties for Ni-Cu Alloys in Coherent Potential Approximation

The density of states and local density of states for ferromagnetic Ni-Cu alloys are calculated in the coherent potential approximation. The Hubbard model is assumed and the Hartree-Fock approximation is used. The calculated densities of states for alloys are found to be strongly smoothened out by a rather large potential difference between Ni and Cu atoms. From the calculated results of the density of states, bulk and local magnetic moments and low temperature specific heat coefficient γ are calculated. Moreover, the high-field spin susceptibility at 0 K is calculated. These results are compared with the experimental ones and a qualitative agreement is obtained.

Influence of local fluctuations of concentration on the electronic density of states in disordered binary alloys

The authors present a method which improves the results of the coherent potential approximation (CPA) generally used to study the electronic properties of disordered alloys. They replace the disordered medium by an effective one characterized by a local coherent potential which varies from place to place and depends on the particular alloy configuration. The local electronic density of states is determined by equations similar to that found in the CPA, and then conveniently averaged over all the possible configurations in order to obtain the alloy density of states. As opposed to the CPA, this method yields tails in the allowed energy bands. As an illustration for the method, the authors calculate the electronic density of states of a simple one-dimensional model and compare it to the exact density of states and to the results of CPA calculation.

Shielding and Density of States in Alloys

Shielding and Density of States in Alloys

Perturbation-variation potential for disordered systems

A novel definition of a disordered alloy potential is introduced by a variation-perturbation method on the alloy density of states. Connection is made with the single site CPA.

Effects of Off-Diagonal Randomness in the Tight-Binding Model of Random Binary Alloys: Weak-Coupling Limit

The single-band tight-binding model of random binary alloys is studied in the limit of second-order self-consistent perturbation theory, allowing the off-diagonal, as well as the diagonal, matrix elements of the model Hamiltonian to be dependent upon alloy composition. Fluctuations of both types of matrix elements about their configurational averages are assumed to be small but comparable, so that the randomness introduced into both parts of the Hamiltonian must be treated on an equal footing. Only nearest-neighbor hopping between constituent atoms (type $A$ or $B$) is considered, with the hopping integrals parametrized by the three numbers $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ for $A\ensuremath{-}A$, $B\ensuremath{-}B$, and $A\ensuremath{-}B$ hopping, respectively. The single-particle alloy spectrum and the alloy density of states are obtained from an equation for the effective single-particle self-energy using standard techniques and with the dependence on model parameters explicitly displayed. With the assumption that $\ensuremath{\gamma}=\frac{1}{2}(\ensuremath{\alpha}+\ensuremath{\beta})$, the theory is found to be rather simply characterized by the wave-vector-dependent displace ment ${E}_{A}(\mathrm{k})\ensuremath{-}{E}_{B}(\mathrm{k})$ of the pure constituent spectra ${E}_{A}(\mathrm{k})$ and ${E}_{B}(\mathrm{k})$.