Lattice specific heat and local density of states of Ni-based dilute alloys at low temperature

A detailed theoretical study of the low-temperature lattice specific heat of Ni-based dilute alloys has been carried out. Lattice Green’s function method has been used to calculate the local density of states of substitutional impurities and lattice specific heat in different alloys. The resonance condition has been investigated for possible occurrence of resonance modes. Except in NiCr and NiMn, low-frequency resonance modes have been obtained in all the alloys. However, no localized mode was obtained. The impurity-induced increase in lattice specific heat is explained on the basis of the obtained resonance modes. The calculation shows an excellent agreement with the measured lattice specific heat in these alloys     

On calculation of electronic states in disordered alloys

A new representation of the Green function for a disordered multicomponent substitution alloy is proposed, using which it is easy to separate one-, two-, etc. contributions to this Green function in corresponding equations. Due to this representation we succeed in obtaining common expressions for the configurationally averaged electron Green function with allowance for the electron scattering on clusters formed by an arbitrary number of atoms. The approach used is equally appropriate to described alloys both in the weak binding model, and in the tight binding one.     

Note on the perturbation calculation of the spin correlation function of dilute magnetic alloys

Using graphical perturbation technique, nonoscillatingJ ²/r ³ andJ ³lnk _Fr/r³-contributions for the spin correlation function of dilute magnetic alloys are calculated. The results are in complete agreement with Müller-Hartmann's results, based on the exact solution of the Nagaoka equations.     

The density of states in two-dimensional dilute Heisenberg ferromagnets near the percolation threshold

Two-dimensional dilute Heidenberg ferromagnets with the magnetic atom concentration near the percolation threshold are considered. The density of magnetic excitation states at zero temperature is obtained by the scaling theory for the percolation problem.     

Electronic density of states of binary alloys within thev-cluster-approximation

The properties of the 1-cluster-approximation (1 CA) as an extension of the CPA which preserves translational invariance are investigated. The electronic density of states of one-dimensional disorderedAB-alloys is calculated analytically in the limit of low concentrations within the 1 CA; for arbitrary concentrations the electronic densities of states of one-dimensional and of three-dimensional simple cubicAB-alloys are evaluated numerically. In the impurity band region the 1 CA shows more structure than the CPA even in the low-concentration-limit in agreement with the exact calculations for finite crystals. The 1CA-self-energy, however, is not analytic; in particular it has a branchpoint of order 2 off the real energy axis.Work performed within the research program of the Sonderforschungsbereich 125 Aachen-Jülich-Köln     

On the calculation of electron density of states in disordered metals

It is demonstrated that in a full nonlocal pseudopotential calculation of the density of states, the results from the standard formula, $\text{N(E) = N(E}{}_{\mathrm{<mtext>FE</mtext>}}\text{)k (}\text{?E}{}_{\mathrm{<mtext>k</mtext>}}\text{?k}\text{)}{}^{\mathrm{?1}}$, do not differ significantly from those using the Green-function method and are reliable for simple liquid metals, despite the fact that there is an uncertainty in treating E_k as a dispersion function.     

The asymptotic region of the charge density oscillation around impurities in dilute copper-based alloys

The wipe-out number for the first order quadrupole effect in the magnetic resonance of the matrix nuclei was measured in dilute copper-based alloys to study the asymptotic behaviour of the charge density oscillation around different impurities. The measured values are Cu-Zn: 490 ± 25; Cu-Au: 880 ± 40; Cu-Ni: 1250 ± 125; Cu-Pd: 1200 ± 100; Cu-Pt: 1900 ± 100.     

On the Tc of dilute alloys

The equation for the critical temperature T_c, of a dilute superconducting alloy due to Markowitz and Kadanoff (MK) is generalized to include renormalization effects due to the electron-phonon interaction. Such corrections constitute a 50% effect in weak coupling superconductors like aluminum while in strong coupling systems like lead this correction gives a factor of 2.5. The mean square anisotropy parameter appearing in the T_c equation is also generalized to remove the separability assumption of the electron-phonon interaction. Some consequences of these two corrections to the analysis and systematization of data for dilute superconducting alloys is discussed.     

Properties of magnetically dilute alloys

Recently it has been shown by analysing Mössbauer spectra of dilute alloys of iron in gold that the anisotropic part of the exchange interaction between iron near neighbours determines the direction of magnetisation of such iron moments at low temperatures. Estimates of the important interactions between moments, which are here identified as the isotropic and anisotropic parts of the near neighbour exchange, and the isotropic exchange between further neighbours via the conduction electrons, have been obtained from available experimental data. The near neighbour exchange causes spins within random clusters linked as near neighbours to align parallel (the isotropic part) and to prefer certain directions in the cluster (the anisotropic part). The further neighbour exchange is responsible for the cooperative magnetic transition shown in Mössbauer and small field susceptibility results. The final model leads to qualitative interpretations of the temperature and field dependence of the susceptibility, and is able to reproduce the hysteresis loops observed at low temperatures for samples cooled in different fields. The effects of the interactions on the entropy and specific heats are discussed, and it is shown that the linear specific heat contributions at low temperatures occur because of the anisotropic interactions.     

Magnetoresistance of dilute magnetic alloys

The magnetoresistance of dilute magnetic alloys is calculated. The difficulties appearing in the calculation of scattering times within the density wave model are discussed and overcome semi-empirically.     

Local atomic magnetic moments and density of states in iron-nickel-chromium alloys

The electronic structure and local atomic magnetic moments of components in iron-nickel-chromium alloys with bcc lattice are calculated using a coherent potential combined with the Hartree-Fock approximation. The computations agree well with available experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 20–24, October, 1982.     

Impurity-impurity interactions in dilute alloys

The interaction between impurities of two solute elements in metal hosts leads to a non-random distribution of impurity atoms among the lattice sites. Experimental studies of this distribution provide quantitative information on the energy involved in the formation of an individual impurity-impurity pair. It is shown how this energy can be extracted from PAC and Mössbauer spectra. A review of the experimental results for several metallic matrices is given. The binding energy of the impurity-impurity pairs is discussed in terms of a correlation with the heat of formation for corresponding alloys.     

On the magnetoresistance in dilute magnetic alloys

Many-particle contributions to extended Suhl equations are considered to derive a magnetoresistance, which saturates for nonzero magnetic fields below the unitatarity limit in agreement with experiments.     

The calculation of transport properties and density of states of disordered solids

A computational method is presented for the calculation of the conductivity tensor and the density of states of disordered solids. This is a more general and detailed discussion of an algorithm which has been applied to d.c. and a.c. conductivity, Hall effect and density of states of various systems.     

Galvanomagnetic effects of dilute nitrogen-iron alloys

Galvanomagnetic effects of the dilute nitrogen-iron alloys have been investigated in detail between 4.2 and 250 K. The specific resistivity increment by solute nitrogen in pure iron is determined experimentally as 8.9 μΩcm/at.%. It is revealed that Kohler plots for the transverse magnetoresistance of quenched and unquenched specimens lie on the same curve, while it is not so for the longitudinal magnetoresistance, and Kohler plots for the longitudinal effect of single crystals containing solute nitrogen, are different from one another for each crystal axis (100), (110) and (111). Furthermore, a substantial deviation from Matthiessen's rule was found out by the measurement of the temperature dependence of the electrical resistivity of the alloy between 4.2 and 250 K.The Matthiessen's rule parameter is about seven.Kohler's rule holds for the Hall resistivity. In the low-field limit the plots do not go through the origin. The tangent of the Hall angle is 1.0 × 10^?2 for quenched specimens. This non-zero value can be explained with the asymmetric scattering proposed by Smit. The extraordinary Hall constant is proportional to the ohmic resistivity.     

Asymptotic region of charge density oscillation around impurities in dilute copper based alloys—II

The wipe-out number for the first order quadrupole effect in the NMR of matrix nuclei was measured in dilute Cu-Ag, Cu-Ga and Cu-Ge alloys in order to study the asymptotic behavior of the charge density oscillation around different non-transitional metal impurities. The measured values of the wipe-out numbers were compared with the predictions from Friedel's theory by using the phase shifts calculated by Blatt, Kohn-Vosko, Alfred-Van Ostenburg. “Empirical” phase shifts (η₀, η₁, η₂) for the Cu-Zn, Cu-Ga and Cu-Ge alloys were determined on the basis of Faber-Ziman diagrams. The values of the experimentally determined first order wipe-out number, the residual resistivity, and the Knight shift measured on the smelts, were described by these phase shifts.