Local coherent potential approximation

An attempt to describe the concentration dependence of the DOS at the Fermi level in NbC Ni (x = 1.0, 0.75, 0.25, 0.12, and 0) was undertaken by Nikiforov and Kolpachev (1988) and Kolpachev and Nikiforov (1988). They used a multiple scattering method in the local coherent potential approximation. Variations in the solid solution composition caused the greatest changes in the nonmetal p-symmetry states and in the part of the d-band located close to the Fermi level. At the same time, comparison of the DOS of the calculated clusters and photoelectron spectra of Nb ternary alloys obtained by Ichara and Watanabe (1981) shows that the calculations only roughly reproduce the peculiarities of the NbC jNi c valence spectrum the energy separation of the p-d- and d-bands is considerably overestimated and there are some additional peaks in the DOS which are not seen in the experimental spectrum. 

We have used the multisublattice generalization of the coherent potential approximation (CPA) in conjunction with the Linear-MufRn-Tin-Orbital (LMTO) method in the atomic sphere approximation (ASA). The LMTO-ASA is based on the work of Andersen and co-workers and the combined technique allows us to treat all phases on equal footing. To treat itinerant magnetism we have employed for the local spin density approximation (LSDA) the Vosko-Wilk-Nusair parameterization". 

ANG AO ATA BF CB CF CNDO CPA DBA DOS FL GF HFA LDOS LMTO MO NN TBA VB VCA WSL Anderson-Newns-Grimley atomic orbital average t-matrix approximation Bessel function conduction band continued fraction complete neglect of differential overlap coherent-potential approximation disordered binary alloy density of states Fermi level Green function Flartree-Fock approximation local density of states linear muffin-tin orbital molecular orbital nearest neighbour tight-binding approximation valence band virtual crystal approximation Wannier-Stark ladder... 

Figure 4. Schematic representation of the coherent potential approximation for a substu-tionally disordered alloy. The vertical strip in (h) denotes the effective self-energy, which is a complex quantity, to be determined by the compatibility requirement between die local and average description.

The LSGF method on the other hand is an order-IV method for calculation of the electronic system. It is based on a supercell (which may just be one unit cell) with periodic boundary conditions, see Fig.(4.5), and the concept of a Local Interaction Zone (LIZ), which is embedded in an effective medium, usually chosen to be the Coherent Potential Approximation medium (see next chapter). For each atom in the supercell, one uses the Dyson equation to solve the electronic structure problem as an impurity problem in the effective medium. The ASA is employed as well as the ASA+M correction described above. The total energy is defined to... 

Contents H.Matsuda Atoms as Constituents of Matter. - T. Tsuneto System of Protons and Electrons. - T. Tsuneto Helium. - T. Tsuneto Superfluid Helium 3. - T.Matsubara Metals. - T.Matsubara Non-metals. - T.Matsubara Localized Electron Approximation. - T.Murao Magnetism. - T.Murao Magnetic Properties of Dilute Alloys - the Kondo Effect. - H.Matsuda Random Systems. - F. Yonezawa Coherent Potential Approximation (CPA).- References.- Subject Index. 

In the second method, going beyond Bom, we examined the density of states within the coherent potential approximation (CPA) which takes into account multiple scattering processes. One might think that on this level impurity states are introduced in the gap. However, we find [16] that the existence of such localized impurity states strongly depends on the relative strength of site vs. bond impurity. Only states in the gap due to disorder can be found if the site amplitude f/s is stronger than the bond amplitude Ufc. Since CPA is an effective medium theory this result might be questionable in one dimension. 

The anion sublattice is occupied only by As atoms, while the cation sublattice is occupied by Ga and Mn atoms, and also by As antisite defects. We consider only substitutional disorder on the cation sublattice which in turn is described within the coherent potential approximation (CPA) . We thus neglect local environment effects and lattice relaxations. 

LOCAL CHARGE DISTRIBUTIONS IN METALLIC ALLOYS A LOCAL FIELD COHERENT POTENTIAL APPROXIMATION THEORY... 

In this paper we shall develop a new version of Coherent Potential Approximation theory (CPA). We apply a local external field and study the response of the mean field CPA alloy. Because of the fluctuation-dissipation theorem, the response to the external field must be equal to the internal field caused by electrostatic interactions. This new theoretical scheme, avoiding the consideration of specific supercells, will enable us to explore a broad range of fields and clarify certain aspects of the mentioned qV relations. 

The local spin density approximation and coherent potential approximation used in the first principles calculation may not be adequate to handle the strong correlation and lattice relaxations. This is particularly so for thin films prepared by different deposition techniques under nonequilibrium conditions. In (Zn,Mn)0, the 3d electron onsite Goulomb interaction Uis estimated to be 5.2 eV, which is comparable... 

The present paper is devoted to the theoretical formulation and numerical implementation of the NDCPA. The dynamical CPA is a one-site approximation in which variation of a site local environment (due to the presence, for example, of phonons with dispersion) is ignored. It is known from the coherent potential theory for disordered solids [21], that one can account in some extension the variation of a site local environment through an introduction of a nonlocal cohcn-cnt potential which depends on the difference between site... 

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