Coherent potential approximation

About: Coherent potential approximation is a research topic. Over the lifetime, 1930 publications have been published within this topic receiving 36805 citations.

Zero-temperature spin dynamics of a random two-dimensional XY model

Abstract: We study the zero-temperature spin dynamics of a random-exchange two-dimensional [ital XY] model using both exact numerical methods and an approach based upon the coherent potential approximation (CPA). The model, which presents a mixed-phase to spin-glass phase transition, consists of a ferromagnetic host with nearest-neighbor bonds [ital J] to which one substitutes impurity bonds of strength [minus][lambda][ital J] at concentration [ital x]. In both phases, the long-wavelength magnetic excitations of this system are spin waves with a linear spectrum. The [ital x] and [lambda] dependence of the spin-wave velocity is determined numerically by calculating the stiffness constant of the system with a new transfer-matrix algorithm. The stiffness constant and the spin-wave velocity decrease rapidly with increasing [ital x]. The two quantities are smooth across the phase boundary, and they saturate in the spin-glass phase. At shorter wavelengths, the [ital q] dependence of the energies and lifetimes of the spin waves in the ferromagnetic and spin-glass states differs qualitatively, reflecting the morphological differences that exist between the spin configurations characteristic of the two phases. Line shapes and linewidths depend strongly on the polarization of the excitations. Whereas out-of-plane modes stay propagative at all concentrations, in-plane ones are inhomogeneously broadened and aremore » overdamped at long wavelengths. There is good overall agreement between the exact numerical results and those obtained using the CPA.« less

Many-body theory of electronic excitations in random substitutional alloys

Abstract: The many-electron Green's function scheme is combined with theories for substitutional disorder, in particular the coherent potential approximation (CPA). Emphasis is placed on both insulating and metallic solids, for which the homogeneous electron gas description is not valid. To this a new set of formally exact equations is derived for the calculation of the disorder and Coulomb contributions to the electron self-energy. One of the equations, determining the disorder contribution as a functional of the average self-consistent propagatorG, can be treated by standard methods such as the CPA. The other equation, determining the Coulomb contribution ∑ c as a functional ofG is solved at three levels of approximation: (i) At the simplest level, disorder-induced correlations are neglected, resulting in ∑ c [G]=∑0[G], where ∑0 is the Coulomb self-energy functional. It is argued that this is already a reasonable approximation in an insulator. (ii) At the second level, disorder-induced correlations within the particle-hole propagators are taken into account in the expression for ∑ c . This approximation is particularly relevant for metals, since it generates the diffusion pole. (iii) At the third level, the correlations responsible for the Altshuler-Aronov anomalies observed in the density of states of disordered metals are additionally incorporated. From the self-energy, expressions for the linear response to an electromagnetic field are derived. A practical trunction to first order in the screened Coulomb interaction is suggested. A comparison with some previously advanced theories, in particular the Korringa-Kohn-Rostoker-CPA (KKR-CPA), is made.

Ferromagnetism versus superconductivity in ramdom alloys

Abstract: Magnetic, superconducting narrow-band alloys are studied within a random lattice model. Coulomb repulsion and contact-type attraction of the itinerant electrons, and the indirect exchange between local magnetic moments are taken into account. Averaging the free energy expansion in coherent potential approximation, the Ginzburg-Landau functional is derived with two biquadratically coupled order parameters. Coherent orderings and concentration-dependent critical temperatures are determined. The coefficients of theq2-terms, i.e. the stiffness and diffusion constants, involve vertex corrections due to impurity scattering. The coexistence criterion resulting from the 4th-order couplings cannot be fulfilled, referring to a first order transition between ferromagnetism and superconductivity. Tetracritical behaviour is found by including the direct exchanges between the localized spins.

Neutron and Mössbauer Studies of Atomic Magnetic Moments in a Ternary Alloy Fe–Co–Ni

Abstract: The magnetic moments of Co atom in Fe–Co–Ni alloys have been determined over whole concentration range of the fcc structure by polarized neutron diffuse scattering experiment. The results support qualitatively the theoretical calculations under the coherent potential approximation. The magnetic moments of Fe atoms have also been obtained in the alloys by means of Mossbauer effect. The results lie in between the theoretical values obtained either by the coherent potential or by the rigid band approximation.