Electronic structure and short-range order of amorphous and crystalline alloys in the coherent locator approximation

Model systems are considered to investigate the dispersion and damping of phonon-like elementary excitations in structural disordered chains. An approximate separation is carried out between the dynamical and structural disorder. For a rectangular distribution of the force constants the concept of quasistationary plane-wave-like excitations is inappropriate at the band edges. The influence of random equilibrium positions on the complex dispersion relation is also investigated. The quasi-crystalline approximation is generalized to include the quasiparticle damping and the low-temperature specific heat excess is briefly discussed.



Modellsysteme werden betrachtet, um die Dispersion und Dampfung phononahnlicher Elementaranregungen in strukturell ungeordneten Ketten zu untersuchen. Eine naherungsweise Trennung zwischen der dynamischen und strukturellen Unordnung wird vorgenommen. Fur eine Rechteckverteilung der Kraftkonstanten ist das Konzept der quasistationaren ebenen Wellen ahnlichen Anregungen an den Bandkanten ungeeignet. Der Einflus der zufalligen Gleichgewichtslagen auf die komplexe Dispersionsrelation wird ebenfalls untersucht. Durch Einschlus der Quasiteilchen-Dampfung wird die quasi-kristalline Naherung verallgemeinert und der Exzes der spezifischen Warme fur tiefe Temperaturen wird kurz diskutiert.

Dispersion and Damping of Phonon-Like Excitations in Disordered Systems

The properties of the bond CPA (BCPA) have been investigated in the Goldstone case and for pure off-diagonal disorder. The results have been compared with those obtained using other methods. The moments of the density of states obtained using different CPA theories have been compared with the exact ones. The numerical comparison of the BCPA and Lage-Stinchcombe (LS) theory in the case of pure off-diagonal disorder shows that unlike LS theory the BCPA gives a gap and the delta -like peak at E=0 for the concentration below the approximate percolation concentration which is in rough agreement with computer simulations. The properties of the gap and peak have been obtained analytically. The BCPA appears to be a useful working approximation for the Goldstone and off-diagonal disorder problem but not for the site diagonal case usually treated in the SCPA.

https://iopscience.iop.org/article/10.1088/0022-3719/15/19/009/pdf

A comparison of bond and site CPAS for dilute systems

A coherent potential approximation is developed for a distribution of transfer integrals (off-diagonal disorder) in a single band on a dilute close-packed lattice with first and second neighbour interactions. The moments of the density of states are evaluated and compared to exact results up to order six. It is suggested that this model provides a reasonable approximation to the structurally disordered material with the same distribution of transfer integrals arising via the pair distribution function. Results are computed for distributions based on experimental observation in liquid Fe and on those computed by the Percus-Yevick method for a liquid of hard spheres.

T Fujiwara

Papers

A CPA theory of excitations in structurally disordered materials using a lattice model