New theoretical method to study densities of states of tetrahedrally coordinated solids

A new simple method is proposed to calculate local densities of states of arbitrary tetrahedrally coordinated solids. It involves the selection of a finite cluster of atoms connected to an infinite Bethe lattice of coordination four. The method is accurate, is easily handled numerically, and converges fast. Low-order approximations yield sufficient information which is susceptible to consistent physical interpretation. This has been made for the diamond, BC-8 and ST-12 structures in terms of the ring topology around a given atom. Comparison with exact calculations is very good and the ring interpretation is physical and conceptually appealing.     

New theory of binary alloys with short-range order properties

A new method for treating the electronic structure of binary alloys is presented. It is based on the study of a finite size cluster connected at its edges to a Bethe-lattice of the same coordination number. An illustrative example is presented. It includes concentration sequences which are (i) random, (ii) with a tendency to segregation and (iii) with a tendancy to form binary compounds. Energy gaps and localized states appear naturally in the method.     

Metal-insulator and order-disorder phase transitions: Magnetite

A thermodynamic model is presented for a system which exhibits both metal-insulator and order-disorder phase transitions. It is found that transition temperatures could be much lower than those predicted by pure order-disorder -only considerations. A variety of behaviors, including first-order and second-order transitions and critical points is found as the parameters change. The model explains qualitatively the experimental results for magnetite and suggests other possible behaviors for closely related systems.     

Electronic surface properties of group V semimetals

We have studied the electronic surface properties of the (111) surface of group V semimetals (As, Sb, Bi) using a Green's function formalism in the tight-binding representation. We find that no surface states appear unless the surface layer relaxes, but any relaxation is enough to produce bona fide surface states.     

Atomic ordering and ferromagnetism in binary alloys

Atomic and magnetic ordering in binary alloys are studied within the mean field approximation. The influence of magnetism on the spatial order-disorder critical temperature and, conversely, the influence of chemical order on the Curie temperature are analyzed in detail. Phase diagrams for various cases are presented. In particular we find that the interplay of the two order parameters yields in some cases unusual sequences of phases as a function of temperature. For example we obtain a possible sequence with increasing temperature of spatially ordered ferromagnet → spatially ordered paramagnet → spatially disordered ferromagnet → disordered paramagnet.