J.L. Morán-López

The local electronic densities of states at the surface of disordered alloys

Abstract The local densities of states at a clean as well as at an adsorbate-covered surface of a disordered alloy is studied within the tight-binding approximation using the first terms in a continued fraction series for the electronic Greens function. The dependence of the adatom local densities of states on the alloy composition, adatom position, special local atomic environment around the adatom and the hopping integral between the adatom and its nearest surface atoms is given. Also the binding energy of an adatom is calculated as a function of the alloy composition. We discuss the significance of electron charge transfer for chemisorption and in general for the determination of the electronic density of states at the surface.

Theory for the electronic structure at the surface of transition metal alloys

A tight binding theory for d electrons at the surface of transition metal alloys which play an important role in catalysis is presented. An atomic layer dependent electronic self energy due to atomic disorder scattering is introduced and determined by using the continued fraction method and by using the Bethe-Peierls approximation extension on the coherent potential approximation. The authors also discuss s-d hybridization at the surface, and show how the electronic disturbance at the surface due to adatoms, can be determined.

On the selective catalytic behaviour of the Cu-NI alloy surface☆

Abstract The selective catalytic behaviour of the Cu x Ni 1− x alloy surface is studied. The concentration dependence of the catalytic activity with respect to chemical reactions involving cracking and non-cracking of C-C bonds is calculated for a simple model for C-C bonds. Surface segregation and local environment effects on chemisorption both calculated from a microscopic electronic theory using tight-binding approximation are taken into account.

Changes in work function due to charge transfer in chemisorbed layers

Abstract In a simple tight-binding model well suited to transition and noble metals, we calculate the work function change due to adsorption of alkali metals. A decrease is found to be caused by ionic bonds formed on the surface through charge transfer between the two metals. The local density of states and the number of electrons in the substrate and the chcmisorbed layer are evaluated for various alkali metals and coverages. The method is applicable in both the high and low coverage limits in contrast to existing theories.

Energetics of giant fullerenes and Matrjoschka structures

Abstract The electronic structure of the giant fullerene family C n , n = 60( ν + 1) 2 , was calculated for ν = 0,1,2,3,4. The calculation was carried out within the tigh-binding Hamiltonian in the Hartree-Fock approximation. We present results for the energy per atom as a function of size. An analytic expression is derived for the energy of giant fullerenes of arbitrary size in terms of the energy per pentagon, and plain and folded hexagons occuring in the truncated icosahedra. We computed also the energy of formation of multi-shelled matrjoschka structures and found that those are the most stable forms of carbon clusters.

The s and p character of the electronic structure of C20, C60 and C70

Abstract The electronic structure of the carbon small clusters, C20, C60 and C70, is calculated. The calculation is carried out within a Hubbard like Hamiltonian in which s- and p-electrons are taken into account. Results are obtained for the selfconsistent densities of states by means of the recursion method and are discussed and compared with those of diamond and the hexagonal lattice.

Magnetic Properties of Cubo-Octahedral Ising Clusters

The magnetic properties of large Ising clusters with cubo-octahedral structures are studied. The Ising Hamiltonian is solved in the mean field approximation and the magnetization and the Curie temperature are calculated for clusters with up to 2057 atoms. It is found that the dependence of T c with respect to the total number of atoms (N) follows a finite scaling behaviour δ = 1 − T c (N)/T c (∞) = aN −λ. We find that λ = 0.593 and a = 2.924, for the case in which the magnetic coupling constants between spins are independent of their location in the cluster. However, these quantities change considerably when the coupling constant between surface spins is assumed to be different from the rest. These results clearly show the particular behaviour of finite aggregates. We present the magnetization profiles of the sites lying on the central plane of a cluster with 923 spins. We compare our results with some Monte Carlo simulation for compact clusters and with mean field calculations using Open image in new window Figure 1 A cubo-octahedral cluster with 561 spins. For clearness we show only the spins in the front faces. The central plane is shown by dashed lines. the average coordination number. We finally comment very briefly on the dependence of T c for clusters with different geometrical structures.

Electronic theory for the metal-nonmetal transition in simple metal alloys

The metal-nonmetal transition in liquid alloys such as Cs1-xAux and Li1-xPbx is explained as resulting from concentration dependent electron charge transfer causing short-range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing, the volume change, the electrical conductivity and the thermoelectric power are given.

Oxygen chemisorption effects on the spatial atomic distribution of CuNi, CuPd and NiPt nanostructures

Abstract The spatial atomic distribution in cubo-octahedral CuNi, CuPd and NiPt clusters with a total number of atoms, N = 147, in the presence of chemisorbed oxygen, is studied. The equilibrium atomic configuration is obtained by calculating the free energy within the regular solution model and by assuming that the surface of the cluster is covered by oxygen atoms. Depending on the interaction between oxygen and the cluster components, the atomic distribution in the cluster can be completely modified as compared to the case of clusters with a clean surface. We present result for the temperature dependence of the concentration at the different shells around the central atom.

Electronic theory for the metal-non-metal transition in simple metal alloys

The metal-non-metal transition in liquid alloys such as Cs1−xAux and Li1−xPbx is explained as resulting from concentration dependent electron charge transfer causing short range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing and the volume change are given.