F. Ducastelle

Segregation and Ordering at Surfaces of Transition Metal Alloys: The Tight-Binding Ising Model

From the electronic structure of the disordered alloy we derive an effective Ising Hamiltonian for segregation and ordering processes at transition metal alloy surfaces. In this tight-binding Ising model (TBIM), the Hamiltonian contains a linear term, quasi-concentration-independent, which proves to be very close to the difference in surface tensions between pure constituents and a quadratic one involving effective pair interactions larger at the surface than in the bulk. The former explains the success of the popular phenomenological approaches based on surface tension arguments and the latter could be of prime importance in surface ordering processes.

Correlation effects on Auger spectra in unfilled d band metals

The authors present a study of the influence of correlations on the two-particle spectra of transition metals using a generalization of Cinis method (1977). These two-particle spectra are shown to be strongly sensitive to the presence of empty electron states in the d band. Indeed, as soon as the d hole concentration differs from zero, the XVV Auger spectra are much broader than the quasi-atomic shape observed on Cu. The U values which account for this effect are found to be in agreement with those deduced from experimental photoemission data.

Wetting of Antiphase Boundaries by the Disordered Phase in CoPt3

In situ transmission electron microscopy experiments coupled with resistivity measurements have been performed on CoPt3. The antiphase boundaries separating different L12 (Cu3Au-type) ordered domains are shown to be progressively wetted by the disordered phase when approaching the critical order-disorder transition from below. This is in excellent qualitative agreement with previous cluster variation method calculations by Kikuchi and Cahn.

Variational and Mean Field Formulations of the Cluster Variation Method and of the Path Probability Method

It is shown that the path probability method of Kikuchi is a genuine mean field treatment of the kinetic Ising model. The theory for the spin-flip dynamics is presented in a general form, completely consistent with that already put forward for the cluster variation method

Thermodynamics of Surfaces and Interfaces

Order-disorder transitions are frequent in alloys. The homogeneous ordered phase characterized by long-range order parameters which vanish in the disordered phase. In fact, local order parameters can also be defined on a scale large enough compared to the size of the relevant unit cells, and they vary in space in the presence of defects.

Influence of Coulomb correlations on the electronic excitations of transition metal surfaces: application to Mo(100) and Ni(100), (110) and (111)

Presents a study of the influence of correlations on the electronic optical excitations of transition metal surfaces. A Hubbard model in the band limit is solved by a second-order perturbation method previously developed for bulk metals. Excitation spectra of Mo(100) and Ni(100), (110) and (111) are shown to differ strongly from their respective one-electron local densities of states. This explains some features of experimental UV photoemission spectra of Mo.

On the temperature dependence of the surface sandwiches observed in PtNi and AgNi alloys

PtNi and AgNi alloys present experimentally striking surface segregation behaviours. They are successfully interpreted within a recently developed theory coupling electronic structure (TBIM: Tight-Binding Ising Model) and thermodynamics (mean field approximation formulated as an Area Preserving Map: APM). Furthermore, this approach predicts unusual phase transitions of the concentration profiles as a function of temperature.