Ph. Lambin

Band structure of YBa2Cu3Ox in relation with the oxygen vacancy distribution

Abstract The electronic band structure and the density of states of the YBa2Cu3Ox superconductor are computed for x = 7 using the extended Huckel scheme. Two ordered arrangements of the oxygen vacancies in the basal Cu-O planes have been considered. It is shown that the oxygen vacancy distribution plays a major role in the electronic structure of this compound and deeply influences the density of states at the Fermi level.

Theory of electron energy loss spectroscopy at the surface of a planar stratified medium, application to semiconductor superlattices

Abstract The nonretarded, macroscopic surface and interface optical vibrational modes of a layered material are theoretically investigated via their electron energy loss spectrum (EELS) in the reflection geometry. For this purpose, a continued-fraction representation of the relevant effective dielectric function of an arbitrarily stratified medium is obtained. For a semi-infinite superlattice two distinct types of vibrational excitations are predicted. The first gives rise to weak EELS continua and consists in Bloch-like interface modes propagating throughout the layered medium. The second type is responsible for strong EELS lines outside the above continua and is associated with evanescent, Fuchs-Kliewer-like surface modes of the superlattice. Application to a model GaAs/InAs superlattice is given for illustration.

Synthesis parameters affecting the bulk composition and superconducting properties of YBaCuO-based compounds

Abstract Different preparation routes of the high critical temperature superconducting YBa2Cu3O6.5+x material from solid reactant powders Y2O3, BaCO3 and CuO have been explored. Portions of precursor were heated/annealed/cooled under various atmospheres (O2, air, N2, …) and the resulting solid intermediate or final phases characterized by X-ray diffraction and bulk analysis (PIXE). Results of long and short heat treatments confirm the importance of the oxygen stoichiometry and then O2 partial pressure in order to obtain a superconducting phase. The basic reactions were quantitatively studied performing repeated short thermal treatments (heating/cooling cycles) of the same precursor and collecting thermogravimetric and differential thermal analysis data. On heating, the orthorhombic-tetragonal phase transition of YBa2Cu3O6.5+x is found as a first endotherm at 850°C, not accompanied by any weight variation. Upon cooling the corresponding exotherm is seen at 765°C. The high temperature stabilized tetragonal phase reversibly loses oxygen in a series of sharp steps (DTA) at selected temperatures.

Relationship between oxygen stoichiometry and crystal structure in YBa2Cu3O6.5+x precursors to high-Tc superconducting materials

Abstract Possible mechanisms for the oxygen intercalation in the ternary mixed-Cu-valence YBa2Cu3O6.5+x oxide are considered, which account for the observed cell symmetry transformation driven by the oxygen concentration. These mechanisms are consistent with the formation of both single-and double-cell superstructures of ordered oxygen vacancies that have been encountered in crystallographic investigations of the high-Tc superconductor YBa2Cu3O≈7.

New scattering-theoretic approach to elastic one-electron tunneling through spatially localized barriers: application to scanning tunneling microscopy

Abstract One-electron elastic tunneling through nonseparable, localized barriers is treated as a problem in Potential Scattering Theory, using the technique of localized Green functions. The method is illustrated by the calculation of the current in a model plano-spherical junction representative of the Scanning Tunneling Microscope whose lateral resolution is discussed. The new technique is applicable to several other localized barrier problems and allows one to test the accuracy of approximate, one-electron tunneling theories.

Experimental foundation of the Gabor-Nelson theory applied to boundaries which are non-insulating

In order to found the application of the Gabor-Nelson theory to non-insulating boundaries, we have used a network which we have divided into two parts: a core energized by a source sink pair and an appendage, the conductivity of which may or may not differ from that of the core. By ignoring the appendage and by applying the Gabor-Nelson method to the restricted perimeter as if it were totally insulating, we stress the errors made in computing the dipole strength, orientation and position and how they are influenced by the dipole eccentricity, by its orientation with respect to the junction between the added portion and the core, and by a change in conductivity between the same compartments. Finally, we restore the dipole characteristics by using the appropriate correction derived from theory. Comparing the later results to those obtained by applying the Gabor-Nelson method to the whole insulating boundary leads to the conclusion that the correction is founded and must be taken into account.

Electronic theory of surface segregation in CuNi alloys

Abstract A calculation of the surface composition of CuNi alloys based on their electronic properties is proposed. Without any adjustable parameters, the theory predicts a surface enrichment in Cu quite consistent with the experimental results. Bulk and surface local densities of states are also given.

On the Homomorphic Cluster CPA

Among the various extentions of the CPA, the homomorphic cluster CPA (HCPA) was presented recently as the only approximation (i) having the correct analytical properties, (ii) taking multiple scattering into account and (iii) preserving the symmetry of the original lattice. Though very appealing mathematically, we show in this paper that it does not give the right position for the impurity states in the dilute limit with non zero diagonal disorder. Consequently, even if HCPA satisfies points (i) to (iii) and even if it gives the correct dilute limit for purely off-diagonal disorder, it is to be rejected as a method to study systems with non negligible diagonal disorder.

Theory of relaxation of hot electrons and oscillatory photoconductivity in polar semiconductors

Abstract The Boltzmann equation is solved for the relaxation of out-of-equilibrium electrons due to optical and acoustical phonons. The first stage of the relaxation, governed by the optical phonons, is quite fast and leads to a non Maxwellian distribution. The acoustical phonons are responsible for the second stage of the relaxation, much slower and leading to a Maxwellian distribution. Comparison is made with the experimental results for GaAs. The stationary situation encountered in the study of the oscillatory photoconductivity and photoluminescence is also studied. It is shown that the electron population has an oscillatory behaviour versus the energy at which they are photo-injected.