Gilles Santi

Nesting Properties and Anisotropy of the Fermi Surface of LuNi{sub 2}B{sub 2}C

The rare-earth nickel borocarbides, with the generic formula RNi{sub 2}B {sub 2}C , have recently been shown to display a rich variety of phenomena. Most striking has been the competition between, and even coexistence of, antiferromagnetism and superconductivity. We have measured the Fermi surface (FS) of LuNi{sub 2}B {sub 2}C , and shown that it possesses nesting features capable of explaining some of the phenomena experimentally observed. In particular, it had previously been conjectured that a particular sheet of FS is responsible for the modulated magnetic structures manifest in some of the series. We report the first direct experimental observation of this sheet. (c) 1999 The American Physical Society.

S- AND D-WAVE SYMMETRIES OF THE SOLUTIONS OF THE ELIASHBERG EQUATIONS

Abstract We examine the different possible symmetries of the superconducting gap obtained by solving the Eliashberg equations. We consider an electron-phonon interaction in a strong coupling scenario. The Coulomb pseudopotential plays the crucial role of providing the repulsion needed to favour the d -wave symmetry. But the key parameter that allows very anisotropic solutions even with very strong coupling is the small angular range of the interaction due to predominantly electron-phonon forward scattering that is found in the high- T c superconductors. We find both s - and d -wave solutions whose stability depends mainly on the angular range of the interaction.

The role of thermal disorder on the electronic structure in high-Tc compounds

Abstract The electronic structures of YBa 2 Cu 3 O 7 , HgBa 2 CuO 4 and Bi 2 Sr 2 CaCu 2 O 8 are calculated for lattice distortions corresponding to different temperatures. Such distortions, including the zero-point motion, are found to be responsible for band broadenings and modifications of the Fermi surface. The density-of-states (DOS) near the Fermi energy, E F , shows important changes for some modes involving atomic z -displacements as well as for in-plane distortions within the CuO 2 layer. In addition our calculations show that the electron–phonon coupling ( λ ) will be much enhanced for such modes. The consequences of this result on superconducting and normal state properties are discussed.

Existence of both s and d-wave solutions of eliashberg equations

We solve Eliashberg equations in the case of strong anisotropic electron-phonon coupling and low energy cutoff. In a simplified model representing the Fermi Surface of (CuO2)n planes, we exhibit ad-wave solution as well as a generalizeds-wave one.

Solutions of the Eliashberg equations for models of electron-phonon coupling

We solve the Eliashberg equations for models based on the electron-phonon pairing mechanism. The coupling parameters are chosen to be consistent with the Fermi surface of the high-Tc oxides as obtained in band calculations. The attractive coupling parameters are taken to be similar to the calculated electron-phonon interaction and the repulsive ones are assumed to be of Coulomb type. We show that boths- andd-wave solutions exist provided a repulsive interaction is included in the model. The key feature allowing for a near degeneracy of these solutions is that the electron-phonon processes are dominated by forward scattering (i.e. the attractive coupling between distant parts of the FS is small). We discuss the conditions for existence and stability as well as physical consequences for both types of solutions.