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Résumé |
The discovery of superconductivity with a critical temperature Tc of 26 K in fluorine-doped LaOFeAs in early 2008 has initiated a so-called "iron age" in the field of superconductivity. In a few months, several new iron-based superconductors, with Tc's as high as 55 K, have been found. Their crystal structures show a common motive, characterized by a square Fe lattice, surrounded by distorted tetrahedra of pnictogen or chalchogen atoms.
Besides superconductivity, iron-based superconductors show a spin density wave (SDW) transition accompanied by a lattice distortion. This points to a possible unconventional origin of superconductivity, in analogy with the high-Tc cuprates, although there is no general consensus on the pairing mechanism.
The main experimental findings concerning iron-based superconductors will be first reviewed and discussed here. Then Density Functional theory will be used to describe the electronic and vibrational properties of LaOFeAs, which is a prototype compound for iron pnictides.
Using linear response calculations, I will show that the standard Migdal-Eliashberg theory fails to account for the observed critical temperature.
Using an ab-initio effective tight-binding Hamiltonian, based on Fe d and As p Wannier orbitals,derived from NMTO downfolding, I will then analyze the origin of the band structure of iron pnictides, and use this tight-binding model to discuss the origin of magnetism in Fe based superconductors and its itinerant nature
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