POUCHARD, Michel; VILLESUZANNE, Antoine; DEMOURGUES, Alain

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POUCHARD, Michel
Institut de Chimie de la Matière Condensée de Bordeaux [ICMCB]

VILLESUZANNE, Antoine
Institut de Chimie de la Matière Condensée de Bordeaux [ICMCB]

DEMOURGUES, Alain
Institut de Chimie de la Matière Condensée de Bordeaux [ICMCB]

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Through a 2-dimensionnal tight-binding crystal orbital approach and a (CuO2)2 square unit cell of parameter a, we show that a Cu2+/O2- -> Cu+/O- charge transfer is likely to occur at the M($\pi/a$, $\pi/a$) point of the Brillouin zone, for O4 groups with antibonding b1g symmetry. This approach emphasises the role of oxygen-oxygen interactions in avoiding the nesting of the Fermi surface and agrees with its observed topology. At the M point of the Brillouin zone, oxygen atoms are strongly dissymmetric ("Janus" atoms) and link copper atoms with different environments (a1g vs. b1g symmetries). Further hole doping generates two situations: two holes (S=0) and/or a single hole (S=+/- 1/2) in the O4 b1g groups ($\sigma$ or $\pi$), with a possible equilibrium between them; the former can be considered as a "hole lone pair" by analogy with electron lone pairs. Mulliken-Jaffe electronegativity considerations justify the nearly zero U Hubbard parameter value. The mixing of the pair-occupied with pair-iunoccupied wave functions is realised via an electronic hamiltonian in place of the electron-phonon coupling of the pristine BCS theory.< Réduire

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Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB) - UMR 5026