POUCHARD, Michel; VILLESUZANNE, Antoine

doi:10.3390/condmat5040067

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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]

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Condensed Matter. 2020-12, vol. 5, n° 4, p. 67 (30 p.)

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From a tight-binding approach to the instability of nonbonding electronic states, along a double-well potential, we consider here how the coupling of these states with a phonon mode can open a superconducting gap at the Fermi level. The alternation of broken- and unbroken-symmetry states, along the phonon breathing distortion, induces the mixing of band-edge states on a very short timescale, according to the noncrossing rule of chemists. We show that this mixing may generate cationic and anionic disproportionation. The negative U mechanism is thus justified here, leading to the mixing of occupied and unoccupied pair states, for the opening of a 2Δ superconducting gap. The closeness of broad σ * and narrow π * bands in the vicinity of the Fermi level should favor the superconducting phase over the insulating or metallic state, in agreement with Micnas et al.’s studies. We applied this approach to several families of superconducting materials, i.e., doped strontium titanate, high-TC cuprates and iron selenide.< Réduire

Mots clés en anglais

nonbonding electrons

noncrossing rule

negative-u mechanism

phonons

BCS theory

disproportionation

hole pairing

cuprates

titanates

iron selenide

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Are superconductivity mechanisms a matter for chemists?

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