A chemist's view of chemical bonding in the mechanism of high temperature superconductivity

TL;DR

This paper presents a chemical bonding perspective on high-temperature superconductivity, emphasizing oxygen interactions and charge transfer mechanisms in CuO2 planes, challenging traditional electron-phonon coupling models.

Contribution

It introduces a crystal orbital approach highlighting oxygen-oxygen interactions and charge transfer effects as key factors in high-Tc superconductivity.

Findings

Charge transfer occurs at the M point in the Brillouin zone.

Oxygen atoms act as dissymmetric 'Janus' atoms linking copper atoms.

Hole doping leads to different electronic configurations in oxygen groups.

Abstract

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.