Again `why layered, square-planar, mixed-valent cuprates alone?' - further pursuit of the `chemical' negative-Uroute to the HTSC mechanism

This paper is a consolidated presentation of why the author believes the most promising way forward as regards the mechanism for high-temperature superconductivity remains with the `chemical negative-U' interpretation, first sketched out in 1987. Many other mechanisms have been proposed over the years but none address head on the problem as to why the phenomenon is so tightly confined to the current small subset of mixed-valent cuprates. Tc in Hg-1223 under pressure is more than 450% greater than from any other class of superconductor. By concentrating on the materials side, and how this controls evolution in real space and k-space properties, a more tailored outlook can be gained. This includes understanding how cuprates find themselves in a unique position with regard to chemical bonding, to the delocalization of charge and to the decay of magnetism. It entails appreciation of the fine-tuning of metallization which is achieved by the choice of counter-ions, and the part which the latter play in stabilizing cuprate valencies and crystal structures. By emphasizing the fundamental role of the periodic table in this physics I wish to see the `chemical' side to what is involved given greater consideration. It is clear whether in dealing with the HTSC materials by transport measurements or nmr or optical properties or ARPES or neutron scattering that they are `marginal metals'. A formal Fermi liquid approach has to be pushed to the limit - and beyond. It is a pity that more high-level cluster calculations, drawing on quantum chemistry, have not been employed to attack the problem. Nowhere is that effort more greatly missed than in the need to corroborate formally the shell-closure negative-U understanding of HTSC adhered to throughout the course of the present work. I have made double-loading shell-closure fluctuations, with their termination of pdσ/σ* bonding/antibonding interaction, and the concomitant collapse in the elevated position of the crucial dx2-y2 state, the lynch pin to all HTSC events. As the paper describes, the layered square-planar crystal structure adds its own essential contribution toward the configuring of a highly restrictive set of conditions. I have attempted to incorporate a broad enough sweep of experimental results to demonstrate just how restrictive those conditions are. These any satisfactory interpretation of HTSC has necessarily and readily to embrace.