Mixed valence model for superconductivity

Abstract

Superconductivity often occurs in crystals with one active electron per site with charge density wave (CDW) or spin density wave (SDW) as 'mother state'. It is proposed that superconductivity is possible when the differences in equilibrium geometry and energy between the diabatic CDW and SDW states are so small that there is interaction between them via the zero point vibrations. Electron pairing in real space is directly related to oxidation states being different in two units. Three valence states in succession have to be stable (ground state or low-energy excited states) and we therefore refer to this mixed valence model as the MV-3 model. Examples are chosen from bismuthates, cuprates, and fullerides. The theory is simple and straightforward and offers solutions to other important problems as well, for example for A 3 C 60 (A = K, Rb), that (1) there are no magnetic moments in crystal phase, and (2) that these systems are superconducting metals while A 4 C 60 are insulators.

Conductivity in Cuprates Arises from Two Different Sources: One-Electron Exchange and Disproportionation

“…The remarkable fact that cuprates, oxidatively or reductively doped, are important superconductors is after all understandable. It may be traced back to the occurrence of three different oxidation states for copper, Cu(I), Cu(II), and Cu(III), with approximately the same free energy [55,56].…”

“…10 is going to exist at all as a basis for the explanation. A small U ad requires that three oxidation states are possible with similar geometry and with a low free energy for disproportionation [55,56].…”

Effect of Pressure on Superconducting Properties

“…Negative U is a central point in the theory of superconductivity of the present author [21]. Excitation over a positive U is connected to insulator-metal transition [7].…”

“…For U ad ≈ 0, the electron 6s 2 pair in Tl + may move to a Tl 3+ site. This hypothesis [7,21] hardly allows any other explanation than the one of Fig. 1b, c. It is in fact the energy to obtain interaction between the charge separated and spin-coupled states.…”

“…The Bloch model, on the other hand, is a model for single configurations at the Hartree-Fock or DFT level, but leaves out correlation effects of multi-configuration type. "Mott-Hubbard physics" [7] is known in chemistry as disproportionation [20,21]. This is a quite common phenomenon where an electron in an atom is promoted to the neighbouring atom.…”