Theory of high-T_Csuperconductivity: transition temperature

Abstract

The superconducting transition temperatures of high-T C compounds based on copper, iron, ruthenium and certain organic molecules are discovered to be dependent on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. 1 Optimal transition temperature, denoted as T C0 , is given by the universal expression k B T C0 = e 2  / ℓζ; ℓ is the spacing between interacting charges within the layers, ζ is the distance between interacting layers and  is a universal constant, equal to about twice the reduced electron Compton wavelength (suggesting that Compton scattering plays a role in pairing). Non-optimum compounds in which sample degradation is evident typically exhibit T C < T C0 . For the 31+ optimum compounds tested, the theoretical and experimental T C0 agree statistically to within  1.4 K. The elemental high T C building block comprises two adjacent and spatially separated charge layers; the factor e 2 / ζ arises from Coulomb forces between them. The theoretical charge structure representing a room-temperature superconductor is also presented.

Superconducting interaction charge in thallium-based high-TC cuprates: Roles of cation oxidation state and electronegativity

“…Superconductivity in these systems breaks down as intralayer Coulomb repulsion and charge scattering become increasingly important with decreasing ℓ. Equation (1) has been validated and β confirmed through extensive study of experimental results for high-T C compounds based on cuprate [7,9], ruthenate, rutheno-cuprate [7], iron-pnictide [7], iron-chalcogenide [8], organic [7], and intercalated group-5-metal nitride-chloride [27,28] structures, with T C0 ranging from 6.3 K to 145 K.…”

“…By adopting this model that takes the two types of charge reservoirs into account, it has been shown that the optimal transition temperature T C0 , corresponding to the highest T C for a given compound structure, is given by the algebraic equation [7], where ζ is the interaction distance (measured along the transverse or hard axis), A is the basal-plane area per formula unit, σ/A is the areal charge density per type I layer per formula unit for participating charges and β (= 0.1075 ± 0.0003 eV Å 2 ) is a universal constant, and where e -2 β is approximately twice the reduced electron Compton wavelength. Superconductivity in these systems breaks down as intralayer Coulomb repulsion and charge scattering become increasingly important with decreasing ℓ.…”

“…Rule (2a) originally gave the γ-factor for inner-layer cations in +3 oxidation states substituting for Cu +2 [7]. However, single-TlO-layer cuprates may contain admixtures of Tl +1 and Tl…”

“…As detailed in prior work, it is essential to model high-T C compounds as layered structures comprising two types of charge reservoirs [7][8][9]. The non-CuO 2 structure, which commonly provides charge doping from metal cations associated with oxygen anions, is designated as type I and the structure containing CuO 2 layers is designated as type II.…”

Concerning the superconducting gap symmetry in YBa2Cu3O7 − δ, YBa2Cu4O8, and La2 −xSrxCuO4determined from muon spin rotation in mixed states of crystals and powders

“…12 In applied fields other studies have shown sub-gap peaks at ~5.5 meV for tunnelling into the vortex core region [71,72]; these were interpreted as evidence of bound quasiparticles in a discrete energy state that theoretically is allowed only for a nodeless gap, noting that d-wave pairing symmetry would yield a continuum in quasiparticle states [23]. Coexisting hole and electron carriers found in Hall effect studies [73] have recently been identified with the two spatially separated electronic bands involved in high-T C pairing; manifestations of superconductivity is considered to have dependence on relative band sensitivities in experiment as discussed in [74].…”