Chiral plaquette polaron theory of cuprate superconductivity

Abstract

Ab initio density functional calculations on explicitly doped La2−xSrxCuO4 find that doping creates localized holes in out-of-plane orbitals. A model for cuprate superconductivity is developed based on the assumption that doping leads to the formation of holes on a four-site Cu plaquette composed of the out-of-plane A1 orbitals apical O pz, planar Cu d3z2−r2, and planar O psigma. This is in contrast to the assumption of hole doping into planar Cu dx^2−y^2 and O psigma orbitals as in the t-J model. Allowing these holes to interact with the d^9 spin background leads to chiral polarons with either a clockwise or anticlockwise charge current. When the polaron plaquettes percolate through the crystal at x[approximate]0.05 for La2−xSrxCuO4, a Cu dx^2−y^2 and planar O psigma band is formed. The computed percolation doping of x[approximate]0.05 equals the observed transition to the "metallic" and superconducting phase for La2−xSrxCuO4. Spin exchange Coulomb repulsion with chiral polarons leads to d-wave superconducting pairing. The equivalent of the Debye energy in phonon superconductivity is the maximum energy separation between a chiral polaron and its time-reversed partner. This energy separation is on the order of the antiferromagnetic spin coupling energy, Jdd~0.1 eV, suggesting a higher critical temperature. An additive skew-scattering contribution to the Hall effect is induced by chiral polarons and leads to a temperature dependent Hall effect that fits the measured values for La2−xSrxCuO4. The integrated imaginary susceptibility, observed by neutron spin scattering, satisfies omega/T scaling due to chirality and spin-flip scattering of polarons along with a uniform distribution of polaron energy splittings. The derived functional form is compatible with experiments. The static spin structure factor for chiral spin coupling of the polarons to the undoped antiferromagnetic Cu d^9 spins is computed for classical spins on large two-dimensional lattices and is found to be incommensurate with a separation distance from (pi/a,pi/a) given by deltaQ[approximate](2pi/a)x, where x is the doping. When the perturbed x^2−y^2 band energy in mean field is included, incommensurability along the Cu-O bond direction is favored. A resistivity ~T^(µ+1) arises when the polaron energy separation density is of the form ~Deltaµ due to Coulomb scattering of the x^2−y^2 band with polarons. A uniform density leads to linear resistivity. The coupling of the x^2−y^2 band to the undoped Cu d^9 spins leads to the angle-resolved photoemission pseudogap and its qualitative doping and temperature dependence. The chiral plaquette polaron leads to an explanation of the evolution of the bilayer splitting in Bi-2212.

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