Copper oxide superconductors: A distinguishable thermodynamic state

Abstract

The temperature dependence of the resistivity and Seebeck coefficient for the two p-type systems La2CuO4+δ, 0≤δ≤0.09, and La2−xSrxCuO4, 0≤x≤0.3, are reported and interpreted in the context of overall phase diagrams. Above room temperature, the La2CuO4+δ system tends to lose oxygen at 1 atm O2; superconductive samples exhibit a first-order loss of oxygen above 500 K to revert to the antiferromagnetic phase. Below a transition temperature Ts≊300 K, compositions with 0<δ<0.05 undergo phase segregation to an antiferromagnetic and a superconductive phase; the superconductive phase appears to undergo a further dynamic segregation into hole-rich and hole-poor domains in the interval Tc<T<Tρ≊100 K. In the system La2−xSrxCuO4, the holes move diffusively, with a ΔHm=0, above Tl≊300 K for the compositions 0<x≤0.21; the system undergoes a transition from a p-type two-dimensional conductor to an n-type three-dimensional conductor in the interval 0.22≤x<0.35. Compositions with 0<x≤0.12 are metastable in the range Tc<T<Tl where the holes continue to move diffusively, but charge fluctuations appear in the range Tc<T<Tρ≤150 K. Compositions with 0.15≤x≤0.2 appear to undergo a transition from a polaronic gas to a polaronic (Luttinger) liquid on cooling through Tl; superconductive pairs are condensed from the homogeneous polaronic-liquid normal state at Tc. The origin of the unusual electron-lattice interactions in the normal state of the superconductive compositions is attributed to a coexistence of ionic and covalent bonding at a transition from more ionic to covalent Cu:3dx2-y2-O:2pσ bonding in the CuO2 sheets; through the transition the orbital hybridization and Hubbard U parameter vary sensitively with both the Cu-O bond length and the formal local oxidation state at a Cu atom.

Source-reported materials — not catalogue approval

Similar papers