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Zinc and strontium doping in high-temperature superconductors: NMR and DFT study

We study two well-known doping scenarios of high-Tc superconductors. Zinc (Zn) doping in the copper-oxide plane efficiently lowers the superconducting (SC) Tc, without introducing carriers into the plane, as indicated by ARPES. Conversely, doping La2CuO4 with strontium (Sr) out of the plane does introduce carriers into the plane, eventually bringing about a SC transition. Experimentally, 4% of in-plane Cu in an optimally doped YBCO powder was replaced by the 67Zn isotope, lowering Tc to 57 K with a sharp Meissner transition. The pure nuclear quadrupole resonance (NQR) spectrum of 67Zn shows a greatly increased local charge compared to bulk Cu sites, as well as a significant asymmetry of the Zn site, in line with STM observations. Spin-lattice relaxation measured directly on the Zn nuclei indicates they are in an insulating environment, in agreement with ARPES data. An unusual complex relaxation is observed below 200 K, tentatively ascribed to a two-stage process. In the first stage, the Zn spin is quickly relaxed to local ZnO4 “molecular” states, which then relax slowly to nearest-neighbor (nn) Cu spins in the second stage. The first stage is substantiated by cluster and DFT calculations, which give the correct order of magnitude for the ZnO4 spectrum splitting. Weak-field NMR measurements on Cu yield further information about the second stage, with both nn and next-nn Cu signals resolvable. The hole charge on the nn Cu is found to be somewhat different than in the bulk, corroborating our second-stage scenario, consistently with observation of localized spins on nn Cu in previous NMR experiments. We conclude that Zn destroys the SC state by a Coulombic “domino effect” which pushes a significant number of surrounding sites back towards the parent-compound configuration, as suggested originally by Mazumdar. The alternative scenario, that the strong oscillations observed by STM around the Zn site come from the superconducting metal scattering directly on Zn, is excluded by our data. Theoretically, we investigate the effects of Sr in La2CuO4 by a bulk DFT calculation of the charge distribution. We find that the charge arrives in the plane by a similar “domino effect, ” a Coulomb-induced redistribution of orbital charge, such that the CuO2 plane receives part of the charge of each introduced strontium from out-of-plane atoms, while the other part is self-doped by a charge transfer from insulating to metallic orbitals of the atoms in the plane. The part which the plane receives from outside is similarly due to a cascade of inter-orbital charge transfers, amounting to dielectric screening by the ionic orbitals, in which lanthanum barely participates. The self-doping introduces axial asymmetry, around the in-plane Cu-Cu (bridging) axis, in the insulating part of the charge distribution on the planar O sites, dominantly consisting of the two O p-orbitals perpendicular to the bridging axis. The in-plane (metallic) charge distribution on the O sites, dominated by the third O p-orbital, remains axially symmetric along the bridging axis. The orbital charge transfers stop at the nearest CuO2 planes, beyond which there is no effect of introduced strontium, consistently with metallic screening by the planes.

high-temperature superconductors; doping; NQR; DFT

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