Effects of Sr and Zn Doping on the Metallicity and Superconductivity of LSCO and YBCO
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Sr doping in La 2CuO 4 is investigated by a Kohn-Sham DFT + U calculation. The Sr charge remains highly localized, confirming earlier calculations. The bulk of the charge in the nodal metal is delocalized in situ in the CuO 2 plane. Standard Kohn-Sham DFT+U reproduces the metal-insulator transition and growth of Fermi-surface arcs, as observed in angle-resolved photoemission spectroscopy (ARPES), when not restricted to the paramagnetic solution space. Substituting planar Cu by isotopically pure 67Zn in yttrium barium copper oxide (YBCO), we measure the nuclear quadrupole resonance (NQR) signal on the Zn site itself for the first time. Zn creates large insulating islands, extending at least to the nearest-neighbor Cu site, thus confirming that doping induces an orbital delocalization transition in the plane, which is locally reversed by Zn substitution. Calculations show that the D4 symmetry-breaking observed in NQR is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the low temperature tetragonal (LTT) transition in lanthanum strontium copper oxide (LSCO).
KeywordsHigh-temperature superconductivity Doping DFT NQR
Conversations with S. Barišić, A. Dulčić, S. Mazumdar, and D. Pavuna are gratefully acknowledged, as well as the help of T. Cvitanić with sample preparation. DKS thanks the organizers for an invitation and a stimulating time at the ECRYS-2011 Workshop on Electronic Crystals.
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