Microscopic study of local structure and charge distribution in metallic La2CuO4+δ

We employ NMR and NQR spectroscopy as probes of local structure and charge environments in metallic La2CuO4+δ (Tc = 38 K). We discuss the effect of annealing the sample at various temperatures Ta (Tc < Ta < 300 K) on the superconducting Tc. The dependence of Tc on Ta indicates that annealing allows the development of structural order which is important for Tc. The 139La quadrupole frequency νQ in the in metallic phase is smaller than in the undoped material. This is unexpected and may indicate a smaller charge on the apex oxygen in the doped material and thus a different distribution of charge between the La-O layer and the planes. We find that the presence of doped holes induces a distribution of displacements of the apex oxygen off of the vertical La-Cu bond axis. The distribution in displacements extends from zero to the value observed in lightly doped (antiferromagnetic) La2CuO4+δ. This directly demonstrates considerable inhomogeneity in the crystal structure of the La-O layer. Copper NQR spectroscopy shows that there are two distinct copper sites in the CuO2 planes and thus that either the structure or the charge distribution in the planes is inhomogeneous as well. These inhomogeneities are the intrinsic response of the crystal to doped holes; they are not the result of distortions of the lattice due to the presence of interstitial oxygen atoms. (orig.)