Atomic structure of GdBa2Cu3O7-δ superconductor thin films on NdGaO3(0 0 1)Probed by X-ray standing waves and photoelectron spectroscopy

We characterized the structure of GdBa2Cu3O7-δthin films grown on NdGaO3(0 0 1) substrates using X-ray standing waves combined with photoelectron spectroscopy. The O 1s and Cu 2p core-level spectra display components that exhibit distinct X-ray standing wave modulations, which we identify as arising from the well-ordered (intrinsic) parts of the oxide films, but also contain nearly unmodulated components arising from reacted, degraded phases (non-intrinsic). The large mean-free paths of high kinetic-energy photoelectrons permits, in addition, the detection of the Nd and Ga atoms near the buried interface below a 3.5-nm film. Our measurements reveal an essentially bulk-terminated NdGaO3 surface at the interface. By considering a pseudomorphically strained, orthorhombic Y123 structure for the 3.5-nm film, we find that our X-ray standing wave data are consistent with an interface composed of CuO-NdO planes, where the Cu-Nd distances are determined to be 0.18 nm. The analysis rules out the possibility for the film to start its stacking with a BaO plane. The X-ray standing wave measurement for the GdBa2Cu3O7-δ valence band yields results nearly identical to the intrinsic part of the Cu 2p core level and therefore confirms the major contribution of Cu 3d to the valence band in high-temperature cuprates. For thicker films, the standing wave analysis on a 17.5-nm sample indicates that the film is partially relaxed, in good agreement with the previous X-ray diffraction results. Repeating the measurements at 60 K suggests that the in-plane lattice mismatch between the film and substrate increases with decreasing temperature.