Similarities and differences between nickelate and cuprate films grown on a SrTiO₃ substrate

The recent discovery of superconductivity in Sr-doped NdNiO₂ films grown on STO started a novel field within unconventional superconductivity. Our results revealed a strong lattice reconstruction in the case of NNO/STO and CCO/STO, resulting in a polar film. To avoid the polar catastrophe, the NiO₂/ CuO₂ surface to the vacuum reconstructs as well. Furthermore, we also observed a 2D electron gas at the interface between NNO/CCO and STO, caused by the polar discontinuity. For NNO/STO the two-dimensional electron gas extends over several layers, while for CCO/STO this electronic rearrangement is very localized at the CCO/STO interface. The electronic reconstruction found at the interface involves a strong occupation of the Ti 3d_xy state. In both cases, there is a significant electronic charge transfer from the surface Ni/Cu layers to the Ti interface layer. By introducing magnetism and electronic correlation, we observed that the d_3z2 orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems.

[1] Y. Zhang, L. F. Lin, W.-J. Hu, A. Moreo, S. Dong and E. Dagotto (to appear in PRB).