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 SrTiO3 started a novel field within unconventional superconductivity. To understand the similarities and differences between nickelate and cuprate layers on the same SrTiO3 substrate, here based on the density functional theory we have systematically investigated the structural, electronic, and magnetic properties of NdNiO2/SrTiO3 and CaCuO2/SrTiO3 systems. Our results revealed a strong lattice reconstruction in the case of NdNiO2/SrTiO3, resulting in a polar film, with the surface and interfacial NiO2 layers presenting opposite displacements. To avoid the polar catastrophe, the NiO2 surface to the vacuum reconstructs as well. However, for CaCuO2/SrTiO3, the distortions of those same two CuO2 layers were in the same direction. In addition, we found this distortion to be approximately independent of the studied range of film thickness for the nickelate films. Furthermore, we also observed a two-dimensional electron gas at the interface between NdNiO2 and SrTiO3, caused by the polar discontinuity, in agreement with recent literature. For NdNiO2/SrTiO3 the two-dimensional electron gas extends over several layers, while for CaCuO2/SrTiO3 this electronic rearrangement is very localized at the interface between CaCuO2 and SrTiO3. 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 or Cu layers to the Ti interface layer. The interfacial Ni and Cu layers are hole and electron doped, respectively. By introducing magnetism and electronic correlation, we observed that the d(3z2-r2) 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. Furthermore, we also observed a strong magnetic reconstruction at the NdNiO2 surface to vacuum layer where magnetism is basically suppressed.