Magnetic and ligand field properties of copper at the interfaces of (CaCuO2)n/(SrTiO3)nsuperlattices

Using high-resolution resonant inelastic x-ray scattering (RIXS) excited at the Cu ${L}_{3}$ edge we have investigated the ligand field ($dd$) and magnetic excitations in (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{n}$ superlattices ($n=2,3$) and compared them to those of a 14-nm-thick CaCuO${}_{2}$ film. The $dd$ excitation spectrum reveals a pyramidal coordination of Cu ions at the CaCuO${}_{2}$/SrTiO${}_{3}$ interfaces. In the three samples, spin excitations are in the form of dispersing magnons, with similar spectral intensity but reduced dynamics in SLs with respect to pure CaCuO${}_{2}$. By fitting the dispersions within linear spin wave theory we have obtained the leading term of the in-plane superexchange parameters: $J=127$ meV, 138 meV, and 157 meV for $n=2$, 3 SLs, and CaCuO${}_{2}$, respectively. These results demonstrate that the antiferromagnetic order is preserved in these SLs down to very small cuprate layer thickness and despite the chemical and structural alterations at the interface. This finding opens the way to the production of artificial Cu-based high-temperature superconductors where the charge reservoir layer is constituted by the interface itself.