In oxide materials, atomic bonds are usually ionic, which means that electrons sit either on the metal ions or on the oxygens. However, in some compounds such as nickel or copper perovskites covalent bonding is favored: electrons are shared between the metal and the oxygens. Often the situation is in fact intermediate, and the fine tuning between ionic and covalent bonding plays a key role in the emergence of high-Tc superconductivity in cuprates. In the ongoing quest for novel high-Tc superconductors built from oxide multilayers, characterizing and tuning the level of covalence in the different oxide layers is therefore as important as controlling the overall number of electrons leaking from one layer to the other.
In our study (supported by ERC grant #615759 “MINT”) just out in Nature Physics , we show that at the interface between a nickel perovskite and an ionic material (here a titanium perovskite oxide, GdTiO3), electrons are transferred into the nickelate in an amount regulated by the local level of covalence. With weakly covalent NdNiO3, the total transferred charge is the largest while with LaNiO3, the strong covalent character thwarts electron transfer: added electrons tend to disturb the covalence level, which costs energy, and the material thus tries to keep covalence unchanged. Interestingly, while our interfaces do not show superconductivity they develop a novel ferromagnetic-like state, which does not exist in bulk nickelates, and whose properties are tuned by the covalence level. Further work is needed to clarify the magnetic coupling mechanism and achieve conductivity at the interface, but our finding identifies covalence as a new knob to guide research on correlated oxide heterostructures towards its holy grail.
Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces
M. N. Grisolia et al, Nature Phys. 12, 484 (2016)