Resolving the polar interface of infinite-layer nickelate thin films

Thickness-Dependent Interface Polarity in Infinite-Layer Nickelate Superlattices

“…Owing to the residual oxygen near the interfaces, no compressive strain may exist at the STO/NNO interfaces, while the NNO inner layers are likely under a small compressive strain. Another point is that a distinct extension of the out-of-plane lattice spacing to ∼0.4 nm occurs between the final SrO layer and the first NdO layer at partial interfaces, which is in agreement with a reported NNO single film grown on a STO substrate . This is associated with the proportion of Ni/Ti intermixing according to the DFT calculations …”

“…Another point is that a distinct extension of the out-of-plane lattice spacing to ∼0.4 nm occurs between the final SrO layer and the first NdO layer at partial interfaces, which is in agreement with a reported NNO single film grown on a STO substrate . This is associated with the proportion of Ni/Ti intermixing according to the DFT calculations …”

“…He et al verified by DFT calculations that an atomic reconstruction is more energetically favorable than an electronic reconstruction and predicted an interface configuration with residual apical oxygen atoms as well as Ni displacements. A single intermediate Nd­(Ti,Ni)­O 3 layer was observed by atomic-resolution EELS in a NdNiO 2 single film grown on a (001)-oriented SrTiO 3 single crystal, indicating that atomic reconstruction at the polar interface mitigates the polar instability . In addition, superlattice structures were proposed to introduce hole doping in their infinite-layer stacks through interface engineering, providing the possibility of achieving superconductivity in nickelates, without disorder introduced by alkaline-earth doping.…”

“…The residual apical oxygen atoms are visible in the first NdO x layer at the interfaces, which is beneficial to alleviating the strong polar discontinuity at NNO/STO interfaces by providing an extra electron to compensate for the built-in electrostatic field according to the theoretical calculations. , The observed residual apical oxygen columns toward the center of the NdO x layer can also contribute to suppressing the polar instability. Additionally, we note that there is an asymmetrical distribution of the residual oxygen at the bottom and top interfaces, which is associated with the extent of elemental intermixing because Ti intermixing into Ni sites increases the bonding energy and hinders the removal of apical oxygen . This is in agreement with the above EELS mapping results, which shows that the proportion of cation intermixing is not homogeneous.…”

“…That might enhance the electronic transport of the SL_8_4 sample . Moreover, the excess charges can be accommodated by the Ni 3d orbital due to Jahn–Teller distortions at interfaces, which is the possible reason the Ti valence tends to be 4+ . In addition, we find that atomic steps were formed at the interfaces in both SL_8_4 and SL_4_2 samples as shown in Figure S7, accompanying a step distribution of the apical oxygen at the interfaces as displayed in Figure S8.…”