Dimensionality-controlled interface properties in infinite-layer nickelate superlattices

Abstract text (incl. figure legends and references)

The interface polarity plays a vital role in physical properties of oxide heterointerfaces since it can enforce the reconstruction of the electronic and atomic structure. A strong polar interface possibly exists in recently discovered nickelate superconductors and the superconductivity is absent in its bulk material, attracting increasing attentions on interface effects [1-4]. This motivated our investigation of the electronic and atomic structure at atomic scale at the interfaces of infinite-layer nickelates. Recently, we reported that advancedfour-dimensional scanning transmission electron microscopy (4D-STEM) technique can provide clear phase-contrast imaging of the oxygen sublattice [5].

Here, by employing 4D-STEM and atomic-resolution electron energy-loss spectroscopy (EELS), we systematically studied the oxygen distribution and atomic structure at the interface of infinite-layer NdNiO2 (NNO) and SrTiO3 (STO). Figure 1a shows an HAADF image of a 4NNO/2STO superlattice, where the STO and NNO layers can be easily identified due to their distinctly different mean inner potentials. It has been well accepted that the infinite layer of NNO can be synthesized by the topotactic reduction method. In order to understand the deintercalation process of oxygen ions and the variation of oxygen concentration in NNO layers, we acquired all oxygen positions by Gaussian fitting and then extracted the oxygen intensity map as displayed in Figure 1d. The corresponding intensity line profile of apical oxygen is presented in Figure 1e, identifying the reduction procedure about the local symmetry variations from octahedral to square planar.

By calculating the white-line ratio, we obtained the valence variations of Ni and Ti ions across the interfaces as shown in Figure 2. The valence of Ti intermixed on Ni sites for the 4NNO/2STO superlattice tends to be 3+ while it almost maintains 4+ for the 8NNO/4STO superlattice (Figure 2c and 2f). The Ni valence varies between 1+ and 2+ since it depends on the extent of deintercalation of the apical oxygen in the NNO inner layer. An apparent gradient variation of Ni valence is visible in the NNO layer for the 8NNO/4STO superlattice. Furthermore, we will discuss the effects of elemental intermixing, cation distortion, and oxygen distribution as well as occupancy on the observed valence variations near the interfaces.

References

[1] D. F. Li, et al. Nature 2019, 572 (7771), 624-627.

[2] B. H. Goodge, et al. arXiv:2201.03613, 2022.

[3] B. Geisler et al. Phys. Rev. B 2020, 102 (2), 020502.

[4] R. He et al. arXiv:2006.00656, 2020.

[5] C. Yang et al. Nano Lett. 2021, 21 (21), 9138-9145.

[6] This project has received funding from the European Union"s Horizon 2020 research and innovation programme under Grant Agreement 823717 – ESTEEM3.

Figure 1 Atomic structure of a 4NNO/2STO superlattice. (a) An overview HAADF image of a 4NNO/2STO superlattice. (b) An enlarged HAADF image and (c) the iCoM image obtained in the region marked by a yellow dashed box in (a). (d) The oxygen intensity map calculated from the oxygen columns in (c). (e) The extracted intensity line profile of apical oxygen columns from the white dashed line marked in (d).

Figure 2 Valence variations of Ni and Ti ions in 4NNO/2STO and 8NNO/4STO superlattices. HAADF images of (a) the 4NNO/2STO superlattice and (d) the 8NNO/4STO superlattice. (b) Ni L3/L2 intensity ratio and (c) Ti L3/L2 intensity ratio of the 4NNO/2STO superlattice. (e) Ni L3/L2 intensity ratio and (f) Ti L3/L2 intensity ratio of the 8NNO/4STO superlattice. The references of Ni3+ and Ni1+ valences are from our NdNiO3 and NdNiO2 samples, respectively. The Ni2+ reference is from a NiO film in the Gatan EELS database. The Ti4+ reference is from our SrTiO3 sample.