Thickness-Dependent Interface Polarity in Infinite-Layer Nickelate Superlattices

Abstract

The interface polarity plays a vital role in the physical properties of oxide heterointerfaces because it can cause specific modifications of the electronic and atomic structure. Reconstruction due to the strong polarity of the NdNiO2/SrTiO3 interface in recently discovered superconducting nickelate films may play an important role, as no superconductivity has been observed in the bulk. By employing four-dimensional scanning transmission electron microscopy and electron energy-loss spectroscopy, we studied effects of oxygen distribution, polyhedral distortion, elemental intermixing, and dimensionality in NdNiO2/SrTiO3 superlattices grown on SrTiO3 (001) substrates. Oxygen distribution maps show a gradual variation of the oxygen content in the nickelate layer. Remarkably, we demonstrate thickness-dependent interface reconstruction due to a polar discontinuity. An average cation displacement of ∼0.025 nm at interfaces in 8NdNiO2/4SrTiO3 superlattices is twice larger than that in 4NdNiO2/2SrTiO3 superlattices. Our results provide insights into the understanding of reconstructions at NdNiO2/SrTiO3 polar interfaces.

Direct observation of strong surface reconstruction in partially reduced nickelate films

“…There is a ∼ 0.2 Å displacement of Ni atoms in the NiO 2 layer at the NdNiO 2 /SrTiO 3 interface 29 . This is similar to our previous experimental results measured by STEM at the interfaces in the 8NdNiO 2 /4SrTiO 3 superlattice, although residual oxygen is present at the interface 14 . In the case of the partially reduced sample, the electrostatic effect is particularly pronounced in the unit cells near the surface, where the polarity is significantly enhanced.…”

“…Polarity at a perfect NdNiO 2 /SrTiO 3 interface induces 2DEG formation due to strong occupation of interfacial Ti according to theoretical calculations 27 , which is similar to the LaAlO 3 /SrTiO 3 superconducting system 30 . However, the experimental results show that the residual oxygen, elemental mixing, atomic reconstruction, and Ni valence modulations can compensate for the polar discontinuity induced by the interface polarity at the NdNiO 2 /SrTiO 3 interface 14 . In terms of surface polarity, theoretical calculations predict a high Ni e g orbital polarization of 35% due to the elimination of out-of-plane Ni-O bonding associated with polar distortion 27 .…”

“…Polarity at interfaces and surfaces in complex oxide thin films plays a critical role in their physical and chemical properties such as ferroelectricity 1 – 7 , superconductivity 8 , magnetism 9 , and catalysis 10 . By controlling the crystal plane orientation and termination during film growth, polar surfaces, and interfaces can be obtained, where structural distortions form due to the interaction of short-range covalency and long-range electrostatic effects 11 – 14 . At surfaces, the abruptly reduced coordination significantly alters the lattice and electronic structures, potentially affecting the overall physical and chemical properties of the thin oxide material.…”

Real‐Space Observation of Potential Reconstruction at Metallic/Insulating Oxide Interface

“…Indeed, the differences of the mean-inner potential gives rise to a strong COM at the interface between AlN and GaN, but it is also shown that dynamical scattering can redistributed the intensity in such a way that the COM sign can even be reversed. [22] Yang et al [24] reported the electrostatic characteristic of a grain boundary in a SrTiO 3 bi-crystal. They used two sets of collection semi-angles, 20 mrad for atomical resolved electric field investigation and a 6 mrad for the large-scale electric field measurement as encountered next to the grain boundary.…”

Magnetism and Superconductivity in the t–J Model of La3Ni2O7 Under Multiband Gutzwiller Approximation

“…Considering that Ni + is isoelectronic with Cu 2+ , the search for unconventional superconductivity in nickelates has become one major focus in condensed matter community. [1][2][3][4][5][6][7][8][9][10][11] Although superconductivity has been discovered in several nickelates, such as hole-doped infinite-layer nickelates Nd1−𝑥Sr𝑥NiO2, the maximum of transition temperature 𝑇c is just over 30 K and superconductivity only appears in thin films. Recently, evidences of superconductivity were observed in single crystal La3Ni2O7 under pressure, and the transition temperature 𝑇c can reach 80 K. [12] This experimental discovery immediately attracts great attention, and provides a new platform for researches of high-𝑇c superconductivity.…”