Direct observation of infinite NiO₂ planes in LaNiO₂ films

Abstract

Epitaxial thin films of LaNiO2, which is an oxygen-deficient perovskite with “infinite layers” of Ni1+O2, were prepared by a low-temperature reduction of LaNiO3 single-crystal films on NdGaO3 substrates. We report the high-angle annular dark-field and bright-field scanning transmission electron microscopy observations of infinite NiO2 planes of c-axis-oriented LaNiO2 epitaxial thin films with a layer stacking sequence of NiO2/La/NiO2. Resistivity measurements on the films show T 2 dependence between 400 and 150 K and a negative Hall coefficient.

Electronic structure of the parent compound of superconducting infinite-layer nickelates

“…Infinite-layer NdNiO2 became the first such nickelate superconductor following the recent discovery of superconductivity in Srdoped samples [11]. The undoped parent compound, produced by removing the apical oxygen atoms from the perovskite nickelate NdNiO3 using a metal hydride-based soft chemistry reduction process [10,[18][19][20], appears to be a close sibling of the cuprates-it is isostructural to the infinitelayer cuprates with monovalent Ni 1+ cations and possesses the same 3d 9 electron count as that of Cu 2+ cations in undoped cuprates. Yet, as we will reveal, the electronic structure of the undoped RNiO2 (R = La and Nd) remains distinct from the Mott, or charge-transfer, compounds of undoped cuprates, and even other nickelates.As a reference, we first discuss the electronic structure of canonical nickelates, NiO and LaNiO3.…”

Two-band model for magnetism and superconductivity in nickelates

“…The lattice constant along the z-axis in NbNiO 2 is only about 3.4Å, much smaller than that in cuprates. This leads to the dispersion along the zaxis from the Nd 5d z 2 -orbital [1,36]. The Nd-originated electron pockets are found by the LDA+U calculations in previous works [23,25,26] and also in recent works [3][4][5][6][7][8][9][10].…”

Aspects of the synthesis of thin film superconducting infinite-layer nickelates

“…This corresponds to approximately 3 unit cells of unconverted Nd0.8Sr0.2NiO3 (001), which can be attributed to the interfacial layers as previously observed. 22,23 In comparison to the partial decomposition of the uncapped film upon reduction (Figure 6(a)), the crystallinity of the film with SrTiO3 capping layer shows significant improvement, with essentially the entire film transformed to the infinite-layer phase. The optimal reduction condition varies as a function of film crystallinity and the thickness of the capping layer, which appears to act as a diffusion barrier to oxygen deintercalation.…”

“…22 Depending on the reduction conditions, decomposition of the infinite-layer phase at the upper region of the film was also observed. 23 These results indicate the need of careful optimization of the reduction conditions, and, perhaps, adjustments in the structural design of the film to promote single-phase stabilization.…”