[Paper Review] Electronic structure and magnetism in infinite-layer nickelates RNiO$_2$ (R= La-Lu)

Jesse Kapeghian, Antía S. Botana|arXiv (Cornell University)|Jul 29, 2020

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This first-principles study reveals that the in-plane Ni-O distance is the dominant control parameter for electronic and magnetic properties in infinite-layer nickelates RNiO₂ (R = La-Lu), with all compounds exhibiting a metallic antiferromagnetic ground state. Unlike cuprates, this state features a flat Ni-dz² band pinned at the Fermi level, alongside dₓ²⁻y² character, indicating a multiorbital, high-spin Ni-d⁸⁺δ configuration that persists across the lanthanide series despite structural changes.

ABSTRACT

Using first-principles calculations, we analyze the evolution of the electronic structure and magnetic properties of infinite-layer nickelates RNiO$_2$ (R= rare-earth) as R changes across the lanthanide series from La to Lu. By correlating these changes with in-plane and out-of-plane lattice parameter reductions, we conclude that the in-plane Ni-O distance is the relevant control parameter in infinite-layer nickelates. An antiferromagnetic ground state is obtained for all RNiO$_2$ (R=La-Lu). This antiferromagnetic state remains metallic across the lanthanide series and is defined by a multiorbital picture with low-energy relevance of a flat Ni-d$_{z^2}$ band pinned at the Fermi level, in contrast to cuprates. Other non-cuprate-like properties such as the involvement of R-$d$ bands at the Fermi level, a large charge transfer energy, and a suppressed superexchange are robust for all RNiO$_2$ materials.

Motivation & Objective

• To understand how electronic structure and magnetism evolve in RNiO₂ (R = La-Lu) across the lanthanide series.
• To identify the key structural parameter governing electronic and magnetic responses in infinite-layer nickelates.
• To determine whether the non-cuprate-like behavior observed in La/NdNiO₂ persists across the full R series.
• To assess the stability of the antiferromagnetic ground state and the role of Ni-dz² bands in RNiO₂ materials.

Proposed method

• First-principles density functional theory (DFT) calculations using the GGA and GGA+U functionals were performed for RNiO₂ across R = La-Lu.
• Structural parameters were optimized using PAW pseudopotentials, with R-4f electrons treated as core states.
• Electronic structure and magnetic properties were analyzed by varying in-plane and out-of-plane lattice constants in LaNiO₂ to isolate their effects.
• Band structure, density of states (DOS), and magnetic moments were computed to identify active orbitals and electronic correlations.
• The role of U in the GGA+U approach was systematically explored to assess the emergence of high-spin states and flat bands.
• Comparative analysis of lattice parameter changes and their impact on band dispersion and Fermi level features was conducted.

Experimental results

Research questions

• RQ1 How does the electronic structure of RNiO₂ evolve as R changes from La to Lu?
• RQ2 Which lattice parameter— in-plane or out-of-plane—most strongly controls the electronic and magnetic properties in RNiO₂?
• RQ3 Does the antiferromagnetic ground state in RNiO₂ remain metallic and multiorbital, as in La/NdNiO₂, across the lanthanide series?
• RQ4 What is the role of the Ni-dz² orbital in the magnetic and electronic response of RNiO₂, and is it robust across different R ions?
• RQ5 Can the flat d_z² band pinned at the Fermi level, previously reported in NdNiO₂, be generalized to other RNiO₂ compounds?

Key findings

• The in-plane Ni-O distance is the dominant control parameter for electronic and magnetic properties in RNiO₂, while out-of-plane lattice changes have negligible effects.
• All RNiO₂ compounds (R = La-Lu) exhibit a metallic antiferromagnetic ground state, contrary to the insulating behavior of cuprates.
• A flat Ni-dz² band is pinned at the Fermi level across all R ions and lattice parameters, indicating strong orbital character distinct from cuprates.
• The magnetic moment in LaNiO₂ decreases slightly from ~0.7 μB (GGA) to ~0.6 μB upon 4% in-plane lattice reduction, consistent with reduced hybridization.
• Within GGA+U, increasing U from 0 to 5.4 eV increases the Ni magnetic moment from ~0.7 μB to ~1.3 μB, indicating a LS-to-HS transition.
• The flat d_z² band feature is robust across all R ions and lattice parameters, suggesting universal instability toward charge, spin, or lattice order in the absence of doping.