Ground-state oxygen holes and the metal-insulator transition in the negative charge-transfer rare-earth nickelates.

Valentina Bisogni,Thorsten Schmitt,Pavlo Zubko,Shadi Balandeh,Raoul Scherwitzl,Robert J Green,George Sawatzky,Jean-Marc Triscone,Vladimir N Strocov,Yaobo Huang,Marta Gibert,Sara Catalano

doi:10.1038/ncomms13017

Abstract

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO3 thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation.

Highlights

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years
The negative charge-transfer picture is at the base of recent charge or bond disproportionation model theories where, as first suggested by Mizokawa[26], the disproportioned insulating state is characterized by alternating Ni 3d8 (n 1⁄4 0) and Ni 3d8L2 (n 1⁄4 2) sites arranged in a lattice with a breathing-mode distortion[27,28,29,30]
We will focus on 30 nm thick NdNiO3 film grown on (110)-oriented NdGaO3 substrate under tensile strain conditions ( þ 1.6% of strain) as a representative example of bulk ReNiO3 in general

Summary

Introduction

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. The intriguing perovskite nickelates family ReNiO3 (with Re 1⁄4 rare-earth)[1,2,3,4] have garnered significant research interest in recent years, due to the remarkable properties they exhibit These include a sharp metal to insulator transition (MIT) tunable with the Re radius[2], unusual magnetic order[5] and the suggestion of charge order[6] in the insulating phase. To get a full understanding of the MIT and of the unique physical properties of the rare-earth nickelates, it is crucial to investigate the ground-state electronic structure in this class of materials To this purpose, we stress that while both positive and negative charge-transfer interpretations introduced above can be described as highly covalent, there are striking inherent differences between the two. The electronic structure and the character of the gap are vastly different in the two scenarios: a Á j3d7i þ b Á j3d8Li with a O 2p–Ni 3d-like gap or 3d8Ln with a O 2p–O 2p-like gap (refer to Fig. 1a,b, respectively)

Methods

Results

Conclusion