Hole doping in a negative charge transfer insulator

TL;DR

This study investigates how hole doping influences the bond disproportionation and metal-insulator transition in NdNiO3, revealing that increased doping suppresses the BD phase and clarifies its role in the MIT.

Contribution

It provides experimental and theoretical evidence that hole doping alters charge transfer energy and suppresses bond disproportionation, clarifying its role in the MIT of $RE$NiO$_3$.

Findings

Doped holes localize near Ca$^{2+}$ ions at low doping levels.

Increasing Ca concentration raises the charge transfer energy ($ riangle$).

Suppression of bond disproportionation occurs above a critical doping level.

Abstract

$RE$NiO$_3$ is a negative charge transfer energy system and exhibits a temperature-driven metal-insulator transition (MIT), which is also accompanied by a bond disproportionation (BD) transition. In order to explore how hole doping affects the BD transition, we have investigated the electronic structure of single-crystalline thin films of Nd$_{1-x}$Ca$_x$NiO$_3$ by synchrotron based experiments and {\it ab-initio} calculations. For a small value of $x$, we find that the doped holes are localized on one or more Ni sites around the dopant Ca$^{2+}$ ions, while the BD state for the rest of the lattice remains intact. The effective charge transfer energy ($\Delta$) increases with Ca concentration and the formation of BD phase is not favored above a critical $x$, suppressing the insulating phase. Our present study firmly demonstrates that the appearance of BD mode is essential for the MIT and settles a long-standing debate about the role of structural distortions for the MIT of the $RE$NiO$_3$ series.