Mass divergence type metal-insulator transition in charge transfer rare-earth nickelates

Abstract

Experimental observables around the metal-insulator (M-I) transition are key to unveiling the underneath mechanism that induces the transition. From the heuristic argument, two different characteristics of charge carriers are observed on approaching the M-I transition point from the metallic side: (i) the Mott insulators in which the carrier effective mass diverges, and (ii) charge-transfer insulators in which the carrier-number vanishes. The rare-earth nickelates exhibiting M-I transition have recently emerged as materials of enormous contemporary relevance; however, with no conclusive understanding of the origin of their M-I transition. Here we have investigated the charge-transfer phase of two prototypical systems ${\text{Pr}}_{1\ensuremath{-}x}{\text{Nd}}_{x}{\text{NiO}}_{3}$ ($x=0$, 0.5) to construct a unified picture of the metal-insulator transition in the presence of electron and hole bands. Contrary to the existing understanding, our analysis of two-band framework modeled to the terahertz (THz) optical conductivity of the two systems suggests the effective mass divergence upon approaching the transition from the metallic regime as the cause of electron correlations; and resultantly, an unambiguous manifestation of a negative charge-transfer insulating ground state is established in rare-earth nickelates.