Pseudogap-related charge dynamics in layered-nickelate R2-xSrxNiO4 (x sim 1)

TL;DR

This study explores the charge dynamics near the metal-insulator transition in layered-nickelate R2-xSrxNiO4, revealing pseudogap formation linked to charge correlations and its impact on carrier behavior.

Contribution

It demonstrates the persistence of charge ordering and the evolution of the pseudogap in layered-nickelates near the critical doping, combining spectroscopy techniques.

Findings

Charge ordering persists up to critical doping.

Pseudogap develops with decreasing carrier density.

Coherent carriers are suppressed in a momentum-dependent manner.

Abstract

Charge dynamics and its critical behavior are investigated near the metal-insulator transition of layered-nickelate R2-xSrxNiO4 (R=Nd, Eu). The polarized x-ray absorption spectroscopy experiment clearly shows the multi-orbital nature which enables the x2-y2-orbital-based checkerboard-type charge ordering or correlation to persist up to the critical doping region (x sim 1). In the barely metallic region proximate to the charge-ordered insulating phase, the nominal carrier density estimated from the Hall coefficient markedly decreases in accord with development of the pseudogap structure in the optical conductivity spectrum, while the effective mass is least enhanced. The present findings combined with the results of recent angle-resolved photoemission spectroscopy show that the pseudogap in the metal-insulator critical state evolves due to the checkerboard-type charge correlation to extinguish the coherent-motion carriers in a characteristic momentum (k)-dependent manner with lowering temperature.