Insulator-to-metal transition in the pyrochlore iridates series (Eu_1-xBi_x)2Ir2O7 probed using Hard X-ray Photoemission Spectroscopy

TL;DR

This study investigates the insulator-to-metal transition in the (Eu_1-xBi_x)2Ir2O7 series using Hard X-ray Photoemission Spectroscopy, revealing that Bi 6p - Ir 5d hybridization drives the transition while all cations retain their valence states.

Contribution

The paper provides new insights into the electronic structure changes and hybridization mechanisms underlying the insulator-to-metal transition in pyrochlore iridates.

Findings

Bi contributes to the density of states at the Fermi energy in the metallic region.

All cations retain their nominal valence states throughout the series.

Bi 6s peak remains unchanged and is deep in the valence band.

Abstract

Eu2Ir2O7, a candidate Weyl semimetal, shows an insulator-to-metal transition as a function of Bi substitution at the Eu site. In this work, we investigate the (Eu_1-xBi_x)2Ir2O7 series via Hard X-ray Photoemission Spectroscopy (HAXPES), where substitution of larger Bi3+ for Eu3+ is reported to result in an anomalous lattice contraction (20 %) for low Bi doping (3.5 %). Using HAXPES, we confirm that all the cations retain their nominal valence state throughout the series. The asymmetric nature of Bi core-level spectra for compositions in the metallic region indicates that Bi contributes to the density of states at the Fermi energy in this doping range. The valence band spectra shows that the Bi 6s peak is unaltered throughout the series and is situated deep within the valence band. based on these observations we argue that Bi 6p - Ir 5d hybridization drives the insulator-to-metal transition.