Experimental Observation of Incoherent-Coherent Crossover and Orbital Dependent Band Renormalization in Iron Chalcogenide Superconductors

TL;DR

This study uses ARPES to explore how electronic correlations and orbital-dependent band renormalization evolve in iron chalcogenide superconductors, revealing an incoherent-coherent crossover linked to selenium content.

Contribution

It provides the first systematic ARPES investigation of correlation effects and orbital-dependent band renormalization across the Fe$_{1+y}$Se$_x$Te$_{1-x}$ series, elucidating their role in superconductivity.

Findings

Incoherent to coherent crossover observed with increasing selenium.

Effective mass of d$_{xy}$ orbital decreases significantly with selenium.

Orbital-dependent correlation changes align with theoretical predictions.

Abstract

The level of electronic correlation has been one of the key questions in understanding the nature of superconductivity. Among the iron-based superconductors, the iron chalcogenide family exhibits the strongest electron correlations. To gauge the correlation strength, we performed systematic angle-resolved photoemission spectroscopy study on the iron chalcogenide series Fe$_{1+y}$Se$_x$Te$_{1-x}$ (0$<$x$<$0.59), a model system with the simplest structure. Our measurement reveals an incoherent to coherent crossover in the electronic structure as the selenium ratio increases and the system evolves from the weakly localized to more itinerant state. Furthermore, we found that the effective mass of bands dominated by the d$_{xy}$ orbital character significantly decreases with increasing selenium ratio, as compared to the d$_{xz}$/d$_{yz}$ orbital-dominated bands. The orbital dependent change in the correlation level agrees with theoretical calculations on the band structure renormalization, and may help to understand the onset of superconductivity in Fe$_{1+y}$Se$_x$Te$_{1-x}$.