Observation of universal strong orbital-dependent correlation effects in iron chalcogenides

TL;DR

This study reveals that iron chalcogenide superconductors exhibit universal, orbital-dependent strong correlation effects, especially in the dxy orbitals, regardless of their Fermi surface differences, indicating proximity to an orbital-selective Mott phase.

Contribution

The paper provides direct ARPES evidence of universal orbital-selective correlations in FeCh superconductors, highlighting their proximity to an orbital-selective Mott phase and challenging existing theoretical models.

Findings

Strong correlation effects in FeChs at low temperatures.

Orbital-selective renormalization of dxy bands.

Temperature-induced loss of spectral weight in dxy orbitals.

Abstract

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide (FeCh) superconductors, the only iron-based family in proximity to an insulating phase. Here, we use angle-resolved photoemission spectroscopy (ARPES) to measure three representative FeCh superconductors, FeTe0.56Se0.44, K0.76Fe1.72Se2, and monolayer FeSe film grown on SrTiO3. We show that, these FeChs are all in a strongly correlated regime at low temperatures, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi-surface topologies. Furthermore, raising temperature brings all three compounds from a metallic superconducting state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that FeChs display universal orbital-selective strong correlation behaviors that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase (OSMP), hence placing strong constraints for theoretical understanding of iron-based superconductors.