Role of the orbital degree of freedom in iron-based superconductors

TL;DR

This review discusses how the orbital degree of freedom influences the electronic properties and phase transitions in iron-based superconductors, highlighting experimental ARPES findings and their implications for understanding high-temperature superconductivity.

Contribution

It provides a comprehensive summary of ARPES studies on orbital-dependent phenomena in FeSCs, emphasizing the importance of multi-orbital effects often neglected in theoretical models.

Findings

Orbital-dependent electron correlations vary systematically across FeSC families.

Different Fe 3d orbitals play distinct roles in nematic and magnetic transitions.

Implications for superconductivity are linked to orbital physics.

Abstract

Almost a decade has passed since the serendipitous discovery of the iron-based high temperature superconductors (FeSCs) in 2008. The question of how much similarity the FeSCs have with the copper oxide high temperature superconductors emerged since the initial discovery of long-range antiferromagnetism in the FeSCs in proximity to superconductivity. Despite the great resemblance in their phase diagrams, there exist important disparities between FeSCs and cuprates that need to be considered in order to paint a full picture of these two families of high temperature superconductors. One of the key differences lies in the multi-orbital multi-band nature of FeSCs, in contrast to the effective single-band model for cuprates. Due to the complexity of multi-orbital band structures, the orbital degree of freedom is often neglected in formulating the theoretical models for FeSCs. On the experimental side, systematic studies of the orbital related phenomena in FeSCs have been largely lacking. In this review, we summarize angle-resolved photoemission spectroscopy (ARPES) measurements across various FeSC families in literature, focusing on the systematic trend of orbital dependent electron correlations and the role of different Fe 3d orbitals in driving the nematic transition, the spin-density-wave transition, and implications for superconductivity.