Orbital occupancy and charge doping in iron-based superconductors

TL;DR

This study uses advanced microscopy and spectroscopy to reveal universal behaviors in electron occupancy and magnetic moments across iron-based superconductors, highlighting the role of charge doping and spin fluctuations in their superconductivity.

Contribution

It provides new microscopic insights into orbital occupancy, magnetic moments, and doping mechanisms across different families of iron-based superconductors, clarifying longstanding debates.

Findings

Local magnetic moments increase near optimal doping

Charge doping influences superconductivity in 122 arsenides

Orbital occupancy varies among different superconductor families

Abstract

Iron-based superconductors (FBS) comprise several families of compounds having the same atomic building blocks for superconductivity, but large discrepancies among their physical properties. A longstanding goal in the field has been to decipher the key underlying factors controlling TC and the various doping mechanisms. In FBS materials this is complicated immensely by the different crystal and magnetic structures exhibited by the different families. In this paper, using aberration-corrected scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we observe a universal behavior in the hole concentration and magnetic moment across different families. All the parent materials have the same total number of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies due to different orbital occupancy. Although the common understanding has been that both long-range and local magnetic moments decrease with doping, we find that, near the onset of superconductivity, the local magnetic moment increases and shows a dome-like maximum near optimal doping, where no ordered magnetic moment is present. In addition, we address a longstanding debate concerning how Co substitutions induces superconductivity in the 122 arsenide family, showing that the 3d band filling increases a function of doping. These new microscopic insights into the properties of FBS demonstrate the importance of spin fluctuations for the superconducting state, reveal changes in orbital occupancy among different families of FBS, and confirm charge doping as one of the mechanisms responsible for superconductivity in 122 arsenides.