Optical and Raman spectroscopy studies on Fe-based superconductors

TL;DR

This review summarizes optical and Raman spectroscopy studies on Fe-based superconductors, highlighting gap formation, phonon behavior, and the interplay of magnetic and superconducting states, advancing understanding of their complex electronic properties.

Contribution

It provides a comprehensive overview of optical and Raman findings on Fe-based superconductors, emphasizing the role of gaps, phonons, and competing phenomena in these materials.

Findings

Evidence of gap formation in spin-density wave and superconducting states

Identification of s-wave pairing in hole-doped BaFe₂As₂

Classification of zone-center phonons and their interactions

Abstract

A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re=rare-earth element) and AFe$_2$As$_2$ (A=alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe$_2$As$_2$. Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron-phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the \emph{T}-dependent mid-infrared peak at 0.6-0.7 eV, electronic correlation, and the similarities/differences between high-\Tc cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.