Infrared study of the iron-based superconductors

Iron-based superconductors are discovered by Japanese scientists Hosono, which have layered structures and characterized by multi-orbital nature. Although a large amount of investigations have been done with different experimental techniques on various families of iron-based superconductors, the most important issue concerning the superconducting mechanism remains controversial, and much more efforts need to be done in order to resolve this issue. Infrared spectroscopy is an important technique to study the electron excitations near the Fermi surface and lattice dynamics in condensed matter, it has played an important role in the early studies of superconductivity in determining the superconducting gaps and electron-phonon couplings. In this paper, we first make a brief introduction to the experimental techniques of infrared spectroscopy, and then we review some recent progress in the investigation of iron-based superconductors with infrared optical spectroscopy. The content includes: (1) superconducting gap in iron-based superconductors; (2) quantum critical and non-Fermi-liquid behavior in (Ba,K)Fe2As2; (3) orbital-selective physics in KFe2As2; (4) lattice dynamics, and (5) nematicity in iron-based superconductors. At last, we summarize what can be further explored by infrared spectroscopy in understanding of superconducting mechanism in iron-based superconductors.