Pairing insights in iron-based superconductors from scanning tunneling microscopy

• Recent STM contributions to the understanding of the gap symmetry in iron-based superconductors (Fe-SCs) are reviewed.
• STM reveals generic bosonic excitations in the four major families of Fe-SCs.
• The sub-gap bound states at single impurities match with a s± wave electron pairing picture.
• The rotational symmetry breaking in both parent and superconducting states supports an orbital ordering phase in Fe-SCs.
• Material-dependent vortex core states and shapes have been resolved in Fe-SCs.

Scanning tunneling microscopy (STM) has made tremendous progress in the study and understanding of both classical and unconventional superconductors. This has motivated a rapidly growing effort to apply the same techniques to the iron-based high-Tc superconductors since their discovery in 2008. Five years have brought exciting advances in imaging and spectroscopic investigation of this new class of materials. In this review, we focus on several recent STM contributions to the identification of the gap symmetry and pairing glue. We highlight the unique capabilities and challenges still ahead for STM studies of iron-based superconductors.