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Abstract

Identifying the pairing symmetry is a crucial step towards uncovering the superconducting mechanism. The pairing symmetry and interactions leading to pairing in the iron-based high-temperature superconductors are under debate. In this thesis work, the pairing symmetry of LiFeAs, a stoichiometric superconductor in the iron-based family, is studied by scanning tunneling microscopy. The tunneling conductance spectrum in a defect-free region shows two nodeless superconducting gaps. In addition, a dip-hump above-gap structure was observed, indicating coupling between the superconducting carriers and bosonic modes. Defect bound states were measured for iron-site defects. The bound states are pinned to the gap edge of the small superconducting gap, consistent with theoretical predictions for a sign-changing pairing symmetry. Finally, the observed Bogoliubov quasiparticle interference associated with scattering from defects provides compelling evidence for an s+- pairing symmetry in LiFeAs.