Directly Visualizing Bogoliubov Quasiparticle Interference of LiFeAs: a Way Toward Understanding Superconductivity in Iron Pnictides*

Imaging quasiparticle interference (QPI) is a way to probe the electronic states of a wide range of materials. In superconductors, QPI of Bogoliubov excitations is directly sensitive to the sign changes of the superconducting order parameter. In this talk, I present our investigation of QPI in superconducting LiFeAs by means of scanning tunneling microscopy/spectroscopy, angle resolved photoemission spectroscopy, and multi-orbital tight binding calculations. Using this combination we identify intra- and interband scattering vectors between the hole ($h$) and electron ($e$) bands in the QPI maps. Bogoliubov QPI, with a clear antisymmetric phase at positive and negative bias voltages near the superconducting gap, is revealed in the spatial modulations of the local density of states. The observation of both h-h and e-h scattering intensity variations is exploited using scattering selection rules for Bogoliubov quasiparticles. From this we infer an $s+-$ gap structure, where a sign change occurs in the superconducting order parameter between the $e$ and $h$ bands.