Friedel oscillations of the superconducting gap in FeSe

The spatial variation of the superconducting gap of FeSe has been studied by spectroscopic-imaging scanning tunneling microscopy. The apparent gap amplitude defined by the energy separation between the quasi-particle coherence peaks exhibits spatial periodic oscillations. We found that the wavelength of the oscillations agrees with the wavelength of the normal-state quasi-particle interference pattern at the Fermi energy. Such Friedel oscillations of the superconducting gap may be related to the extremely small Fermi surfaces of FeSe in the BCS-BEC crossover~[1]. [1] S. Kasahara {\it et al.}, PNAS Early Edition, Nov. 6, 2014, doi:10.1073/pnas.1413477111