Visualizing Superconducting Gap Modulation Induced by Pair-Breaking Scattering Interference in Bulk FeSe

Spatially periodic modulations of the superconducting gap have been recently reported in diverse materials and are often attributed to the exotic superconducting state of Cooper-pair density waves (PDWs). An alternative mechanism, termed pair-breaking scattering interference (PBSI), was proposed to produce gap modulations without invoking PDWs. Here, we search for signatures of PBSI in bulk FeSe, which hosts no PDWs, using scanning tunneling microscopy with superconductive tips, enabling enhanced energy resolution and Josephson tunneling. Subsurface magnetic scatterers with Yu-Shiba-Rusinov states and reduced Josephson current are identified in FeSe, around which we observe particle-hole symmetric gap modulations. Those modulations have wavevectors consistent with intrapocket PBSI. We further demonstrate that phase-referenced quasiparticle interference imaging offers an independent and direct probe of PBSI beyond gap mapping. These results establish that gap modulations commonly attributed to PDWs can arise from PBSI, motivating further investigation of the intriguing gap modulation phenomenology.