Title: Observation of intra-unit-cell superconductivity modulation

Abstract: In unconventional high-temperature (high-Tc) superconductors, the symmetry-breaking electronic orders intertwined with the superconductivity provide important clues for understanding the nature of the unconventional pairing mechanism. Recently, an exotic superconducting order showing spatially periodic order parameter modulations and translational symmetry breaking, namely the pair density wave (PDW) state, has attracted broad attention. Without breaking translational symmetry, point group symmetry breaking may also induce superconductivity modulations on different atom sites within a single unit cell. However, the intra-unit-cell superconductivity modulation has never been carefully investigated before. Here, using scanning tunneling microscopy/spectroscopy, we report the observation of intra-unit-cell superconductivity modulations in the superconducting gap size and the coherence peak sharpness in monolayer high-Tc Fe(Te,Se) films epitaxially grown on SrTiO3(001) substrates. Further analysis shows that the maxima and minima in the superconductivity modulation are centered at the crystallographic locations of the Te/Se atoms, revealing the breaking of the glide-mirror symmetry of the Te/Se atoms in monolayer high-Tc Fe(Te,Se) films grown on SrTiO3(001). Our findings provide precise microscopic information of superconductivity within the lattice unit cell and indicate that the p-orbital electrons of the Te/Se atoms also play an important role in Cooper pairing in unconventional high-Tc iron-based superconductors.