Observation of intra-unit-cell superconductivity modulation

In unconventional high-temperature (high-T_c) superconductors, the pair density wave state, an exotic superconducting order showing spatially periodic order parameter modulations with the period of several unit cells and translational symmetry breaking, has attracted broad attention. One open question is if the superconductivity can be modulated within the single unit-cell. Here, using scanning tunneling microscopy/spectroscopy, we report the observation of intra-unit-cell superconductivity modulations in monolayer high-T_c Fe(Te,Se) films epitaxially grown on SrTiO₃(001). The superconductivity modulations are characterized by the superconducting gap size and the superfluid density obtained by scanning Josephson tunneling. Further analysis shows that the local maxima and minima in the superconducting gap 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-T_c Fe(Te,Se) films grown on SrTiO₃(001). Our findings provide precise microscopic information of superconductivity within the lattice unit cell and may promote the understanding of unconventional high-T_c superconductivity.