Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

The superconducting ground state of kagome metals AV₃Sb₅ (where A stands for K, Rb, or Cs) emerges from an exotic charge density wave (CDW) state that potentially breaks both rotational and time reversal symmetries. However, the specifics of the Cooper pairing mechanism, and the nature of the interplay between these two states remain elusive, largely due to the lack of momentum-space (k-space) superconducting energy gap structure. By implementing Bogoliubov quasiparticle interference (BQPI) imaging, we obtain k-space information on the multiband superconducting gap structure Δ _SC ⁱ (k) in pristine CsV₃Sb₅. We show that the estimated energy gap on the vanadium \({d_{x{y \mathord{\left/ {\vphantom {y {{x^{\rm{2}}} - {y^{\rm{2}}}}}} \right.} {{x^{\rm{2}}} - {y^{\rm{2}}}}}}}\) $$d_{xy/x^{\text{2}}-y^{\text{2}}}$$ orbital is anisotropic but nodeless, with a minimal value located near the M point. Interestingly, a comparison of Δ _SC ⁱ (k) with the CDW gap Δ _CDW ⁱ (k) obtained by angle-resolved photoemission spectroscopy (ARPES) reveals direct k-space competition between these two order parameters, i.e., the opening of a large (small) CDW gap at a given momentum corresponds to a small (large) superconducting gap. When the long-range CDW order is suppressed by replacing vanadium with titanium, we find a nearly isotropic energy gap on both the V and Sb bands. This information will be critical for identifying the microscopic pairing mechanism and its interplay with intertwined electronic orders in this kagome superconductor family.