Nodeless electron pairing in CsV 3 Sb 5 -derived kagome superconductors.

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order and lattice geometry 1-9 . Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive 10-17 . In particular, consensus on the electron pairing symmetry has not been achieved so far 18-20 , in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic and orbital-independent superconducting gap in the momentum space of two exemplary CsV 3 Sb 5 -derived kagome superconductors-Cs(V 0.93 Nb 0.07 ) 3 Sb 5 and Cs(V 0.86 Ta 0.14 ) 3 Sb 5 -using ultrahigh-resolution and low-temperature angle-resolved photoemission spectroscopy. Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Our comprehensive characterizations of the superconducting gap provide indispensable information on the electron pairing symmetry of kagome superconductors, and advance our understanding of the superconductivity and intertwined electronic orders in quantum materials.