The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, 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 CsV3Sb5-derived kagome superconductors -- Cs(V0.93Nb0.07)3Sb5 and Cs(V0.86Ta0.14)3Sb5, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). 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. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking charge density wave (CDW) in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.
In this work, we utilize an ultrahigh resolution and low temperature laser-ARPES, together with a chemical substitution of V in CsV3Sb5 that raises Tc, to precisely measure the gap structure in the superconducting state. CsV3Sb5 crystallizes in a layered structure with V atoms forming a two-dimensional kagome net. At low temperatures, the material exhibits a CDW transition at TCDW ~ 93 K, and eventually becomes superconducting at Tc ~ 3 K. To finely tune the competition between superconductivity and CDW, we take Nb and Ta to substitute V in CsV3Sb5. The Cs(V0.93Nb0.07)3Sb5 sample exhibits a Tc of 4.4 K and a TCDW of 58 K, while the Cs(V0.86Ta0.14)3Sb5 sample exhibits a higher Tc of 5.2 K, but no clear CDW transition. Strikingly, the gap structures of both samples are isotropic, regardless of the disappearance of CDW, hinting at a robust nodeless pairing in CsV3Sb5-derived kagome superconductors.
Ultrahigh-resolution ARPES measurements were performed in a laser-based ARPES setup at the ISSP, University of Tokyo, which consisted of a continuous wave laser (excitation energy = 5.8 eV) provided from OXIDE Corporation and a vacuum ultraviolet laser (excitation energy = 6.994 eV), a Scienta Omicron HR8000 hemispherical analyser, and a sample manipulator cooled by decompression-evaporative the liquid helium. The sample temperature was varied from 2 to 7 K, and the energy resolution for the superconducting gap measurements was better than 0.6 meV for 5.8-eV laser and 1.5 meV for 6.994-eV laser. High resolution of the hemispherical analyser was necessary to resolve the superconducting gaps in those materials with very low Tc.