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The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order and lattice geometry1–9. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive10–17. In particular, consensus on the electron pairing symmetry has not been achieved so far18–20, 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 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. 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.The authors report that with the use of angle-resolved photoemission spectroscopy, nodeless electron pairing in CsV3Sb5-derived kagome superconductors can be observed directly. [ABSTRACT FROM AUTHOR]

In a classical Bardeen-Cooper-Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature ([T.sub.c]), giving rise to a gap in the electronic energy spectrum. In the high-[T.sub.c] copper oxide superconductors, however, a pseudogap (1-8) extends above [T.sub.c]. The relationship between the pseudogap and superconductivity is one of the central issues in this field (9-17). Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive (10-16). The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing., The traditional approach of using spectral features measured by angle-resolved photoemission spectroscopy (ARPES) to determine whether or not the pseudogap consists of a state of preformed pairs has failed to [...]

In a classical Bardeen–Cooper–Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature (Tc), giving rise to a gap in the electronic energy spectrum. In the high-Tc copper oxide superconductors, however, a pseudogap extends above Tc. The relationship between the pseudogap and superconductivity is one of the central issues in this field. Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive. The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing. [ABSTRACT FROM AUTHOR]

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., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)