Probing Small Chern Fermi Pockets in a Kagome Superconductor

The non-magnetic kagome superconductor family AV₃Sb₅ (A = Cs, K, Rb) is renowned for its giant anomalous Hall effect (AHE) and a variety of density-wave orders. Theoretically, the charge-density-wave (CDW)–induced band folding is predicted to create very small Fermi pockets carrying nonzero Chern numbers which contribute to the large AHE. Experimentally resolving these features, however, has been challenging due to the exceptionally high momentum-space resolution required. Here, we employ spectroscopic-imaging scanning tunneling microscopy (SI-STM) to uncover both the existence of such topological Fermi pockets and their distinctive two-fold symmetry. In addition, we use angle-resolved photoemission spectroscopy (ARPES) to directly visualize the formation of these pockets driven by a three-dimensional CDW transition, in agreement with theoretical predictions. Furthermore, we point out that modulations of the superconducting gap in the superconducting state generate an in-plane Cooper-pair density wave at the same q-vectors. These findings reveal how Fermi pockets arising from a parent CDW state can give rise to emergent additional density waves.