Fig. 1: Fermi surface of CsV₃Sb₅ and Ta substituted CsV₃Sb₅ measured at low temperature (25 K).

a, b Crystal structure of CsV₃Sb₅ (a) and Ta substituted CsV₃Sb₅ (b). c, d Fermi surface of CsV₃Sb₅ (c) and CsV_2.6Ta_0.4Sb₅ (d) measured with 56 eV photons, which probe the electronic structure in the Γ-K-M plane³³ (Supplementary Fig. S2). Characteristic features are identified in both compounds, including an electron-like Fermi pocket around the Γ point, double-triangular Fermi surface sheets centered at the K point and their shared corners at the M point. e Schematic of the original Fermi surface (solid magenta line), and folded Fermi surface (orange dash line) by the CDW order. The in-plane wavevectors of the CDW are shown by black arrows. f Fermi surface sheets around Γ and K points of CsV₃Sb₅ and CsV_2.6Ta_0.4Sb₅, respectively. Same as those in (c) and (d), but shown in an expanded scale. The dashed lines are a guide for the eye. The same magenta dashed circle is appended to both Fermi surface sheets around Γ point for a quantitative comparison. The electron-like Fermi pocket becomes slightly larger in the CsV_2.6Ta_0.4Sb₅ sample.