Multiple energy scales and anisotropic energy gap in the charge-density-wave phase of the kagome superconductor CsV 3 Sb 5

Kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, and Cs) exhibit superconductivity at 0.9--2.5 K and charge-density wave (CDW) at 78--103 K. Key electronic states associated with the CDW and superconductivity remain elusive. Here, we investigate low-energy excitations of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ by angle-resolved photoemission spectroscopy. We found an energy gap of 50--70 meV at the Dirac-crossing points of linearly dispersive bands, pointing to an importance of spin-orbit coupling. We also found a signature of strongly Fermi-surface and momentum-dependent CDW gap characterized by the larger energy gap of maximally 70 meV for a band forming a saddle point around the $M$ point, the smaller (0--18 meV) gap for a band forming massive Dirac cones and a zero gap at the $\mathrm{\ensuremath{\Gamma}}/A$-centered electron pocket. The observed highly anisotropic CDW gap which is enhanced around the $M$ point signifies an importance of scattering channel connecting the saddle points, laying foundation for understanding the nature of CDW and superconductivity in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$.