Existence of an ultraflat band in the charge density wave state of 4Hb−TaS1.3Se0.7

A flat band is an electronic state with quenched kinetic energy and an extremely high density of states. In such a state, a small electron-electron correlation can drive multiple intriguing quantum phenomena. By using angle-resolved photoemission spectroscopy, we report the existence of a dispersionless flat band with a sharp quasiparticle peak below ${E}_{\mathrm{F}}$ in the charge density wave state of $4{H}_b\text{\ensuremath{-}}{\mathrm{TaS}}_1.3{\mathrm{Se}}_0.7$. This band extends over the $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ reconstructed Brillouin zone. In combination with the density functional theory calculation, we identify that the flat band is due to the formation of the Star of David at the top-most $T$ layer. In addition, the scanning tunneling spectroscopy observes a sharp peak above ${E}_{\mathrm{F}}$. We attribute it to the electron-correlation-driven splitting, forming a small gap of $\ensuremath{\sim}45$ meV. Our observation of the flat band and splitting in $4{H}_b\text{\ensuremath{-}}{\mathrm{TaS}}_1.3{\mathrm{Se}}_0.7$ provides a platform to manipulate the electron-correlation-related quantum phenomena at a low energy scale.