Existence of an ultraflat band in the charge density wave state of 4H_b-TaS_1.3Se_0.7

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 EF in the charge density wave state of 4Hb-TaS1.3Se0.7. This band extends over the ./ ./ 13 ? 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 EF. We attribute it to the electron-correlationdriven splitting, forming a small gap of -45 meV. Our observation of the flat band and splitting in 4Hb-TaS1.3Se0.7 provides a platform to manipulate the electron-correlation-related quantum phenomena at a low energy scale.