关键词: angle-resolved photoemission spectroscopy, metal-insulator transition, transition metal dichalcogenides

Abstract: The 1$T$-TiSe$_{2}$ is a two-dimensional charge-density-wave (CDW) material that attracts great interest. A small band gap locates at the Fermi level separating the Ti d-bands and Se p-bands, which makes 1$T$-TiSe$_{2}$ a promising candidate for realizing excitonic condensation. Here, we studied the band gap in 1$T$-TiSe$_{2 }$ using angle-resolved photoemission spectroscopy (ARPES). Instead of only focusing on the in-plane band dispersions, we obtained the detailed band dispersions of both conduction and valance bands along the out-of-plane direction. We found that the conduction and valance bands split into multiple sub-bands in the CDW state due to band folding. As a result, the band gap between the Ti d-bands and Se p-bands reduces to $\sim 25 $ meV and becomes a direct gap in the CDW state. More intriguingly, such band gap can be further reduced by the rubidium deposition. The band structure becomes semimetallic in the rubidium-doped sample. Meanwhile, exotic gapless behaviors were observed at the p-d band crossing. Our result characterized the band gap of 1$T$-TiSe$_{2}$ in three-dimensional Brillouin zone with unpreceded precision. It also suggests a closing of band gap or a potential band inversion in 1$T$-TiSe$_{2}$ driven by rubidium deposition.

Key words: angle-resolved photoemission spectroscopy, metal-insulator transition, transition metal dichalcogenides