Observation of Gapped Topological Surface States and Isolated Surface Resonances in PdTe$_2$ Ultrathin Films

The superconductor PdTe$_2$ is known to host bulk Dirac bands and topological surface states. The coexistence of superconductivity and topological surface states makes PdTe$_2$ a promising platform for exploring topological superconductivity and Majorana bound states. In this work, we report the layer-by-layer molecular beam epitaxy growth and spectroscopic characterization of high quality PdTe$_2$ films with thickness down to 3 monolayers (ML). In the 3 ML PdTe$_2$ film, we observed spin-polarized surface resonance states, which are isolated from the bulk bands due to the quantum size effects. In addition, the hybridization of surface states on opposite faces leads to a thickness-dependent gap in the topological surface Dirac bands. Our photoemission results show clearly that the size of the hybridization gap increases as the film thickness is reduced. The success in growing high quality PdTe$_2$ films by state-of-art molecular beam epitaxy technique and the observation of surface resonances and gaped topological surface states sheds light on the applications of PdTe$_2$ quantum films in spintronics and topological computation.