Dimensional crossover and band topology evolution in ultrathin semimetallic NiTe₂films

Nickel ditelluride (NiTe₂), a recently identified Type-II Dirac semimetal possessing topological Dirac fermions close to the Fermi energy, is forecasted to show emergent two-gap superconductivity in the single-layer phase as well as pronounced dimensionality-mediated electronic tunability. Confirming these tantalizing phenomena necessitates fabricating ultrathin NiTe₂ films to unearth the hotly debated underlying physics. By undertaking photoemission band mappings of ultrathin NiTe₂ films grown via molecular beam epitaxy, we unveil spectroscopic proof for the strong thickness-mediated evolutions in the band structures of single-crystalline ultrathin NiTe₂ films. Specifically, when the film thickness is varied from one to five layers, the hybridization gap in the conical topological surface states closes within our experimental energy resolution. Furthermore, comparisons between experimental and first-principles data underline possible inherent difficulties in growing atomically smooth NiTe₂ films in the single-layer phase. Our comprehensive findings not only encourage further examinations of emergent physics in semimetallic NiTe₂ films but also underline the potential obstacles of integrating NiTe₂ into technological devices.