We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface.

Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr₂Ir₂O₇, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr₂Ir₂O₇ is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

We have measured the band dispersion of ultrathin ${\text{Bi}}_2{\text{Se}}_3$ films formed on a silicon substrate using angle-resolved photoemission spectroscopy. An energy gap opening in the topological surface-state bands of the three-dimensional topological insulator ${\text{Bi}}_2{\text{Se}}_3$ is observed due to the hybridization of the top and bottom surfaces. The drastic change in the shape of the band structure and the reversal of the size of the energy gap between 2 and 3 quintuple layer (QL) films indicate that the parity of the conduction and valence bands are exchanged by finite-size effects. The analyses based on an effective four-band model also support the occurrence of a quantum topological phase transition, thus, implying that 2 QL films are trivial while the 3 QL films are in the quantum spin Hall state.

We studied the atomic and electronic structures of ultrathin Bi(111) films grown on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by means of angle-resolved photoemission, first-principles calculations, and low-energy electron diffraction. These Bi films were found to be strained due to the influence of the substrate. Accordingly, the band structure is affected and Bi undergoes a topological phase transition; it is shown that the $Z_2$ topological invariant in three dimensions switches from $+1$ (trivial) to $-1$ (nontrivial or topological). This was clearly confirmed from the change in the surface-state dispersion near the Fermi level. Our discovery offers a method to produce novel topological systems from simple materials.