† These authors contributed equally to this work [Abstract] We performed angle-resolved photoemission spectroscopy studies on a series of FeTe 1-x Se x monolayer films grown on SrTiO 3 . The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron-and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe 1-x Se x monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

The phase transition from a topological insulator to a trivial band insulator is studied by angleresoled photoemission spectroscopy on Bi2−xInxSe3 single crystals. We first report the complete evolution of the bulk band structures throughout the transition. The robust surface state and the bulk gap size (∼ 0.50 eV) show no significant change upon doping for x = 0.05, 0.10 and 0.175. At x ≥ 0.225, the surface state completely disappears and the bulk gap size increases, suggesting a sudden gap-closure and topological phase transition around x ∼ 0.175−0.225. We discuss the underlying mechanism of the phase transition, proposing that it is governed by the combined effect of spin-orbit coupling and interactions upon band hybridization. Our study provides a new venue to investigate the mechanism of the topological phase transition induced by non-magnetic impurities.

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