Strongly correlated superconductor with polytypic 3D Dirac points

Abstract Topological superconductors should be able to provide essential ingredients for quantum computing, but are very challenging to realize. Spin–orbit interaction in iron-based superconductors opens the energy gap between the p-states of pnictogen and d-states of iron very close to the Fermi level, and such p-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe_1−xSe_x making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe_1−xSe_x (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe_1−xSe_x can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the p–d and d–d interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-T_c superconductivity in IBS.