Strongly correlated superconductor with polytypic 3D Dirac points

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 thep-states of pnictogen andd-states of iron very close to the Fermi level, and suchp-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe(1-x)Se(x)making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1-xSex(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-x)Se(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, thep-dandd-dinteractions 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.