Strongly correlated superconductor with polytypic 3D Dirac points

TL;DR

This study identifies FeTe$_{1-x}$Se$_x$ as a superconducting 3D Dirac semimetal with complex topological and electronic properties, highlighting challenges in detecting topological states due to strong correlations.

Contribution

It demonstrates the existence of 3D Dirac points in FeTe$_{1-x}$Se$_x$ and discusses the effects of correlations on topological gaps and surface states.

Findings

FeTe$_{1-x}$Se$_x$ hosts type-I and type-II Dirac points.

Strong correlations reduce the topological gap to sub-meV.

Flat bands at the Fermi level relate to high-T$_c$ superconductivity.

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$_{1-x}$Se$_x$ making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe$_{1-x}$Se$_x$ (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$_{1-x}$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, 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.