Electronic correlations and topological Fermi surface transition in the iron-based chalcogenides

We present results of a theoretical investigation of the electronic structure and phase stability of paramagnetic FeSe obtained within a combination of $ab~initio$ methods for calculating band structure and dynamical mean-field theory. Our results reveal an entire reconstruction of the Fermi surface topology upon a moderate expansion of the lattice (Lifshitz transition), with a change of magnetic correlations from the in-plane magnetic wave vector $(\pi,\pi)$ to $(\pi,0)$. We attribute this behavior to a correlation-induced shift of the Van Hove singularity originating from the $xy$ and $xz/yz$ bands at the M-point across the Fermi level. Our results predict a structural transition of FeSe upon a ca. 10 $\%$ expansion of the lattice volume as well as a topological change of the Fermi surface of FeSe upon partial substitution Se by Te, which is accompanied with a sharp increase of the local moments. We expect that these changes are responsible for the experimentally observed increase of T$_{c}$ in FeSe upon doping with Te. The microscopic origin for superconductivity in this system is then due to a Van Hove singularity close to the Fermi level. This identification may open a new route to increase T$_{c}$ even further.