# Hidden Fermi surface in K$_x$Fe$_{2-y}$Se$_2$: LDA+DMFT study

## TL;DR

This study uses LDA+DMFT calculations to explain the hidden Fermi surface observed experimentally in K$_{0.62}$Fe$_{1.7}$Se$_2$, revealing complex multiorbital features and resolving longstanding questions about its electronic structure.

## Contribution

The paper provides the first theoretical LDA+DMFT evidence supporting the existence of a hidden Fermi surface in K$_x$Fe$_{2-y}$Se$_2$, aligning with recent ARPES experiments.

## Key findings

- Identification of a hidden hole-like Fermi surface near $ ext{Gamma}$-point.
- Discovery of a 'propeller'-shaped Fermi surface structure due to correlations.
- Clarification that the smallest Fermi surface is an artifact of spectral intensity summation.

## Abstract

In this paper we provide theoretical LDA+DMFT support of recent ARPES experimental observation of the so called hidden hole like band and corresponding hidden Fermi surface sheet near $\Gamma$-point in the K$_{0.62}$Fe$_{1.7}$Se$_2$ compound. To some extent this is a solution to the long-standing riddle of Fermi surface absence around $\Gamma$-point in the K$_x$Fe$_{2-y}$Se$_2$ class of iron chalcogenide superconductors. In accordance with the experimental data Fermi surface was found near the $\Gamma$-point within LDA+DMFT calculations. Based on the LDA+DMFT analysis in this paper it is shown that the largest of the experimental Fermi surface sheets is actually formed by a hybrid Fe-3d($xy,xz,yz$) quasiparticle band. It is also shown that the Fermi surface is not a simple circle as DFT-LDA predicts, but has (according to the LDA+DMFT) a more complicated `propeller'-like structure due to correlations and multiorbital nature of the K$_x$Fe$_{2-y}$Se$_2$ materials. While the smallest experimental Fermi surface around $\Gamma$-point is in some sense fictitious, since it is formed by the summation of the intensities of the spectral function associated with `propeller' loupes and is not connected to any of quasiparticle bands.