# Hidden robust presence of a hole Fermi surface in a heavily electron doped iron based superconductor LaFe$_2$As$_2$

## TL;DR

This study reveals that a hole Fermi surface exists in heavily electron-doped LaFe$_2$As$_2$, but is hidden in calculations, and its presence is crucial for understanding the material's superconductivity.

## Contribution

The paper demonstrates that a hole Fermi surface is inherently present in LaFe$_2$As$_2$, despite initial calculations suggesting its absence, highlighting the importance of orbital contributions.

## Key findings

- Hole Fermi surface is present when La d orbitals are hypothetically removed.
- Collapsed phase lacks the hole Fermi surface and is non-superconducting.
- The difference in Fermi surface topology explains superconductivity via spin fluctuations.

## Abstract

We investigate the electronic structure of a recently discovered, heavily electron-doped iron based superconductor LaFe$_2$As$_2$. Although first principles calculation shows apparent absence of hole Fermi surfaces around the $\Gamma$ point, we reveal, by hypothetically removing the La $d$ orbital contribution, that a hole Fermi surface around the $\Gamma$ point is essentially present. In the collapsed phase of LaFe$_2$As$_2$, which is non-superconducting, the hole Fermi surface is found to be absent, and the difference from the uncollapsed superconducting phase can be naturally understood within the spin-fluctuation mediated pairing scenario.