Flat band ferromagnetism without connectivity conditions in the flat band

TL;DR

This paper challenges the traditional understanding that flat band ferromagnetism requires connectivity conditions, showing that ferromagnetism can occur without these conditions due to peculiar behavior of the adjacent band.

Contribution

It demonstrates that flat band ferromagnetism can exist without connectivity conditions, revealing a new mechanism involving the band above the flat band.

Findings

Ferromagnetism can occur without connectivity conditions.

The behavior of the band above the flat band influences magnetic properties.

Traditional connectivity criteria are not always necessary for ferromagnetism.

Abstract

It is known that a system which exhibits a half filled lowest flat band and the localized one-particle Wannier states on the flat band satisfy the connectivity conditions, is always ferromagnetic. Without the connectivity conditions on the flat band, the system is non-magnetic. We show that this is not always true. The reason is connected to a peculiar behavior of the band situated just above the flat band.