Evidence for the band broadening across the ferromagnetic transition in Cr$_{1/3}$NbSe$_2$

TL;DR

This study uses optical spectroscopy to reveal that in Cr$_{1/3}$NbSe$_2$, band broadening occurs across the ferromagnetic transition, challenging the typical exchange splitting explanation.

Contribution

It demonstrates that the band splitting observed is due to band broadening, not exchange splitting, providing new insight into the electronic structure change during ferromagnetic transition.

Findings

Observation of peak splitting below Curie temperature

Identification of band broadening as the cause of splitting

Confirmation that exchange splitting is not responsible

Abstract

The electronic structure of Cr$_{1/3}$NbSe$_2$ is studied via optical spectroscopy. We observe two low-energy interband transitions in the paramagnetic phase, which split into four peaks as the compound enters the ferromagnetic state. The band structure calculation indicates the four peaks are interband transitions to the spin up Cr e$_g$ states. We show that the peak splitting below the Curie temperature is \emph{not} due to the exchange splitting of spin up and down bands, but directly reflects a band broadening effect in Cr-derived states upon the spontaneous ferromagnetic ordering.