Evidence for a band broadening across the ferromagnetic transition of Cr1/3 NbSe2

The electronic structure of Cr1/3 NbSe2 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 eg states. We show that the peak splitting below the Curie temperature is not due to the exchange splitting of spin-up and spin-down bands, but directly reflects a band broadening effect in Cr-derived states upon the spontaneous ferromagnetic ordering.