Nanosheets*

Spin-ordering affects band structure as well as scattering processes in magnetic materials. This is rarely explored in layered ferromagnetic semiconductors. Here we show the band-edge structure and photoresponse of the layered ferromagnet CrSiTe₃ (CST). Single nanosheets are studied by ultrafast mid-IR transient reflectance (TR) and photocurrent (PC) spectroscopy at 300 K (paramagnetic phase) and 10 K (ferromagnetic phase). We find a significant decrease of the fundamental direct bandgap as well as relatively smaller changes in the transition to mid-gap defects upon cooling the sample to 10 K. Density functional theory calculations are consistent with experiments and suggest possible spin-lattice coupling-induced structural changes at low temperatures. We find an enhanced rapid cooling rate of hot carriers in the ferromagnetic phase followed by a slower carrier recombination lasting nanoseconds. Both optical transitions are also seen in photocurrent spectra from CST devices over a broad mid-IR range.