Photothreshold of an $\alpha$-GeS layered crystal: first-principles calculation

The photothreshold of an $\alpha$-GeS layered crystal is calculated from first principles based on the functional density method depending on its thickness. Two neighboring crystal plates consisting of several layers are separated by vacuum 4 layers thick, which corresponds to the doublet unit cell size of a bulk crystal. It is shown that the magnitude of the photothreshold is almost invariable with a crystal thickness larger than 10 layers.