  The concept of valleytronics has recently gained considerable research
attention due to its intriguing physical phenomena and practical applications
in optoelectronics and quantum information. In this study, by employing GW-BSE
calculations and symmetry analysis, we demonstrate that single-layer
orthorhombic SnS and SnSe possess high carrier mobility and exceptional
excitonic effects. Especially, these materials display spontaneous linearly
polarized optical selectivity, a behavior that differs from the
valley-selective circular dichroism observed in the hexagonal lattices.
Specifically, when subjected to a zigzag polarization of light, only the A
exciton (stemming from the X valley) becomes optically active, while the B
exciton (arising from the Y valley) remains dark. The armchair-polarized light
triggers the opposite behavior. This selective optical excitation arises from
the symmetry of the bands under mirror symmetry. Additionally, the study
reveals a strong coupling between valley physics and ferroelectricity in
layered tin chalcogenides, enabling the manipulation of electronic transport
and exciton polarization. Layered tin chalcogenides thus emerge as promising
candidates for both valleytronic and ferroelectric materials.
