Symmetry-driven valleytronics in the single-layer tin chalcogenides SnS and SnSe

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 the Green function-Bethe-Salpeter equation (GW-BSE) calculations and symmetry analysis, we demonstrate that single-layer orthorhombic SnS and SnSe possess remarkable carrier mobility and exceptional excitonic effects. 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 the two-dimensional tin chalcogenide is subjected to a zigzag polarization of light, only the A exciton becomes optically active, while the B exciton 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 exciton polarization. Layered tin chalcogenides thus emerge as promising candidates for valleytronic materials.