The dual-valley PbS monolayer is demonstrated to be a novel platform for exploring topological and valleytronic phenomena by a joint team led by Prof. Yugui Yao (Beijing Institute of Technology) and Prof. Fan Zhang (University of Texas at Dallas), discussed in a recent article in Advanced Materials. The bandgaps of PbS few-layers cover a wide spectrum from the infrared to visible, making them promising for optoelectronics. Intriguingly, although the PbS few-layers are normal insulators, compressive strain can conveniently turn the monolayer into quantum spin Hall insulators. This feature may be utilized for mechanically controlled low-power electronic devices. Furthermore, in spite of the vanishing elliptic dichroism due to the inversion symmetry in the monolayer, optical pumping provides an efficient tool to characterize the topologically distinct phases and to facilitate the realization of charge, spin, and valley Hall effects in optoelectronic transport. Such transport can be tunable by external strain and light ellipticity. Similar results apply to other IV–VI semiconductors.

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