Robust spin-valley coupling in hexagonal monolayers of Tl-based monochalcogenides

Abstract

By comprehensive first-principles calculations, we predict that monolayer Tl-based monochalcogenides TlX (X = S, Se, and Te) are a class of stable direct bandgap semiconductors with band edges lying at a pair of inequivalent valleys. The interplay between inversion symmetry breaking and spin-orbit coupling within active Tl-p_x,y and X-p_x,y states triggers robust spin-valley coupling, yielding sizable valley spin splittings in the conduction and valence bands with complete valley-contrasting physics under time-reversal symmetry. As a consequence, the valley and spin Hall effects naturally occur in the n- and p-doped TlX samples without any contamination from the other part of the Brillouin zone near the Fermi level. Also, valley polarization can be realized by the illumination of circularly polarized light with frequencies in the near-infrared and visible regions. These appealing physical properties and their strain sustainability together with feasible fabrication render monolayer TlX a competitive class of valleytronic materials for valley-based electronic and optoelectronic applications.