Valley polarization reversal and spin ferromagnetism and antiferromagnetism in quantum dots of the topological insulator monolayer bismuthene on SiC

The valley and spin polarizations associated with electronic transport in quantum dots of the large-gap topological insulator (TI) monolayer bismuthene on SiC are investigated in the linear response regime using a minimal tight-binding model that accurately describes the low-energy electronic band structure of this TI. It is found that for zigzag edges the electronic edge states are strongly valley polarized if the Fermi energy lies in the bulk energy band gap. We predict the edge-state valley polarizations to switch between valleys K and K′ as the Fermi energy varies from the top of the valence band to the bottom of the conduction band or if the direction of electric current through the dot is reversed. If the electrostatic potential in the dot is nonuniform, we predict that the valley polarization of an electron can reverse as it travels through the dot. The valley polarization reversal is due to the zigzag edge-state dispersion crossing the center of the Brillouin zone that separates valleys K and K′ and is therefore predicted to be a general phenomenon. Although the spin polarization within the edge states is ferromagnetic, as expected for spin Hall devices, our calculations reveal the out-of-plane component of the spin polarization of the bulk valence band scattering states to be antiferromagnetic, and the direction of the out-of-plane component of the Neel vector to depend on whether the electronic accumulation belongs primarily to valley K or K′.