Valley spin polarization of Tl/Si(111)

The metal/semiconductor hybrid system Tl/Si(111)-$\left(1\times 1\right)$ exhibits a unique Tl-derived surface state with remarkable properties. It lies within the silicon band gap and forms spin-momentum-locked valleys close to the Fermi energy at the $\overline{K}$ and ${\overline{K}}^{\prime }$ points. These valleys are completely spin polarized with opposite spin orientation at $\overline{K}$ and ${\overline{K}}^{\prime }$ and show a giant spin splitting of more than 0.5 eV. We present a detailed preparation study of the surface system and demonstrate that the electronic valleys are extremely robust, surviving exposure to 100 L hydrogen and 500 L oxygen. We investigate the influence of additional Tl atoms on the spin-polarized valleys. By combining photoemission and inverse photoemission, we prove the existence of fully spin-polarized valleys crossing the Fermi level. Moreover, these metallic valleys carry opposite Berry curvature at $\overline{K}$ and ${\overline{K}}^{\prime }$, very similar to ${\mathrm{WSe}}_2$, promising a large spin Hall effect. Thus, Tl/Si(111)-$\left(1\times 1\right)$ possesses all necessary key properties for spintronic applications.