Enhanced Valley Splitting of Transition-Metal Dichalcogenide by Vacancies in Robust Ferromagnetic Insulating Chromium Trihalides.

Recently, single-layer CrI, a member of the chromium trihalides CrX (where X = Cl, Br, or I), has been exfoliated and experimentally demonstrated as an atomically thin material suitable for two-dimensional spintronics. Valley splitting due to the magnetic proximity effect has been demonstrated in a WSe/CrI van der Waals heterojunction. However, the understanding of the mechanisms behind the favorable performance of CrI is still limited. Here, we systematically study the carrier mobility and the intrinsic point defects in CrX and assess their influence on valley splitting in WSe/CrI by first-principles calculations. The flat-band nature induces extremely large carrier mass and ultralow carrier mobility. In addition, intrinsic point defects-localized states in the middle of the band gap-show deep transition energy levels and act as carrier recombination centers, further lowering the carrier mobility. Moreover, vacancies in CrI can enhance ferromagnetism and valley splitting in a WSe/CrI heterojunction, proving that chromium trihalides are excellent ferromagnetic insulators for spintronic and valleytronic applications.