Computational prediction of a two-dimensional semiconductor SnO2 with negative Poisson's ratio and tunable magnetism by doping

Based on first-principles calculations, we predict a stable two-dimensional semiconductor, namely tin dioxide ${\mathrm{SnO}}_{2}$. By investigating its dynamical, thermal, and mechanical properties, we find that ${\mathrm{SnO}}_{2}$ monolayer is an auxetic material with a large in-plane negative Poisson's ratio. Furthermore, our results show that ${\mathrm{SnO}}_{2}$ is an indirect-gap semiconductor with a band gap in the region of 3.7 eV and an extremely high electron mobility, $\ensuremath{\sim}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Interestingly, the band structure of ${\mathrm{SnO}}_{2}$ presents double Mexican-hat-like band edges in the valence bands near the Fermi level. Due to such a unique band feature, a ferromagnetic phase transition takes place with a half-metallic ground state that can be induced by hole doping within a very wide concentration range. Such a magnetic phase can be well explained by the Stoner mechanism. A peculiar feature of the magnetic state is the presence of large magnetocrystalline anisotropy that can switch from in-plane to out-of-plane upon hole doping. Hence, ${\mathrm{SnO}}_{2}$ monolayer can be tuned to be either an $XY$ magnet or an Ising one, with a magnetic critical temperature above room temperature at proper hole concentrations. These findings demonstrate that the predicted phase of ${\mathrm{SnO}}_{2}$ is a rare example of $p$-type magnetism and a possible candidate for spintronic applications.