Intraband Lifshitz transition and Stoner ferromagnetism in Janus PA2As ( A=Si,Ge,Sn , and Pb) monolayers

Abstract

Using first-principles calculations, a class of Janus two-dimensional materials, $\mathrm{P}{A}_{2}\mathrm{As}$ ($A=\mathrm{Si}$, Ge, Sn, and Pb) monolayers, is predicted to be dynamically and thermally stable. The band gap of $\mathrm{P}{A}_{2}\mathrm{As}$ can be linearly modulated by external strain and electric field. The presence of the peculiar sombrero-type band in strained $\mathrm{P}{A}_{2}\mathrm{As}$ results in the intraband Lifshitz transition near the valence band top when including Rashba spin-orbit coupling. The energy range of intraband Lifshitz transition is linearly dependent on electric field, indicating potential electric control of the Fermi surface properties. In particular, hole doping leads to the ferromagnetic transition in $\mathrm{P}{A}_{2}\mathrm{As}$ due to Van Hove singularity and a divergence with $1/\sqrt{E}$ of the density of states. This ferromagnetism can be understood in terms of the Stoner criterion $D$(${E}_{\mathrm{F}}$)${I}_{\mathrm{S}}>1$. A saturated magnetic moment of $1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{hole}$ is obtained in hole-doped ${\mathrm{PSn}}_{2}\mathrm{As}$ and ${\mathrm{PPb}}_{2}\mathrm{As}$ with a wide range of hole densities. The highest Curie transition temperature ${T}_{\mathrm{c}}$ is estimated to be 93.5 K at the mean field level. We demonstrate that hole density and tensile strain are two effective approaches to control the Stoner ferromagnetism in practical applications. Moreover, various electronic transitions, such as semiconductor to half-metal, half-metal to metal, and semiconductor to metal, are revealed in hole-doped $\mathrm{P}{A}_{2}\mathrm{As}$. Finally, we explore the feasibility of realizing ferromagnetism by introducing $p$-type dopants and defects. The experimentally controlled intraband Lifshitz transition and Stoner ferromagnetism demonstrate that Janus $\mathrm{P}{A}_{2}\mathrm{As}$ monolayers have a wide application potential in future electronic and spintronic devices.