First-principles prediction of altermagnetic topological insulators in the hydrogenated Ti2X2O (X = As, Sb) monolayers

Abstract

Altermagnetism, a distinct magnetic phase beyond traditional ferromagnetism and antiferromagnetism, offers a promising platform for realizing novel topological quantum states. Utilizing first-principles calculations, we have systematically investigated the electronic, magnetic, and topological properties of hydrogenated Ti2X2O (X = As, Sb) monolayers. Upon surface hydrogenation, the structural stability is significantly improved, and the formed Ti2X2OH2 monolayers exhibit a typical altermagnetic characteristic. Interestingly, without the spin–orbit coupling (SOC), a Weyl semimetallic behavior is present in the Ti2As2OH2 system, which possesses four Weyl cones and a p–d band inversion around the Γ point. After the inclusion of SOC, a substantial bandgap of 0.15 eV opens at the Fermi level, transforming the Ti2As2OH2 monolayer into an altermagnetic topological insulator. The nontrivial topology is characterized by a quantized spin-Hall conductance, a nonzero spin Chern number, and one pair of helical edge states in the bulk gap. The similar nontrivial behavior also exists in the Ti2Sb2OH2 monolayer, which has a larger gap of 0.19 eV. Through strain engineering, a nontrivial-to-trivial topological transition is induced in the Ti2As2OH2 monolayer, whereas for the Ti2Sb2OH2 system an insulator-to-metal transition will happen. Our study highlights the Ti2X2OH2 systems as compelling candidates for achieving unconventional altermagnetic topological states.