Anisotropy-induced phase transitions in an intrinsic half-Chern insulator Ni2I2.

One crucial target of research on spintronics is to achieve flexibly tunable and highly efficient spin-polarized electronic current. In this work, by using first-principles calculations and topological characterization theories, we propose an intrinsic half-Chern insulator (HCI) in a Ni2I2 monolayer, which possesses 100% spin-polarized topologically nontrivial edge states, distinct from ordinary Chern insulators. Its band gap is formed due to the lifting of the double degeneracy of non-Dirac bands composed of Ni dxz/dyz orbitals. The HCI becomes a half semiconductor (HS) or a combined state of a half metal (HM) and an HCI if biaxial strain is applied. The phase transition is found to be associated with the unique anisotropy of the bands, originating from the diverse orbital distributions and the opposite moving in energy of Ni dxy and dxz/dyz bands under the strain. Our findings demonstrate that the monolayer Ni2I2 is a unique Chern insulator with ideal spintronic properties, supporting versatile applications in spintronic devices with very high spin polarization and extremely low-power dissipation.