Strain-induced half-valley metals and topological phase transitions in M Br2 monolayers (M =Ru ,Os )

The target of valleytronics developments is to manipulate the valley degree of freedom and utilize it in microelectronics as charge and spin degrees of freedom. Based on first-principles calculations, we demonstrate that M Br₂ (M =Ru ,Os ) monolayers are intrinsically ferrovalley materials with large valley polarization up to 530 meV. Compressive strain can induce phase transitions in the materials from ferrovalley insulators to complete valley-polarized metals, called half-valley metals, in analogy to the concept of half metals in spintronics. With the increase of the strain, the materials become Chern insulators, whose edge states are chiral-spin-valley locking. The phase transition is caused by sequent band inversions of the d_{x y}/d_x²−y² and d_z² orbitals at K − and K + valleys, analyzed based on a strained k .p model. Our work provides a pathway for carrying out low-dissipation electronics devices with complete spin and valley polarizations.