Bipolar altermagnetic semiconductor in Ti2Br2O monolayer

The ability to mechanically and electrically manipulate spin degrees of freedom is highly desirable for spintronic devices. Inspired by emerging two-dimensional altermagnets, we propose bipolar altermagnetic semiconductors (BAMSs)—a platform enabling mechanically and electrically switchable full spin-valley polarization in electrons and holes with opposite spins. By constructing a four-band Hamiltonian, we elucidate the electronic structures of BAMSs and establish a classification scheme based on band edge spins, valleys, and carrier types. First-principles calculations demonstrate that the Ti2Br2O monolayer exhibits ideal BAMS behavior under moderate uniaxial strains, featuring excellent structural stability, robust Néel-type antiferromagnetism, and strongly anisotropic spin transport. Remarkably, the unconventional spin Hall effect (noncollinear spin current) arising from its unique anisotropic transport can be realized and well manipulated by combining strain and gate voltage tuning. These findings introduce a class of magnetic semiconductors and highlight Ti2Br2O as a promising candidate for spin manipulation through mechanical and electrical means.