Valley- and Spin-Dependent Electronic and Transport Properties of Two-Dimensional Altermagnetic Titanium-Based Chalcogenide Halides

Abstract

Altermagnets (AMs) combine fully compensated magnetization with momentum-dependent spin splitting, yet intrinsic altermagnetic materials exhibiting exceptional valley characteristics remain scarce. Here, we identify monolayer titanium-based chalcogenide halides, Ti$_2X_2Y$ ($X$ = F, Cl, Br, I; $Y$ = O, S, Se, Te), as a new family of two-dimensional altermagnetic valley materials. These monolayers exhibit robust $d$-wave altermagnetic order, semiconducting band gaps, and pronounced spin-polarized valley characteristics. We show that uniaxial strain breaks the valley degeneracy, inducing giant valley polarization together with a tunable piezomagnetic response. An in-plane electric field generates noncollinear spin currents, while spin--orbit coupling gives rise to the anomalous Hall effect, valley-selective linear dichroism, and the magneto-optical Kerr effect. These findings establish Ti$_2X_2Y$ monolayers as a versatile platform for exploring spin- and valley-dependent electronic, optical, and transport phenomena in two-dimensional altermagnets.