Multi-piezo Effects and Ferroelectricity in a Two-Dimensional Multiferroic Altermagnet Ti2Se2S

Abstract

The exploration of emergent two-dimensional (2D) magnetic phases is essential for advancing next-generation spintronic technologies and deepening the understanding of low-dimensional magnetism. In particular, 2D multiferroic systems offer a pathway toward compact, energy-efficient, and multifunctional devices; however, achieving the intrinsic coexistence and coupling of multiple ferroic orders within a single atomic layer remains a significant challenge. In this work, we propose the Ti 2 Se 2 S monolayer as a promising platform exhibiting ferroic and spin-related phenomena. The system intrinsically lacks inversion symmetry, giving rise to spontaneous out-of-plane ferroelectric polarization with a magnitude of 12 pC/cm, accompanied by a moderate switching barrier (0.3 eV/f.u), ensuring feasible polarization reversal. In addition to its ferroelectric character, the material displays altermagnetic behavior, enabling spin-dependent electronic responses without requiring spin-orbit coupling. Moreover, mechanical deformation plays a crucial role in tuning its properties. Uniaxial strain induces a pronounced valley polarization of 170 meV and allows its reversal depending on the strain direction, demonstrating a clear piezovalley effect.Simultaneously, the combination of strain and carrier doping generates a finite magnetization of 0.024 µβ, revealing a piezomagnetic response. The broken inversion symmetry further enables piezoelectricity, adding another functional degree of control. The coexistence of altermagnetism, ferroelectricity, and multiple strain-driven responses, including piezovalley, piezomagnetic, and piezoelectric effects, highlights the Ti 2 Se 2 S monolayer as a versatile candidate for integrated spintronic and valleytronic applications.