Alterpiezoresponse in Two-Dimensional Lieb-Lattice Altermagnets.

Altermagnetism, featuring alternating spin structures in reciprocal space, has sparked growing interest. Here, we predict novel real-space alternative piezomagnetic and piezoelectric responses in an emerging altermagnetic family of Lieb lattices, specifically transition-metal chalcogenides M₂WS₄ (M = Mn, Fe, Co). The unique S₄T crystal-spin symmetry leads to distinct magnetic and electric responses depending on the direction of applied stress. When subjected to axial stress, they exhibit a giant piezomagnetic response, which is about 1-2 orders of magnitude larger than that of most piezomagnetic materials, while the residual C₂ symmetry suppresses the piezoelectric effect. In contrast, diagonal stress induces an imbalance of oppositely aligned electric-dipole moments and a significant piezoelectric response, while in-plane mirror symmetry inhibits the piezomagnetic effect. This alternative piezoresponse offers an unprecedented opportunity to precisely control electric and magnetic properties independently, opening new avenues for altermagnetic materials in high-fidelity multifunctional memory and sensor applications.