Magnonic Quantum Spin Hall Effect with Chiral Magnon Transport in Bilayer Altermagnets

Abstract: Altermagnetism has attracted considerable interest, yet its associated spintronic phenomena have so far been largely confined to electronic systems. In this work, we uncover a universal symmetry-based strategy for realizing topological altermagnets with the magnonic quantum spin Hall effect, as evidenced by a nonzero spin Chern number and protected helical edge states. Moreover, we demonstrate that chiral magnon splitting in altermagnets gives rise to an intrinsically anisotropic, momentum-resolved thermal Hall response, sharply contrasting with those in ferromagnets and antiferromagnets, thus offering enhanced flexibility for selective manipulation. As a concrete material realization, first-principles calculations and Heisenberg-DM model analysis reveal that V$_2$WS$_4$ bilayer exhibits $d$-wave altermagnetism, integer spin Chern number with helical magnon edge states, and the nonzero momentum-locked thermal Hall conductivity. Our results establish a direct link between topological magnons and altermagnetism, opening new avenues for dissipationless magnonic devices.