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.

Figures (4)

• Figure 1: (a) Distribution of two checkerboard sublattices (orange and purple circles), highlighting an inequivalent closed loop $\mathcal{O}$ (solid orange lines) within a unit cell (black dashed lines), together with the directions of magnetic interactions up to $\mathcal{N}=4$. (b) Schematic illustration of the origin of altermagnetism in the square bilayer and the associated intra- and interlayer symmetry operations. The topological phase diagrams versus (c) $D_{\delta_{1}^{1}}^{z}/J_{\delta_{1}^{1}}$ and $D_{\delta_{2}^{1}}^{z}/J_{\delta_{2}^{1}}$, and (d) $\Delta_{1}$ and $\Delta_{2}$, defined as $\Delta_{1}=J_{\delta_{1}^{2}}^{A}-J_{\delta_{1}^{2}}^{B}$ and $\Delta_{2}=J_{\delta_{2}^{2}}^{A}-J_{\delta_{2}^{2}}^{B}$, respectively.
• Figure 2: Magnon band structures of Heisenberg-DM model under (a) AFM, (b) FM, and (c) AM configurations, with the fat band analysis weighted with the magnonic Berry curvatures. (d) The distribution of momentum-resolved thermal Hall conductivity $\kappa^{M}_{xy}(50, \boldsymbol{k})$ along M$^{\prime}$-$\Gamma$-M for AFM, FM, and AM configurations at 50$K$. The distribution of $\kappa^{M}_{xy}(50, \boldsymbol{k})$ in whole Brillouin zone for AM with (e) $d$-wave and (f) $i$-wave configurations.
• Figure 3: (a) Top and side views of the $V_{2}WS_{4}$ bilayer in layer group $P\bar{4}2_{1}m$. (b) The first Brillouin zone with marked high symmetry points. (c) Electronic band structure of the $V_{2}WS_{4}$ bilayer without spin-orbit coupling. The alternating reciprocal-space spin polarization is clearly visible.
• Figure 4: (a) Magnonic band structure of the $V_{2}WS_{4}$ bilayer. Red and blue colors mark the opposite magnon chiralities. (b) Spin-splitting energy projections given by the energy difference $E_{(L, k)} - E_{_{(R, k)}}$. (c) Distribution of magnonic Berry curvatures of the two bands with opposite chiralities below the gap for $V_{2}WS_{4}$ bilayer. (d) Evolution of magnonic Wannier centers (MWC) for the $V_{2}WS_{4}$ bilayer, indicating a nonzero spin Chern number of $C_{s} = 1$. (e) Magnonic helical edge states of the $V_{2}WS_{4}$ bilayer. (f) The distribution of $\kappa^{M}_{xy}(200, \boldsymbol{k})$ in whole Brillouin zone for $V_{2}WS_{4}$ bilayer.