Altermagnets Enable Gate-Switchable Helical and Chiral Topological Transport with Spin-Valley-Momentum-Locked Dual Protection

Abstract

We establish a unified, symmetry-driven framework that combines the alternating spin splitting of altermagnets with valley topology to realize and electrically interconvert helical and chiral topological phases within a single material platform. We first demonstrate a magnetic analogue of the quantum spin Hall effect in altermagnets, hosting helical spin-valley-momentum-locked (SVML) edge states characterized by a composite spin-valley Chern number Csv = 2. Large-scale quantum transport simulations show these SVML edge states exhibit fully quantized spin conductance robust against nonmagnetic and long-range magnetic disorder, reflecting their dual topological protection, while remaining vulnerable to short-range magnetic disorder. Exploiting that the counterpropagating SVML modes are linked by crystal rotation symmetry, we introduce a gate-tunable sublattice-staggered potential that selectively gaps one valley and converts the helical state into a chiral quantum anomalous Hall phase with Csv = 1, robust against all disorder types. Reversing the potential switches the transmitted spin-valley polarization. Our first-principles calculations identify monolayer V2STeO and VO families as realistic platforms supporting both helical and chiral topological phases and their electrical switching. These results establish altermagnets as electrically programmable platforms for robust topological devices across charge, spin, and valley.

Figures (4)

• Figure 1: (a) Schematic of spin–valley–resolved Weyl points at the $\alpha$ and $\beta$ valleys in AMs. (b) SOC opens topological gaps at both valleys, yielding an AMQSVH phase with helical SVML edge states and fully quantized spin Hall conductivity ($\sigma_s=e/2\pi$). (c) A positive staggered potential ($U>0$) selectively trivializes the gap at the $\alpha$ valley, eliminates its edge state, and produces a $\beta$-AMQAVH phase with spin-up chiral edge states at the $\beta$ valley ($\sigma_{\uparrow}=e^2/h$). (d) Reversing the staggered potential ($U<0$) yields an $\alpha$-AMQAVH phase with spin-down chiral edge states at the $\alpha$ valley ($\sigma_{\downarrow}=e^2/h$).
• Figure 2: (a) 2D square-lattice AM model for Eq. \ref{['eq:tb']}; the black square denotes the unit cell with lattice constant $a$. (b) Calculated $d$-wave AM spin texture in the first Brillouin zone, with valleys labeled $\alpha$ ($\alpha^{\prime}$) and $\beta$ ($\beta^{\prime}$). (c) AMQSVH phase: spin-resolved bands (red/blue), Berry curvature $\Omega$ (black solid), spin Berry curvature $\Omega_{s}$ (black dashed), and the resulting quantized spin Hall conductivity $\sigma_s$. (d) Same as (c) but for the $\alpha$-AMQAVH phase at $U<0$, showing a quantized Hall conductivity $\sigma$. (e) Phase diagram as a function of $U$ and $M$. Parameters are given in the main text and SM SM
• Figure 3: (a) Nanoribbon bands of the AMQSVH phase, showing helical SVML edge states located at $\alpha$ and $\beta$ valleys. (b) Spin-resolved conductance for (a) as a function of Fermi energy $E_F$ and disorder strength $W$, comparing magnetic Anderson (MA), nonmagnetic Anderson (NMA), magnetic long-range (ML), and nonmagnetic long-range (NML) disorder. (c,d) Same as (a,b) but for the $\alpha$-AMQAVH phase with a single chiral edge channel. Parameters taken from Fig. \ref{['fig:model']}.
• Figure 4: (a) Top and side views of monolayer V$_2$TeSO. (b) Spin-resolved bands of (a) without SOC, showing AM Weyl points. (c) Same as (b) but with SOC; the corresponding (spin) Berry curvatures indicate the AMQSVH phase, whose helical edge states are shown in (d). (e) Same as (c) but for Ti substitution on the $V_A$ sublattice, which induces $U<0$ and yields the $\alpha$-AMQAVH phase with chiral edge state (inset). (f) Same as (e) but for Ti substitution on the $V_B$ sublattice ($U>0$), yielding the $\beta$-AMQAVH state.