Strain- and Field-Tunable Nonrelativistic Spin Splitting and Wave-Symmetry-Dependent Spin Transport in Twisted Bilayer Altermagnets

TL;DR

This paper demonstrates that twisting bilayer antiferromagnetic or ferromagnetic 2D materials induces significant nonrelativistic spin splitting and spin transport effects through symmetry breaking, enabling spintronics without relying on spin-orbit coupling.

Contribution

It introduces a novel mechanism for nonrelativistic spin splitting in twisted bilayers, extending altermagnetism concepts to new geometries and symmetry conditions.

Findings

Twisting induces finite nonrelativistic spin splitting in various 2D magnetic materials.

Electric field and strain can tune the magnitude and nature of spin splitting.

Reversible symmetry-driven transitions enhance spin conductivity and control.

Abstract

Magnetism-driven nonrelativistic spin splitting (NRSS) provides a pathway toward efficient, spin-orbit-free spintronics. In centrosymmetric two-dimensional antiferromagnets, spin-polarized transport is symmetry-forbidden due to the combined space-time inversion ($PT$) symmetry. Here, by employing first-principles density functional theory and spin-group symmetry analysis, we demonstrate that twisting two antiferromagnetic or ferromagnetic monolayers of CoCl$_2$, AX$_2$ (A = Mn, V; X = Cl, Br, I), NiF$_2$, NiBr$_2$, FeS, CoS, MnTe$_2$, MnSe$_2$, and RuSe induces finite NRSS even in the absence of spin-orbit coupling. The relative twist breaks $[C_2||P]$ and $[E||C_{nz}]$ symmetries, giving rise to momentum-dependent spin polarization with distinct $d$-, $g$-, and $i$-wave altermagnetic patterns across the Brillouin zone. Using symmetry-invariant $k\cdot p$ modeling, we extract linear spin-splitting coefficients $\alpha^{(1)}$ ranging from 800-1100 meV\AA{}, comparable to SOC-induced Rashba-Dresselhaus strengths observed in noncentrosymmetric semiconductors. An out-of-plane electric field ($\mathcal{E}_z$) introduces Zeeman-type band splitting up to 110 meV at 10 MV/cm, while biaxial strain tunes the NRSS magnitude nearly linearly without altering symmetry. Crucially, the strain $u_{xx-yy}$ reduces the spin point group symmetry and drives reversible $g/i \rightarrow d$ wave-type transitions, resulting in finite spin conductivity and an enhanced spin-splitter angle (up to 18$^\circ$). These results extend the concept of altermagnetism to twisted bilayer geometries and establish a general route for realizing exchange-driven, nonrelativistic spin currents through symmetry engineering without requiring heavy elements or spin-orbit coupling.