Chiral-angle-selected spin splitting and spin-polarized conduction in nanotubes rolled from d-wave altermagnets

Abstract Altermagnets combine compensated collinear magnetic order withmomentum-dependent spin splitting, promising spin-polarized currentswithout stray fields. In two-dimensional (2D) d-wave altermagnets,however, this splitting changes sign between orthogonal directions andvanishes along nodal ones, so the spin polarization of a current dependson its direction relative to the crystal axes, and exploiting it requirescurrents aligned with specific crystallographic directions. Here we showthat rolling a d-wave altermagnet into a nanotube fixes this directionstructurally: although rolling removes the parent C4Tsymmetry, the inherited splitting survives through zone folding as aselection rule set by the chiral angle. Tight-binding, symmetry, andfirst-principles analyses of symmetric and Janus nanotubes show thatantinodal nanotubes are spin split, whereas the nodal nanotube remainsexactly spin degenerate owing to a surviving mirror symmetry combinedwith spin reversal. In V2O nanotubes, the ballistic transmissionpolarization decreases from 39% for the antinodal nanotube to zerofor the nodal one at comparable total transmission. Our results identifynanotubes rolled from altermagnets as geometry-selected spin channels fornanoscale spintronics.