Spin-selective floquet chern topology in a driven anisotropic d-wave altermagnet

Abstract An anisotropic crystal-field (mass) term Bx≠By in a two-dimensional d-wave altermagnet breaks the fourfold symmetry that ties the two spin sectors, so that the d-wave exchange displaces the gap-closing points of the two spins in opposite directions and the two channels enter Chern-insulator windows of different size. This yields unequal Chern numbers C↑≠C↓ without spinorbit coupling or an external field. The parent two-orbital sector is of QiWuZhang form, so the topology is redistributed rather than created, and an isotropic parent keeps C↑=C↓. A circularly polarized drive renormalizes the two channels with distinct photoinduced mass corrections: a converged multi-harmonic FloquetSambe calculation fixes the boundary λc1 below which the spin-selective phase (C↑,C↓)=(+1,0), with normalized spin-Chern index Cs=(C↑−C↓)/2=+1/2, is robust and the first-order 1Ω expansion is quantitatively valid, and the ribbon spectrum there shows one chiral mode per edge in the zero gap, none in the π gap, and a central-replica weight above 0.93. Above λc1 the spin-down channel reaches C↓=+1 through nonperturbative replica hybridization that the 1Ω expansion misses, and at larger driving the central bands become trivial, so the drive delimits rather than creates the imbalance. On one geometric edge this gives a fully spin-polarized chiral charge current, verified at a representative driven point together with a disorder-robust Bott index and a Hall response quantized up to W≃3t0 and kBT≃0.2t0.