Dual Topology as a Fingerprint of Relativistic Altermagnetism in AgF$_2$ Monolayer

Abstract: Altermagnets have emerged as a fertile ground for quantum phenomena, but topological phases unifying different quasiparticles remain largely unexplored. Here, we demonstrate that monolayer AgF$_2$ hosts a dual topological state, driven by a single ferroelastic distortion. This polar transition breaks inversion symmetry and unleashes relativistic spin-orbit effects, simultaneously imparting non-trivial topology to electrons and magnons. The result is valence bands with opposite Chern numbers, $C^E=\pm3$, and a magnon spectrum with a full topological gap and chiral bands, $C^M=\pm1$. This work realizes topological altermagnonics in a tangible material platform, with a clear experimental fingerprint in the transverse thermal Hall effect. The coexistence of fermionic and bosonic topology in AgF$_2$ opens new directions for designing intrinsically hybrid quantum matter.