Symmetry-Protected Bipolar Skin Effect and its Topological Breakdown in Disordered Non-Hermitian Systems

The interplay between non-Hermitian topology and disorder remains a central puzzle in open quantum systems. While the non-Hermitian skin effect (NHSE) is known to be robust against weak perturbations, its fate under strong disorder, particularly in the presence of spin-orbit coupling (SOC), is not fully understood. Here, we uncover a Z_2 topological bipolar skin effect in a non-Hermitian Rashba chain, where spin-up and spin-down eigenstates localize at opposite boundaries. By strictly computing the Lyapunov exponents and introducing a biorthogonal spin-separation index, we map the global phase diagram and reveal a hierarchical breakdown of topology. We demonstrate that the Z_2 skin effect is protected against moderate disorder but collapses into a trivial skin phase before the ultimate onset of Anderson localization. Our results establish a distinct regime of disorder-robust topological non-reciprocity, distinguishable from both the trivial bulk limit and the Anderson localized phase.