In a 2D electron gas lacking inversion symmetry, the Rashba interaction H = α(k × ẑ)·σ splits the Fermi surface into two spin-helical rings. Electrons' spin locks perpendicular to their momentum.
Rashba SOC arises from structural inversion asymmetry (e.g., at heterointerfaces or surfaces). Spin texture is purely tangential (helical) — exploited in spintronics and topological devices.