Unexpected radial and out-of-plane spin texture in the giant Rashba semiconductor BiTeI

BiTeI is a model bulk Rashba semiconductor, with the 3.85 eVÅ record-high Rashba parameter among any known materials. A standard Rashba picture features an in-plane chiral spin texture on a pair of concentric Fermi surfaces, with the strict spin-momentum locking of the quasiparticles. However, for the first time, here we observe the unconventional spin textures beyond classical Rashba physics in BiTeI using high-resolution spin- and angle-resolved photoemission spectroscopy with full 3D vectorial spin detection. By simultaneously resolving all three spin components (P_X, P_Y, and P_Z), we achieve a band-resolved determination of the complete three-dimensional spin-vector texture for both the inner and outer Rashba-split bands. While the tellurium-terminated surface largely preserves the canonical tangential spin texture, the Iodine-terminated surface exhibits pronounced radial and sizable out-of-plane spin components, resulting in an unusual spin configuration. These findings reveal that the spin texture in BiTeI can be fundamentally reconfigured by surface termination, establishing surface engineering as a powerful route to control spin textures beyond the ideal Rashba paradigm.