Very large Rashba coupling by a staggered crystal field in the inversion-symmetric BaNiS$_2$ semi-metal*

By means of a single-crystal angular resolved photoemission spectroscopy study combined with first principles calculations, we give evidence of a giant Rashba coupling $\alpha_R \approx 0.25$ eV \AA~ leading to an energy splitting as large as $\Delta\epsilon \approx 150$ meV in a novel situation of an inversion-symmetric system - the BaNiS$_2$ semi-metal - composed of comparatively light elements. This finding is explained by a huge staggered crystal field $\approx 1.4$ V/\AA~ associated with a peculiar non-symmorphic square-pyramidal structure, which produces a local inversion asymmetry at the Ni site. We show that this very effective mechanism of Rashba coupling enables large changes of the electronic structure of solids without using either heavy elements or external fields.