Peculiar Rashba splitting originating from the two-dimensional symmetry of the surface

A peculiar Rashba effect is found at a point in the Brillouin zone, where the time-reversal symmetry is broken, though this symmetry was believed to be a necessary condition for Rashba splitting. This finding obtained experimentally by photoemission measurements on a Bi/Si(111)-(sqrt(3) x sqrt(3)) surface is fully confirmed by a first-principles theoretical calculation. We found that the peculiar Rashba effect is simply understood by the two-dimensional symmetry of the surface, and that this effect leads to an unconventional nonvortical Rashba spin structure at a point with time-reversal invariance.