Spin-polarized electrons on nonmagnetic surfaces: from the Rashba effect to topological insulators

The spin-orbit interaction in combination with the breaking of spatial inversion symmetry at surfaces can lead to spin-splitting of electronic surface states on non-magnetic metals. For partially filled bands, this so-called Rashba effect leads to pairs of non-degenerate Fermi surfaces with different Fermi wave vectors for opposite spins. It is this effect that is exploited in the concept of the spin field-effect transistor. In topological insulators, the non-trivial topology of the bulk electronic states forces the appearance of surface states, where one of the pair of spin-split electronic bands crosses the Fermi level and the other not. While the non-magnetic character of the surface is still maintained due to time-reversal symmetry, the directions of spin polarization and electron momentum are strictly locked in these non-degenerate surface states, which therefore develop a plethora of unusual properties. Being confined to the outermost atomic layers, these surface states can be studied in detail by angle-resolved photoemission spectroscopy (ARPES) and its spin-resolved version. They can also be manipulated by the means provided by surface science. Our group has built an angle-resolved photoemission apparatus equipped with a three-dimensional spin polarimeter, and introduced a powerful two-step fitting procedure for deriving three-dimensional spin polarization vectors. A survey will be given on recent results from our studies.