  Spin splitting of Rashba states in two-dimensional electron system provides a
promising mechanism of spin manipulation for spintronics applications. However,
Rashba states realized experimentally to date are often outnumbered by
spin-degenerated substrate states at the same energy range, hindering their
practical applications. Here, by density functional theory calculation, we show
that Au one monolayer film deposition on a layered semiconductor surface
beta-InSe(0001) can possess "ideal" Rashba states with large spin splitting,
which are completely situated inside the large band gap of the substrate. The
position of the Rashba bands can be tuned over a wide range with respect to the
substrate band edges by experimentally accessible strain. Furthermore, our
nonequilibrium Green's function transport calculation shows that this system
may give rise to the long-sought strong current modulation when made into a
device of Datta-Das transistor. Similar systems may be identified with other
metal ultrathin films and layered semiconductor substrates to realize ideal
Rashba states.
