  Finding a new class of materials supporting a long spin lifetime is essential
in development of energy-saving spintronics, which is achievable by using a
persistent spin helix (PSH) materials. However, for spintronic devices, the PSH
states with large spin splitting is required for operation at room temperature.
By employing first-principles calculations, we show that the PSH states with
large spin splitting are achieved in the SnSe monolayer (ML) functionalized by
a substitutional halogen impurity. We find the PSH states in the Fermi level
where k-space Fermi surface is characterized by the shifted two loops,
dominated by out-of-plane spin orientations. We clarify the PSH states in term
of an effective k.p Hamiltonian obtained from symmetry consideration. Finally,
large spin-orbit strength in the PSH states with a substantially small
wavelength are found, rendering that this system is promising for the
development of an efficient and high-density scalable spintronic devices
operating at room temperatures.
