  We compute the spectrum of pure spin current injection in ferroelectric
single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the
coherent spin dynamics of optically excited conduction states split in energy
by spin orbit coupling. The velocity of spins is calculated as a function of
incoming photon energy and angle of linearly polarized light within a full
electronic band structure scheme using density functional theory. We find peak
speeds of 250, 210, 180 and 154 Km/s for SnS, SnSe, GeS and GeSe, respectively
which are an order of magnitude larger than those found in bulk semiconductors,
e.g., CdSe and GaAs. Interestingly, the spin velocity is independent of the
direction of polarization of light in a range of photon energies. Our results
demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce
on demand spin-velocity injection for spintronics applications.
