  The ability to control the spin textures in semiconductors is a fundamental
step toward novel spintronic devices, while seeking desirable materials
exhibiting persistent spin texture (PST) remains a key challenge. The PST is
the property of materials preserving a unidirectional spin orientation in the
momentum space, which has been predicted to support an extraordinarily long
spin lifetime of carriers. Herein, by using first-principles density functional
theory calculations, we report the emergence of the PST in the two-dimensional
(2D) germanium monochalcogenides (GeMC). By considering two stable formations
of the 2D GeMC, namely the pure GeX and Janus Ge2XY monolayers (X, Y = S, Se,
and Te), we observed the PST around the valence band maximum where the spin
orientation is enforced by the lower point group symmetry of the crystal. In
the case of the pure GeX monolayers, we found that the PST is characterized by
fully out-of-plane spin orientation protected by C2v point group, while the
canted PST in the y-z plane is observed in the case of the Janus Ge2XY
monolayers due to the lowering symmetry into Cs point group. More importantly,
we find large spin-orbit coupling (SOC) parameter in which the PST sustains,
which could be effectively tuned by in-plane strain. The large SOC parameter
observed in the present systems leads to the small wavelength of the spatially
periodic mode of the spin polarization, which is promising for short spin
channel in the spin Hall transistor devices.
