  The persistent spin helix (PSH) is robust against spin-independent scattering
and renders an extremely long spin lifetime, which can improve the performance
of potential spintronic devices. To achieve the PSH, a unidirectional spin
configuration is required in the momentum space. Here, T-XY (X$\neq$Y=P, As, Sb
and Bi) monolayers with dynamical, mechanical and thermal stabilities are
predicted to intrinsically possess PSH. Due to the $C_{2\upsilon}$ point-group
symmetry, a unidirectional spin configuration is preserved in the out-of-plane
direction for both conduction and valence bands around the high-symmetry
$\Gamma$ point. That is, the expectation value of the spin $S$ only has the
out-of-plane component $S_z$. The application of an out-of-plane external
electric field can induce in-plane components $S_x$ and $S_y$, thus offering a
promising platform for the on-off logical functionality of spin devices. Our
work reveals a new family of T-phase two-dimensional (2D) materials, which
could provide promising applications in spintronic devices.
