  In a semiconductor, collective excitations of spin textures usually decay
rather fast due to D'yakonov-Perel' spin relaxation. The latter arises from
spin-orbit coupling, which induces wave-vector-dependent spin rotations that,
in conjunction with random disorder scattering, generate spin decoherence.
However, symmetries occurring under certain conditions can prevent the
relaxation of particular homogeneous and inhomogeneous spin textures. The
inhomogeneous spin texture, termed as persistent spin helix, is especially
appealing as it enables us to manipulate the spin orientation while retaining a
long spin lifetime. Recently, it was predicted that such symmetries can be
realized in zinc-blende two-dimensional electron gases if at least two
growth-direction Miller indices agree in modulus and the coefficients of the
Rashba and linear Dresselhaus spin-orbit couplings are suitably matched [PRL
117, 236801 (2016)]. In the present paper, we systematically analyze the impact
of the symmetry-breaking cubic Dresselhaus spin-orbit coupling, which
generically coexists in these systems, on the stability of the emerging spin
helices with respect to the growth direction. We find that, as an interplay
between orientation and strength of the effective magnetic field induced by the
cubic Dresselhaus terms, the spin relaxation is weakest for a low-symmetry
growth direction that can be well approximated by a [225] lattice vector. These
quantum wells yield a 30\% spin-helix lifetime enhancement compared to
[001]-oriented electron gases and, remarkably, require a negligible Rashba
coefficient. The rotation axis of the corresponding spin helix is only slightly
tilted out of the quantum-well plane. This makes the experimental study of the
spin-helix dynamics readily accessible for conventional optical spin
orientation measurements where spins are excited and detected along the
quantum-well growth direction.

Mesoscale and Nanoscale Physics