  We show that attractive two-dimensional spinor Bose-Einstein condensates with
helicoidal spatially periodic spin-orbit coupling (SOC) support a rich variety
of stable fundamental solitons and bound soliton complexes. Such states exist
with chemical potentials belonging to the semi-infinite gap in the band
spectrum created by the periodically modulated SOC. All these states exist
above a certain threshold value of the norm. The chemical potential of
fundamental solitons attains the bottom of the lowest band, whose locus is a
ring in the space of Bloch momenta, and the radius of the ring is a
non-monotonous function of the SOC strength. The chemical potential of soliton
complexes does not attain the band edge. The complexes are bound states of
several out-of-phase fundamental solitons whose centers are placed at local
maxima of the SOC-modulation phase. In this sense, the impact of the helicoidal
SOC landscape on the solitons is similar to that of a periodic two-dimensional
potential. In particular, it can compensate repulsive forces between
out-of-phase solitons, making their bound states stable. Extended stability
domains are found for complexes built of two and four solitons (dipoles and
quadrupoles, respectively). They are typically stable below a critical value of
the chemical potential.
