  Spin-momentum locking in topological insulators and materials with
Rashba-type interactions is an extremely attractive feature for novel
spintronic devices and is therefore under intense investigation. Significant
efforts are underway to identify new material systems with spin-momentum
locking, but also to create heterostructures with new spintronic
functionalities. In the present study we address both subjects and investigate
a van der Waals-type heterostructure consisting of the topological insulator
Bi$_{2}$Se$_{3}$ and a single Se-Ta-Se triple-layer (TL) of H-type TaSe$_{2}$
grown by a novel method which exploits an interface reaction between the
adsorbed metal and selenium. We then show, using surface x-ray diffraction,
that the symmetry of the TaSe2-like TL is reduced from D$_{3h}$ to C$_{3v}$
resulting from a vertical atomic shift of the tantalum atom. Spin- and
angle-resolved photoemission indicates that, owing to the symmetry lowering,
the states at the Fermi surface acquire an in-plane spin component forming a
surface contour with a helical Rashba-like spin texture, which is coupled to
the Dirac cone of the substrate. Our approach provides a new route to realize
novel chiral two-dimensional electron systems via interface engineering that do
not exist in the corresponding bulk materials.
