  We have performed spin- and angle-resolved photoemission spectroscopy of
Bi2Te3 and present the first direct evidence for the existence of the
out-of-plane spin component on the surface state of a topological insulator. We
found that the magnitude of the out-of-plane spin polarization on a hexagonally
deformed Fermi surface (FS) of Bi2Te3 reaches maximally 25% of the in-plane
counterpart while such a sizable out-of-plane spin component does not exist in
the more circular FS of TlBiSe2, indicating that the hexagonal deformation of
the FS is responsible for the deviation from the ideal helical spin texture.
The observed out-of-plane polarization is much smaller than that expected from
existing theory, suggesting that an additional ingredient is necessary for
correctly understanding the surface spin polarization in Bi2Te3.
