  We perform spin-resolved and spin-integrated angle-resolved photoemission
spectroscopy measurements on a series of compositions in the BiTl(S1-xSex)2
system, focusing on x-values in the vicinity of the critical point for the
topological phase transition (the band inversion composition). We observe quasi
two dimensional (2D) states on the outer boundary of the bulk electronic bands
in the trivial side (non-inverted regime) of the transition. Systematic
spin-sensitive measurements reveal that the observed 2D states are
spin-momentum locked, whose spin texture resembles the helical spin texture on
the surface of a topological insulator. These anomalous states are observed to
be only prominent near the critical point, thus are possibly related to strong
precursor states of topological phase transition near the relaxed surface.
