  The recently discovered three dimensional or bulk topological insulators are
expected to exhibit exotic quantum phenomena. It is believed that a trivial
insulator can be twisted into a topological state by modulating the spin-orbit
interaction or the crystal lattice via odd number of band inversions, driving
the system through a topological quantum phase transition. By directly
measuring the topological invariants (for the method to directly measure
Fu-Kane {$\nu_0$}, see Hsieh \textit{et.al.,} Science 323, 919 (2009) at
http://www.sciencemag.org/content/323/5916/919.abstract) we report the
observation of a phase transition in a tunable spin-orbit system
BiTl(S{1-d}Se{d})2 (which is an analog of the most studied topological
insulator Bi2Se3, see Xia \textit{et.al.,} Nature Phys. 5, 398 (2009) at
http://www.nature.com/nphys/journal/v5/n6/full/nphys1294.html and Spin-Momentum
locking at
http://www.nature.com/nature/journal/v460/n7259/full/nature08234.html) where
the topological insulator state formation is visualized for the first time. In
the topological state, vortex-like polarization states are observed to exhibit
3D vectorial textures, which collectively feature a chirality transition of its
topological spin-textures as the spin-momentum locked (Z2 topologically
ordered) electrons on the surface go through the zero carrier density point.
Such phase transition and texture chirality inversion can be the physical basis
for observing \textit{fractional charge} (e/2) and other related fractional
topological phenomena.
