  The three-dimensional (3D) topological insulator is a novel quantum state of
matter where an insulating bulk hosts a linearly-dispersing surface state,
which can be viewed as a sea of massless Dirac fermions protected by the
time-reversal symmetry (TRS). Breaking the TRS by a magnetic order leads to the
opening of a gap in the surface state and consequently the Dirac fermions
become massive. It has been proposed theoretically that such a mass acquisition
is necessary for realizing novel topological phenomena, but achieving a
sufficiently large mass is an experimental challenge. Here we report an
unexpected discovery that the surface Dirac fermions in a solid-solution system
TlBi(S1-xSex)2 acquires a mass without explicitly breaking the TRS. We found
that this system goes through a quantum phase transition from the topological
to the non-topological phase, and by tracing the evolution of the electronic
states using the angle-resolved photoemission, we observed that the massless
Dirac state in TlBiSe2 switches to a massive state before it disappears in the
non-topological phase. This result suggests the existence of a condensed-matter
version of the "Higgs mechanism" where particles acquire a mass through
spontaneous symmetry breaking.
