Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Unexpected mass acquisition of Dirac fermions at the quantum phase
  transition of a topological insulator   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.

15
The categories of this paper are: nucl-ex
22
The categories of this paper are: chao-dyn
12
The categories of this paper are: nucl-th
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
3
The categories of this paper are: physics
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
26
The categories of this paper are: bayes-an
24
The categories of this paper are: dg-ga
11
The categories of this paper are: hep-lat
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph