Unexpected mass acquisition of Dirac fermions at the quantum phase transition of a topological insulator

T. Sato, Kouji Segawa, K. Kosaka, S. Souma, K. Nakayama, K. Eto, T. Minami, Yoichi Ando, T. Takahashi

Research output: Contribution to journalArticlepeer-review

196 Citations (Scopus)


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 to realize 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 acquire 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 TlBiSe 2 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.

Original languageEnglish
Pages (from-to)840-844
Number of pages5
JournalNature Physics
Issue number11
Publication statusPublished - 2011 Nov

ASJC Scopus subject areas

  • Physics and Astronomy(all)


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