## Abstract

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(S_{1-x}Se_{x}) 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 language | English |
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Pages (from-to) | 840-844 |

Number of pages | 5 |

Journal | Nature Physics |

Volume | 7 |

Issue number | 11 |

DOIs | |

Publication status | Published - 2011 Nov |

## ASJC Scopus subject areas

- Physics and Astronomy(all)