Opening a Surface Gap Many properties of topological insulators are a consequence of their so-called gapless surface state, in which electrons are protected from back-scattering, thanks to time-reversal symmetry. Breaking the time-reversal symmetry and opening a surface gap offers prospects for studying phenomena relevant to particle physics, such as axion electrodynamics. To achieve this, Chen et al. (p. 659 ; see the Perspective by Franz ) doped the three-dimensional topological insulator Bi 2 Se 3 with magnetic dopants and observed the opening of a surface gap. Simultaneous doping with charge dopants was used to shift the Fermi energy to the inside of the surface gap, thus achieving an insulating gapped Dirac state. Both the size of the gap and the position of the Fermi energy level were tunable by varying the nature and the density of the dopants.

Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator