Topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. The bulk of such materials is insulating but the surface can conduct electric current with well-defined spin texture. In addition, the relativistic energy-momentum relationship of electrons in these materials provides a great opportunity to study the physics of relativity in a condensed matter system with the velocity of massless particles about 200 times slower than the light speed in vacuum.

Unlike other materials where the fragile surface states can be easily altered by details in the surface geometry and chemistry, topological insulators are predicted to have unusually robust surface states due to the protection of time-reversal symmetry. These unique states are protected against all time-reversal-invariant perturbations, such as scattering by non-magnetic impurities, crystalline defects, and distortion of the surface itself, and can lead to striking quantum phenomena such as quantum spin Hall effect, an image magnetic monopole induced by an electric charge, and Majorana fermions (whose anti-particle is itself) induced by proximity effect from a superconductor.

Extracting the electronic and structural properties of topological insulators is essential for both the understanding of the underlying physics and potential applications. As a direct method to study the electron band structures of solids, ARPES can yield rich information of the electronic bands of topological insulators, as demonstrated in our results on the realization of the large gap single Dirac cone topological insulator, Bi_{2}Te_{3}.

### Selected Publications

[1] Y. L. Chen *et al.* Experimental realization of a three-dimensional topological insulator, Bi_{2}Te_{3}. *Science* **325**, 178 (2009)

[2] Y. L. Chen *et al.* Massive Dirac fermion on the surface of a magnetically doped topological insulator. *Science* **329**, 659 (2010)

[3] Z. K. Liu *et al.* Discovery of a three-dimensional topological Dirac semimetal, Na_{3}Bi. *Science* **343**, 864 (2014)