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Topological states of matter including topological insulators received explosive research attention since 2006. Electronic properties in these novel materials such as the unique surface states and the presence of spin polarized current are crucial to therealization of future quantum computation based on topological logic. In the view of electronic structure, the most decisive signature of topologically nontrivial systems is the “Dirac cone” surface electronic states. In usual topological insulators, such a state consists of spin polarized surface electrons and is always a closed contour. Recently, a new form of topological matter, the so-called “topological semimetal”, emerges as a new direction in this field. The valence and conduction bands in these systems touches at discrete points in the momentum space, rendering the system metallic instead of insulating. Importantly, it is pointed out by first principles calculations that the surface states in these semimetals are no longer closed contours but “Fermi arcs” that begins at a touching point of the valence and conduction bands, and ends at another such touching point. This unique spectroscopic signature leads directly to surface transport properties unforeseen in any solid.
Chang Liu, an Associate Professor at the department of physics of SUSTC, recently published a paper in Science in collaboration with his former group members, reporting the experimental discovery of a “Dirac semimetal”. Using angle resolved photo emission spectroscopy (ARPES), they studied the low-lying electronic structure on the side surface (100) of Na3Bi, recognizing two touching points between the valence and conduction bands, along with a pair of Fermi arc surface states linking between these two points. Spin resolved ARPES further revealed that both Fermi arcs consist of highly spin-polarized electrons. This experiment reveals a brand-new form of two dimensional electron gas in which spin-polarized electrons “travel from” the crystal bulk to one of the touching points, then propagate on the crystal surface to the other touching point before they “return to” the bulk. Such form of electron propagation is never observed in other systems.
The above mentioned experiment is led by Prof. Zahid Hasan’s group at Princeton University. His PhD student Suyang Xu and SUSTC’s Chang Liu are equal contributed first authors of this paper.