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Phys Rev Lett. 2016 Feb 12;116(6):066801. doi: 10.1103/PhysRevLett.116.066801. Epub 2016 Feb 9.

Weak Topological Insulators and Composite Weyl Semimetals: β-Bi4X4 (X=Br, I).

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Department of Physics, University of Texas at Dallas, Richardson, Texas 75080, USA.
School of Physics, Beijing Institute of Technology, Beijing 100081, China.


While strong topological insulators (STIs) were experimentally realized soon after they were theoretically predicted, a weak topological insulator (WTI) has yet to be unambiguously confirmed. A major obstacle is the lack of distinct natural cleavage surfaces to test the surface selective hallmark of a WTI. With a new scheme, we discover that β-Bi4X4 (X=Br, I), dynamically stable or synthesized before, can be a prototype WTI with two natural cleavage surfaces, where two anisotropic Dirac cones stabilize and annihilate, respectively. We further find four surface-state Lifshitz transitions under charge doping and two bulk topological phase transitions under uniaxial strain. Near the WTI-STI transition, there emerges a novel Weyl semimetal phase, in which the Fermi arcs generically appear at both cleavage surfaces whereas the Fermi circle only appears at one selected surface.

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