I will describe our recent work on a new topological phase of matter: topological Weyl semimetal. This phase arises in three-dimensional topological (ordinary) insulators, which are close to a phase transition to the ordinary (topological) insulators. Breaking time-reversal symmetry in such materials, for example by doping with sufficient amount of magnetic impurities, leads to the formation of a Weyl semimetal phase, with two (or more) Dirac nodes, separated in momentum space. Such a topological Weyl semimetal possesses chiral edge states and a finite Hall conductivity in the absence of an external magnetic field, proportional to the momentum-space separation of the Dirac nodes. Weyl semimetal demonstrates a qualitatively different type of topological protection: the protection is provided not by a bulk band gap, such as for example in topological insulators, but by the separation of gapless Dirac nodes in momentum space. I will describe a simple way to engineer such materials using superlattice heterostructures, made of thin films of topological insulators.

References: arXiv:1110.1089; Phys. Rev. Lett. 107, 127205 (2011); Phys. Rev. B 83, 245428 (2011).