### Tian Liang

###
(Stanford University)

## Anomalous Transport Properties in Topological Phases of
Matter

The notion of topological phases of matter has become one of the
central fields in modern physics. The past decade has witnessed the
explosion of the theoretical and experimental developments in this
field, expanding from the traditional 2D and 3D TIs (topological
insulators), to now including the topological semimetals, notably
Dirac/Weyl semimetals. The key concepts of the Dirac/Weyl semimetals
are that they consist of 3D Weyl nodes which can be regarded as the
monopoles/anti-monopoles that live in k-space (momentum space),
producing strong Berry curvature (effective magnetic field in k-space).

Recently, one of the new routes to generate and manipulate the
monopoles/anti-monopoles in Weyl semimetals was proposed by Murakami
[1]. The picture of how a gap closes in a semiconductor has been
radically transformed by topological concepts. Instead of the gap
closing and immediately reopening, topological arguments predict that,
in the absence of inversion symmetry, a metallic phase protected by
Weyl nodes persists over a finite interval of the tuning parameter (for
example, pressure P). The gap reappears when the Weyl nodes mutually
annihilate. I will talk about evidence that Pb1−xSnxTe exhibits this
topological metallic phase [2]. Using pressure to tune the gap, we have
tracked the nucleation of a Fermi surface droplet that rapidly grows in
volume with P. In the metallic state, we observe a large Berry
curvature, which dominates the Hall effect. Moreover, a giant
negative magnetoresistance is observed in the insulating side of phase
boundaries, in accord with ab initio calculations. The results confirm
the existence of a topological metallic phase over a finite pressure
interval. Finally, possible future directions and some open questions
are discussed.

[1] S. Murakami, New J. Phys. 9, 356 (2007)

[2] T. Liang et. al., Sci. Adv. 3, e1602510 (2017)