The topological phases of matter have 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 [1].
In this talk, after a brief introduction to the concept of topological phases of matter, I will talk about two examples of Dirac/Weyl semimetals, ZrTe5 [2] and PbSnTe under pressure [3]. The key concepts of the Dirac/Weyl semimetals are that they consist of 3D Weyl nodes which can be regarded as the effective magnetic monopoles/anti-monopoles that live in k-space (momentum space). These Weyl nodes produce strong Berry curvature or equivalently the effective magnetic field in k-space.
This allows the system to manifest large anomalous Hall effect even though the system has no magnetic ordering. In ZrTe5 [2], in-situ double-axis rotator was employed to detect the full 4p solid angular dependence of the anomalous Hall signals, or effectively the strength of the Berry curvature. Interestingly, the system shows large in-plane anomalous signals, the direct evidence of the Berry curvature coming from the Weyl nodes.
In order to further investigate and manipulate the effective magnetic monopoles/anti-monopoles (Weyl nodes) in the momentum space, pressure measurements of Pb1−xSnxTe were performed. Instead of the gap closing and immediately reopening, topological arguments [1] 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 [3]. 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. The results confirm the existence of a topological metallic phase protected by the Weyl nodes over a finite pressure interval.
Finally, possible future directions will be discussed.
[1] S. Murakami, New J. Phys. 9, 356 (2007)
[2] T. Liang et al., Nat. Phys. 14, 451 (2018)
[3] T. Liang et al., Sci. Adv. 3, e1602510 (2017)
Short Bio:
Dr. Tian Liang (梁 田) obtained his Bachelor’s degree in physics from University of Tokyo, Japan in 2009, and his Master’s degree there in 2010. He received his Ph.D. degree in physics from Princeton University, in 2016. After graduation, he moved to Stanford University as a post-doctoral research associate until now. His PhD research focused on the transport and magnetic properties in the topological phases of matter and current research focuses on microwave impedance microscopy, angle-resolved photoemission spectroscopy.
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