Physics

Prof Nitin Samarth
Pennsylvania State University

Abstract:

When time-reversal symmetry is broken in a three-dimensional topological insulator, the twodimensional helical surface Dirac states are replaced by one-dimensional chiral edge states. This is most readily achieved in magnetically-doped tetradymite semiconductor thin films and heterostructures grown by molecular beam epitaxy [1]. Analogous to the well-known quantum Hall edge states, these dissipationless ‘quantum anomalous Hall’ edge states are characterized by a vanishing longitudinal conductance (σxx = 0) and a quantized Hall conductance (σxy = h/e2 ). In strong contrast to the quantum Hall effect, however, the quantum anomalous Hall effect does not require Landau levels: it occurs even in the absence of an external magnetic field and can be observed in highly disordered samples (Drude mobility ~ 200 cm2 /V.s). We provide an overview of the development of this phenomenon, from its conception in theory [2] to its experimental observation [3], with examples from our recent work that probes the interplay between the magnetization and the electronic transport in magnetically-doped topological insulators [4-6]. Finally, we show how epitaxially engineered heterostructures that incorporate different magnetic doping may be used to realize a novel ‘axion insulator’ state [7].

This work is supported by ONR, ARO MURI, and NSF-MIP.

1. N. Samarth, Nature Materials 16, 1068-1076 (2017).

2. R. Yu, W. Zhang, H. J. Zhang, S. C. Zhang, X. Dai, and Z. Fang, Science 329, 61 (2010).

3. C. Z. Chang, et al., Science 340, 167 (2013).

4. A. Kandala, A. Richardella, S. Kempinger, C. X. Liu, and N. Samarth, Nat. Commun. 6, 7434     (2015).

5. E. O. Lachman, et al., Sci. Adv. 1, e1500740 (2015).

6. M. H. Liu, W. D. Wang, A. R. Richardella, A. Kandala, J. Li, A. Yazdani, N. Samarth, and N.        P. Ong, Sci. Adv. 2, e1600167 (2016).