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Physics Colloquium: Weida Wu, "Toward robust quantum anomalous Hall effect"

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Physics Colloquium: Weida Wu, "Toward robust quantum anomalous Hall effect"
When Apr 25, 2018
from 04:00 PM to 05:00 PM
Where MR418N
Contact Name
Contact Phone 212-650-7443
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Physics Colloquium: Weida Wu, Title to be announced

Toward robust quantum anomalous Hall effect
Weida Wu
Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA, Piscataway, NJ, United States.
Abstract:
Quantum anomalous Hall (QAH) systems are of great fundamental interest and of potential application (e.g. quantum computing) because of dissipationless conduction without external magnetic field.1–3 The QAHE has been realized in magnetically doped topological insulator thin films.4–7 There is experimental evidence of chiral majorana fermion modes in QAH-superconductor heterostructure.8 However, full quantization requires extremely low temperature (<50 mK) in the initial works, though it was significantly improved with modulation doping or co-doping of magnetic elements.9,10 Improved ferromagnetism was indicated in these thin films, yet a direct evidence of long-range ferromagnetic order is lacking. In this talk, I will present direct evidence of long-range ferromagnetic order in thin films of Cr and V co-doped (Bi,Sb)2Te3 using low-temperature magnetic force microscopy with in-situ transport. The magnetization reversal process reveals a typical ferromagnetic domain behavior, i.e., domain nucleation and domain wall propagation, in contrast to much weaker magnetic signals observed in the end members, possibly due to superparamagnetic behavior observed in Cr doped TI films.11,12 The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way to high-temperature dissipationless conduction and exotic phenomena such as Axion magnetoelectric effect.13
References:
1.    Haldane, F. D. Model for a quantum Hall effect without Landau levels: Condensed-matter realization of the ‘parity anomaly’. Phys Rev Lett 61, 2015–2018 (1988).
2.    Onoda, M. & Nagaosa, N. Quantized anomalous Hall effect in two-dimensional ferromagnets: quantum Hall effect in metals. Phys. Rev. Lett. 90, 206601 (2003).
3.    Yu, R. et al. Quantized Anomalous Hall Effect in Magnetic Topological Insulators. Science (80-. ). 329, 61–64 (2010).
4.    Chang, C.-Z. et al. Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator. Science (80-. ). 340, 167–170 (2013).
5.    Checkelsky, J. G. et al. Trajectory of the anomalous Hall effect towards the quantized state in a ferromagnetic topological insulator. Nat Phys 10, 731–736 (2014).
6.    Kou, X. et al. Metal-to-insulator switching in quantum anomalous Hall states. Nat Commun 6, 8474 (2015).
7.    Chang, C. Z. et al. High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator. Nat Mater 14, 473–477 (2015).
8.    He, Q. L. et al. Chiral Majorana fermion modes in a quantum anomalous Hall insulator – superconductor structure. Science (80-. ). 299, 294–299 (2017).
9.    Ou, Y. et al. Enhancing the quantum anomalous Hall effect by magnetic codoping in a topological insulator. Adv Mater 30, 1703062 (2017).
10.    Mogi, M. et al. Magnetic modulation doping in topological insulators toward higher-temperature quantum anomalous Hall effect. Appl. Phys. Lett. 107, 182401 (2015).
11.    Lachman, E. O. et al. Visualization of superparamagnetic dynamics in magnetic topological insulators. Sci. Adv. 1, 1500740 (2015).
12.    Lachman, E. O. et al. Observation of superparamagnetism in coexistence with quantum anomalous Hall C = ±1 and C = 0 Chern states. npj Quantum Mater. 2, 70 (2017).
13.    Xiao, D. et al. Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures. Phys. Rev. Lett. 120, 56801 (2018).