Citation of the 2015 (the 3rd ) Nishina Asia Award“The first experimental realization of the quantum anomalous Hall effect”
by Dr. He, Ke
When a magnetic field is applied to a semiconductor or metal, a transverse electric fieldappears in the direction perpendicular to both the electric current and the magnetic field.
This phenomenon is the well-known Hall effect. In many magnetic materials the Hall
effect is observed without applying a magnetic field, as the internal field due tospontaneous magnetization plays the part of an applied field. This is called the“anomalous Hall effect”. Another spectacular manifestation of the Hall effect is the“quantum Hall effect” observed in a two-dimensional electron system under a strong
magnetic field, where the Hall conductivity is quantized to integer multiples of e2/h. This
year’s award is for the discovery of the “quantum anomalous Hall effect”, in which theanomalous Hall effect is quantized. In other words, the phenomenon can be viewed as thequantum Hall effect in the absence of an external magnetic field. The quantum anomalouseffect, which involves dissipationless current in a zero magnetic field, is expected to playa crucial role for low-energy-consumption electronic device applications.
The quantum anomalous Hall effect was theoretically predicted in 1988. In spite ofextensive efforts worldwide, the predicted effect defied experimental verification. Since
2009, Dr. Ke He and his collaborators have carried out systematic studies of material
preparation and characterization of a class of material called “topological insulators”, inorder to realize experimentally the quantum anomalous Hall effect. Armed with his
expertise in thin-film growth by molecular beam epitaxy (MBE), angle-resolvedphotoemission spectroscopy and scanning tunneling microscopy, Dr. He played a crucialrole in the band-engineering and MBE growth of topological insulator samples toward
the experimental realization of the quantum anomalous Hall effect. He managed toprepare a series of Bi2Se3 thin films of well-controlled composition, thickness and quality,with which he successfully demonstrated the evolution, with the number of atomic layers,of the electronic structure of the two-dimensional surface state of the topological insulator.
Through this study, Dr. He and his colleagues clarified the relationships between
magnetism, band structure and topological properties in magnetic topological insulators,
and accumulated the know-how for tuning the materials parameters for the discovery ofthe quantum anomalous Hall effect.
In 2013, the quantum anomalous Hall effect was finally observed experimentally inthin films of Cr-doped (BixSb1-x)2Te3 topological insulators by Dr. He and his
collaborators. Growth of such four-element thin films, with fine-tuning of materials

parameters for the observation of the quantum anomalous Hall effect, was by far the mostdifficult part of the research program. Therefore, the contributions of Dr. He were crucial
for the successful observation of the quantized anomalous Hall effect. This discovery not
only settled the 25-year-long search for the quantum Hall effect without magnetic fields,
but also opened the door for many other novel quantum phenomena associated with
topological insulator materials. This work represents one of the most important
experimental breakthroughs of condensed matter physics in recent years. It is remarkablethat this Asian research group has made such an important breakthrough, leading therapidly developing research field of topological materials. Dr. He played the most crucialrole in this outstanding achievement and is well qualified as a recipient of the Nishina
Asia Award.
References
1. Zhang, Y.; He, K.; Chang, C.-Z.; Song, C.-L.; Wang, L.-L.; Chen, X.; Jia, J.-F.; Fang, Z.;
Dai, X.; Shan, W.-Y.; Shen, S.-Q.; Niu, Q.; Qi, X.-L.; Zhang, S.-C.; Ma, X.-C.; Xue, Q.-K.,“Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensionallimit”, Nature Physics 6, 584 (2010).
2. Zhang, J.; Chang, C.-Z.; Tang, P.; Zhang, Z.; Feng, X.; Li, K.; Wang, L.-l.; Chen, X.; Liu,C.; Duan, W.; He, K.; Xue, Q.-K.; Ma, X.; Wang, Y., “Topology-driven magnetic quantum
phase transition in topological insulators”, Science 339, 1582 (2013).
3. Chang, C.-Z.; Zhang, J.; Feng, X.; Shen, J.; Zhang, Z.; Guo, M.; Li, K.; Ou, Y.; Wei, P.;
Wang, L.-L.; Ji, Z.-Q.; Feng, Y.; Ji, S.; Chen, X.; Jia, J.; Dai, X.; Fang, Z.; Zhang, S.-C.;
He, K.; Wang, Y.; Lu, L.; Ma, X.-C.; Xue, Q.-K., “Experimental observation of thequantum anomalous Hall effect in a magnetic topological insulator”, Science 340, 167(2013).