学术沙龙 （6月28日周四14:30-16:00）

主持人：郭东辉 教授

欢迎广大师生前来参与！

【摘要】

Experimental signatures [1] of chiral Majorana edge mode is captured by the magneto-electric transport measurements in a hybrid system of a quantum anomalous Hall insulator thin film partially capped by a superconductor layer. The external magnetic field serves as a “knob” to tune the system into different topological regimes that allow the degenerate and non-degenerate propagation of Majorana edge modes. This tuning was signified as quantized conductance transitions among e²/h, 0.5e²/h, and 0 as the external magnetic field was swept, which correspond to the topological superconducting phases with Chern numbers of 2, 1, and 0. This phase transition was recently further investigated by the edge tunneling spectra, which show the interference signature of the chiral Majorana fermions. When the Chern number is odd, the single chiral Majorana fermion contributes to a tunneling conductance quantized to 2e²/h. Otherwise conductance dips appear, which is attributed to the destructive interference of the degenerate Majorana fermions.

The second part of this seminar demonstrates the use of antiferromagnets (AFMs) to manipulate the topological order in topological insulator (TI). By constructing heterostructures exhibiting Néel order in an antiferromagnet and ferromagnetic order in a magnetic TI, emergent interfacial magnetic effects were realized, which can be tailored through artificial structural engineering [2]. In another TI/antiferromagnet heterostructure, through a ‘seeding effect’, the pinned spins at the interface leads to a tunable modification of the averaged real-space topological charge. This effect experimentally manifests as a modification of the field-dependent geometric Hall effect when the system is field-cooled along different directions. This heterostructure represents a new platform for manipulating magnetic topological transitions using antiferromagnetic order [3].

References:

1 He, Q. L. et al. Chiral Majorana fermion modes in a quantum anomalous Hall insulator-superconductor structure. Science 357, 294-299, (2017).

2 He, Q. L. et al. Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures. Nature Materials 16, 94-100, (2017).

3 He, Q. L. et al. Exchange-biasing topological charges by antiferromagnetism. Nature Communications, in press (2018).

【个人简介】

Q.L. He received his BS from Sun Yat-sen University in 2011 and PhD from the Hong Kong University of Science and Technology in 2015. He moved to University of California, Los Angeles, as a postdoc researcher for three years. In 2018, he joined ICQM, Peking University as an Assistant Professor. His main research interests lie in the studies of molecular beam epitaxial growth and characterization of topological (crystalline) insulators, magnetic materials, and superconductor. He also devotes to the optoelectronic and magnetoelectric studies of metallic and semiconductor thin films and nanostructures, including quantum dots, nanowires and nanoribbons, as well as structural and chemical analysis of hetero-interfaces.