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Recent advances in next generation semiconductor devices: Topological insulator and spintronics applications
Date: 2015/8/24             Browse: 387

Speaker: Xufeng Kou

Time: August 24th, 9:00-9:45 am

Location: Room 220, Building 8, Yueyang Road Campus


Topological insulator (TI) is a class of novel 2D semiconductors featuring with an energy gap in its bulk band structure and unique Dirac-like metallic states on the surface. The spin of surface electron is locked perpendicular to the momentum owing to the strong spin-orbit coupling, thus making TI the suitable conduction channel for low-power, high-speed spintronics applications. In parallel with the pursuit of the massless Dirac fermions, it is of equal significance to break the time-reversal-symmetry (TRS) of the TI surfaces by introducing the perpendicular magnetic interaction. The intrinsic electrical, magnetic, and optical properties of the magnetic TI system are subject to the interplay between the band topology and the magnetic orders, thus the functionalities of associated physics and applications can be further multiplied.

In this talk, I will first present the investigation of magnetic/electrical properties of magnetic TIs. By doping transition metal element (Cr), we achieve robust ferromagnetic order in the host TI films with the magnetic easy-axis out-of-plane. With additional top-gating capability, we realize the electric-field-controlled ferromagnetism in the magnetic TI systems, and demonstrate such magneto-electric effects can be effectively manipulated, depending on the doping profile engineering. Our results are important for the TRS-breaking topological physics, and they may also lead to novel TI-based multifunctional applications.

In the second part of my talk, I would like to discuss about our recent progress on the quantum anomalous Hall effect (QAHE). With robust magnetism introduced by magnetic doping, we observe the dissipationless edge conduction in the macroscopic magnetic TI devices without the presence of any external magnetic field. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the nonlocal transport measurements. More importantly, we also demonstrate that the stability of the quantized Hall conductance of e2/h is well-maintained as the film thickness varied across the 2D hybridization limit. With additional quantum confinement, we discover the metal-to-insulator switching between two opposite QAHE states, and reveal the universal QAHE phase diagram in the thin magnetic TI systems. The observation of the scale-invariant dissipationless chiral propagation in the magnetic TI system thus makes a major stride towards ideal low-power interconnect applications.


Xufeng Kou received his BS (with honor) in Chu Kochen Honors College and Optical Engineering from Zhejiang University (2009). From 2009 to 2015, he received his MS and PhD in Electrical Engineering from University of California, Los Angeles (UCLA), under the direction of Raytheon Chair Professor Kang L. Wang. Xufeng's research interest includes novel semiconductors (topological insulators and dilute magnetic semiconductors) and their nano-electronics/spintronics applications. During his PhD career, Xufeng has co-authored 38 peer-reviewed journal papers including Nature Mater,Nature Nano., Phys. Rev. Lett., Nano Lett., J. Amer. Chem. Soc., and ACS Nano., with more than 900 citations (h-index of 16). He also holds several awards including the Distinguished PhD Dissertation Award (2015), Qualcomm Innovation Fellowship (2012), and Chinese Outstanding Student Abroad Scholarship (2013).           

SIST-Seminar 15037