From High Chern Number to Fractional Qauntum Anomalous Hall Effect
The quantum anomalous Hall (QAH) effect is a two-dimensional topological insulating state that has quantized Hall resistance of h/Ce2 and vanishing longitudinal resistance under zero external magnetic field, where C is called Chern number. A QAH system carries spin-polarized dissipation-free chiral edge current. Therefore, the QAH effect may have a considerable impact on future electronic and spintronic device applications for ultralow-power consumption. The QAH effect was first realized in the magnetically doped topological insulator (TI) in 2013[1]. More recently, it was also observed in the intrinsic topological magnet MnBi2Te4 and the moiré materials. In this talk, I will introduce our recent research progress on the QAH effect based on the magnetic topological insulator (TI) multilayers. Through fabricating magnetic TI multilayers, we realized the QAH effect with tunable Chern number up to 5. The Chern number of the QAH multilayers can be tuned by varying either the Cr doping concentration or the thickness of the magnetic TI layer[2]. Utilizing this property, we also studied the Chern number change-induced plateau phase transition between the C = 1 to C = 2 QAH states under zero magnetic field[3]. In the last part of this talk, I will also briefly introduce our ongoing STM study of fractional QAH effect devices.
Dr. Yi-Fan Zhao is currently a Kavli ENSI postdoctoral research fellow in the Department of Physics at the University of California, Berkeley. He received his Ph.D. in Physics in 2022 from Pennsylvania State University. Dr. Zhao is an expert in the molecular beam epitaxy (MBE) growth and electrical transport measurement of quantum materials, with a particular focus on the QAH insulators. His recent interests have expanded to include electrical transport and scanning probe study of QAH effect in moiré systems.
[1] C. Z. Chang et al., Science 340, 167 (2013).
[2] Y. F. Zhao et al., Nature 588, 419 (2020).
[3] Y. F. Zhao et al., Phys Rev Lett 128, 216801 (2022).
Probing Recombination in thin Film Quantum DTots
Formation of charged trions is detrimental to the luminescence quantum efficiency of colloidal quantum dot (QD) thin films as they predominantly undergo nonradiative recombination. In this regard, control of charged trion formation is of interest for both fundamental characterization of the quasi-particles and performance optimization. Using CdSe/CdS QDs as a prototypical material system, here we demonstrate a metal-oxide-semiconductor capacitor based on QD thin films for studying the background charge effect on the luminescence efficiency and lifetime. The concentration ratio of the charged and neutral quasiparticles in the QDs is reversibly controlled by applying a gate voltage, while simultaneous steadystate and time-resolved photoluminescence measurements are performed. Notably, the photoluminescence intensity is modulated by up to 2 orders of magnitude with a corresponding change in the effective lifetime. In addition, chip-scale modulation of brightness is demonstrated, where the photoluminescence is effectively turned on and off by the gate, highlighting potential applications in voltage-controlled electrochromics.
I K M Reaz Rahman is a Ph.D. candidate in the Electrical Engineering and Computer Sciences department at UC Berkeley, working under the supervision of Professor Ali Javey. He received a Master’s degree in 2020 and a Bachelor’s degree in 2017, both in Electrical and Electronic Engineering at Bangladesh University of Engineering and Technology before joining UC Berkeley. Reaz’s research interests include study of novel optoelectronic materials at the nanoscale and their application in solid-state electronics. He is working on the control of growth orientation using quantum confinement in layered two-dimensional semiconductors, which could enable enhanced device performance and coherent integration with silicon photonics at room temperature processing. In addition, his work focuses on the optoelectronic properties of these layered semiconductors in nanoscale which could serve as potential quantum emitters.