报告摘要：Excited states of semiconducting materials underpin their physical properties including optical, electronic, and magnetic properties. First-principles computational methods have been vastly developed to simulate the ground state and excited state properties. In this talk, we first show our developed first-principles calculation formalism to simulate the trap-assisted Auger-Meitner (TAAM) recombination rate in semiconductors. The nonradiative recombination mechanism to limit the efficiency of wide-band-gap semiconductors has been a puzzle since the 1950s. With our developed first-principles calculation formalism to calculate the TAAM rate, we have shown that the TAAM process is orders of magnitude faster than previously studied mechanisms for semiconductors with band gaps larger than 2.5 eV [1]. Thus, our work showed the crucial role of TAAM in limiting the efficiency of wide-band-gap optoelectronic devices and solved the decade-long puzzle.

In the second part of the talk, we show our discovery of topological phases in graphene nanoribbons (GNRs), and our developed formalism and methodology in characterizing the topological phase of GNRs of any shape, width, and end terminations. We show our novel design of a boron and nitrogen co-doped GNR with tunable topological phases by laboratory transverse electric fields. Localized topological interface states can be tuned easily by controlling the profile of the transverse electric field [2], which provides new insights in designing quantum dot qubits with tunable interactions. We also show our calculation on the excited electronic and magnetic properties of a nitrogen doped zigzag GNR using GW calculations, and our theoretically predicted band splitting phenomena have led to the first experimental verification of magnetism in GNR [3] which was predicted decades ago.

[1] F. Zhao, M. E. Turiansky, A. Alkauskas, and C. G. Van de Walle, Physical Review Letters 131, 056402 (2023).

[2] F. Zhao, T. Cao, and S. G. Louie, Physical Review Letters 127, 166401 (2021).

[3] R. E. Blackwell, F. Zhao, E. Brooks, J. Zhu, I. Piskun, S. Wang, A. Delgado, Y.-L. Lee, S. G. Louie, and F. R. Fischer, Nature 600, 647-652, (2021). 

报告人简介：Fangzhou Zhao is an Elings Prize Postdoctoral Fellow working in the Materials Department at the University of California, Santa Barbara, and will start research work in Max Planck Institute for the Structure and Dynamics of Matter as a Humboldt fellow starting in 2024. Fangzhou obtained his Ph.D. in Physics from UC Berkeley with Prof. Steven G. Louie in 2021, and he obtained his B.S. degree in Physics from Zhiyuan College, Shanghai Jiao Tong University in 2015. His graduate work consists of topological electronic and optical properties of quasi-one-dimensional nanomaterials like graphene nanoribbons and carbon conjugated systems, as well as GW plus Bethe-Salpeter equation formalism to calculate excited state properties of materials. His postdoctoral work as an Elings Prize Postdoc Fellow focuses on first-principles calculations modeling of nonradiative recombination process in semiconductors. He has recently developed a first-principles calculation formalism and code to calculate the rate of trap-assisted Auger-Meitner recombination in semiconductors and was honored with the Corbett Prize at the 32^nd International Conference on Defects in Semiconductors, and NERSC High Performance Computing Achievement Award for High Impact Scientific Achievement.