报告摘要：

　　In this seminar, I will first introduce the main concepts of bilayer exciton insulator, a new type of charge neutral quantum liquid recently realized in 2D materials. Then I will mostly focus on the electromagnetic responses of bilayer excitonic insulators (EIs) and identify two distinct collective modes: (1) Two gapped plasmon modes couple to the layer symmetric gauge field. The transverse mode is nearly dispersionless in the long wavelength limit, while the longitudinal mode, accounting for total charge fluctuations, has a linear dispersion with velocity proportional to 2D electrical polarizability. (2) A gapless phase (Goldstone) mode and a gapped amplitude mode, associated with the fluctuations of EI order parameter, couple to the layer antisymmetric gauge field. In the long wavelength and low frequency limit, the phase mode behaves like an acoustic phonon with speed inversely proportional to the square root of exciton compressibility. Significantly, its linear dispersion yields a cubic frequency dependence of the real admittance in microwave impedance microscopy (MIM), providing a method to detect the Goldstone mode directly.

References:

[1] SiminNie, Yan Sun, Fritz B. Prinz, Zhijun Wang, HongmingWeng, Zhong Fang, and Xi Dai, Magnetic Semimetals and Quantized Anomalous Hall Effect in EuB6, Phys. Rev. Lett. 124, 076403, (2020).

[2] Jian Peng Liu and Xi Dai, Orbital magnetic states in moiregraphene systems, Nature Review Physics, 3, 367 (2021)

[3] Yuanfeng Xu, Zhida Song, Zhijun Wang, Hongming Weng and Xi Dai, Higher-order Topology of Axion Insulator EuIn2As2, Phys. Rev. Lett. 122, 256402 (2019).

[4] Jianpeng Liu, Zhen Ma, Jinhua Gao and Xi Dai, Quantum valley Hall effect, orbital magnetism, and anomalous Hall effect in twisted multilayer graphene systems, Phys. Rev. X 9, 031021 (2019).

[5] Jianpeng Liu, Junwei Liu and Xi Dai, The pseudo-Landau-level representation of twisted bilayer graphene: band topology and the implications on the correlated insulating phase, Phys. Rev. B 99, 155415 (2019).

报告人简介：

　　Professor Xi Dai obtained his Bachelor and Master’s degree (1993 &1996) from Zhejiang University and PhD (1999) from Institute of Theoretical Physics, CAS in China. After graduation, he did postdoctoral research in HKUST, Boston College and Rutegrs University. From 2004-2007, he worked in University of Hong Kong as a research assistant professor. In 2007, he joined Institute of Physics, CAS as a professor in the theory division. In September 2017, he joined HKUST as a professor in the Department of Physics. Professor Dai received several important awards during the past ten years including the OCPA “Achievements in Asia Award” (2012), the CAS prize for outstanding scientific achievements and the KC Chou foundation “Fundamental Physics Prize” (2014), the American Physical Society’s James C. McGroddy Prize for New Materials (2019), the first class prize of the State Natural Science Award (2023). He is a Fellow of American Physical Society (APS).