Friday Quantum Seminar: Beini Gao

Speaker: Beini Gao (JQI)

Title: Probing Quantum Anomalous Hall States in Twisted Bilayer WSe2 via Attractive Polaron Spectroscopy

Abstract: Moire superlattices in semiconductors are predicted to exhibit a rich variety of interaction-induced topological states. However, experimental demonstrations of such topological states, apart from MoTe2 superlattices [1–8], have remained scarce [9, 10]. Here, we report the first optical detection of quantum anomalous Hall (QAH) states in twisted WSe2 homobilayer (tWSe2). Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2degree tWSe2 and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling ν = 1. Together with a Chern number (C) measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by C = 1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe2 as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes.

[1] J. Cai, E. Anderson, C. Wang, X. Zhang, X. Liu, W. Holtzmann, Y. Zhang, F. Fan, T. Taniguchi, K. Watanabe, et al., Signatures of fractional quantum anomalous hall states in twisted mote2, Nature 622, 63 (2023).

[2] Y. Zeng, Z. Xia, K. Kang, J. Zhu, P. KnuÅNppel, C. Vaswani, K. Watanabe, T. Taniguchi, K. F. Mak, and J. Shan, Thermodynamic evidence of fractional chern insulator in moire mote2, Nature 622, 69 (2023).

[3] H. Park, J. Cai, E. Anderson, Y. Zhang, J. Zhu, X. Liu, C. Wang, W. Holtzmann, C. Hu, Z. Liu, et al., Observation of fractionally quantized anomalous hall effect, Nature 622, 74 (2023).

[4] F. Xu, Z. Sun, T. Jia, C. Liu, C. Xu, C. Li, Y. Gu, K.Watanabe, T. Taniguchi, B. Tong, et al., Observation of integer and fractional quantum anomalous hall effects in twisted bilayer mote 2, Physical Review X 13, 031037 (2023).

[5] Z. Ji, H. Park, M. E. Barber, C. Hu, K. Watanabe, T. Taniguchi, J.-H. Chu, X. Xu, and Z.-X. Shen, Local probe of bulk and edge states in a fractional chern insulator, Nature 635, 578 (2024).

[6] E. Redekop, C. Zhang, H. Park, 246 J. Cai, E. Anderson, O. Sheekey, T. Arp, G. Babikyan, S. Salters, K. Watanabe, et al., Direct magnetic imaging of fractional chern insulators in twisted mote2, Nature 635, 584 (2024).

[7] F. Xu, X. Chang, J. Xiao, Y. Zhang, F. Liu, Z. Sun, N. Mao, N. Peshcherenko, J. Li, K. Watanabe, et al., Interplay between topology and correlations in the second moireÅL band of twisted bilayer mote2, Nature Physics, 1 (2025).

[8] H. Park, J. Cai, E. Anderson, X.-W. Zhang, X. Liu, W. Holtzmann, W. Li, C. Wang, C. Hu, Y. Zhao, et al., Ferromagnetism and topology of the higher flat band in a fractional chern insulator, Nature Physics, 1 (2025). 

[9] B. A. Foutty, C. R. Kometter, T. Devakul, A. P. Reddy, K. Watanabe, T. Taniguchi, L. Fu, and B. E. Feldman, Mapping twist-tuned multiband topology in bilayer wse2, Science 384, 343 (2024).

[10] L. Wang, E.-M. Shih, A. Ghiotto, L. Xian, D. A. Rhodes, C. Tan, M. Claassen, D. M. Kennes, Y. Bai, B. Kim, et al., Correlated electronic phases in twisted bilayer transition metal dichalcogenides, Nature materials 19, 861 (2020).

Pizza and drinks will be served after the seminar in ATL 2117.