Strong Electron–phonon Coupling In Magic-angle Twisted Bilayer Graphene

Result of the Month

a, Illustration of the MATBG moiré Brillouin zones around the K point of top and bottom monolayer graphene Brillouin zones. The grey lines show the moiré Brillouin zones of MATBG. Cuts 1–6 mark the coordinates of ARPES spectra in c. b, Three-dimensional intensity plot of ARPES spectra in the vicinity of the graphene K point, presenting an overview of the band structure of MATBG. c, ARPES dispersion plots taken on superconducting hBN-unaligned MATBG device A (top rows, marked by cuts 1–6 in a and b) and their associated second partial derivative plots (bottom rows). The red and cyan arrows in top and bottom rows highlight the flat band and its first and second replicas, respectively. d, Top: integrated ARPES spectra intensity in the green dashed line box in c. Bottom: the orange curve represents the same data shown in the top panel with the removal of a smooth background (fourth-degree polynomial). The blue curve represents the integrated intensity of the second-derivative plot within the same green dashed line box. Both the orange and the blue curves show an energy spacing between the flat band and replicas of 150 ± 15 meV. e, ARPES dispersion plots taken on a second superconducting hBN-unaligned MATBG, device B, where flat band and replicas are observed. Scale bars, 0.1 Å−1. a.u., arbitrary units. ARPES data presented in this figure was taken with photon energy of 95eV at room temperature.

Author: Cheng Chen, Kevin P. Nuckolls, Shuhan Ding, Wangqian Miao, Dillon Wong, Myungchul Oh, Ryan L. Lee, Shanmei He, Cheng Peng, Ding Pei, Yiwei Li, Chenyue Hao, Haoran Yan, Hanbo Xiao, Han Gao, Qiao Li, Shihao Zhang, Jianpeng Liu, Lin He, Kenji Watanabe, Takashi Taniguchi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Chu Li, Xu Han, Ding Pan, Zhongkai Liu, Xi Dai, Chaoxing Liu, B. Andrei Bernevig, Yao Wang, Ali Yazdani, Yulin Chen Institute: ''Shanghai Tech University, Shanghai, People’s Republic of China & Department of Physics, University of Oxford, Oxford, UK'' Nature
URL: https://doi.org/10.1038/s41586-024-08227-w
Date: 2/2025 Instruments: DA30-L, R4000

The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked considerable research interest. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms, the origin of its superconductivity remains elusive. Here, by utilizing angle-resolved photoemission spectroscopy with micrometre spatial resolution, we reveal flat-band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride substrate. These replicas show uniform energy spacing, approximately 150 ± 15 meV apart, indicative of strong electron–boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hexagonal boron nitride or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat-band replicas in superconducting MATBG are attributed to the strong coupling between flat-band electrons and an optical phonon mode at the graphene K point, facilitated by intervalley scattering. These findings, although they do not necessarily put electron–phonon coupling as the main driving force for the superconductivity in MATBG, unravel the electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which its superconductivity is derived.

Recent advancements in high-throughput X-ray optics, together with high-resolution electron analyser system have empowered us to perform high-quality ARPES measurements with sub-micron spatial precision. This makes it well-suited for unravelling the intricate electronic structure of 2-dimensional artificially-fabricated homo/hetero-structures, such as the MATBG system studied here.

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