A team led by Distingshed Adjunct Professor Chen Yulin and Assistant Professor Chen Cheng from Laboratory for Topological Physics at the School of Physical Science and Technology, ShanghaiTech University, has made a significant discovery in the study of magic-angle twisted bilayer graphene (MATBG). Using spatial- and angle-resolved photoemission spectroscopy (Nano-ARPES), they observed prominent inter-valley electron-phonon coupling effects and identified the corresponding phonon modes. This finding is crucial for understanding the superconductivity mechanism of MATBG system. On December 11, the study was published online in Nature titled “Strong Electron-Phonon Coupling in Magic-Angle Twisted Bilayer Graphene.”
Since its discovery in 2018, MATBG has become a hotspot in condensed matter physics research due to its superconducting properties and strongly correlated electron features. Its superconductivity arises from the flat bands of twisted bilayer graphene at the “magic angle,” which significantly enhance electron interactions, providing a novel platform for studying strongly correlated systems such as Mott insulators and high-temperature superconductors. Furthermore, the unique topological quantum anomalous Hall states in magic-angle graphene offer the potential to realize exotic quantum states like topological superconductivity, with potential applications in quantum computing. However, despite extensive experimental and theoretical studies, MATBG’s precise electronic structures and the origin of its superconductivity remain unresolved.
Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for probing the fine electronic structure of materials. Over the past decades, it has played a pivotal role in unraveling the mechanisms of high-temperature superconductivity and discovering novel topological quantum materials. However, due to the microscale dimensions of MATBG devices, traditional ARPES technique, with a limit of millimeter-scale spatial resolution, cannot be directly applied. Persistent efforts have led to the development of Nano-ARPES, featuring a sub-micrometer spatial resolution. Facilities like the Shanghai Synchrotron Radiation Facility’s (SSRF) Spatial-resolved and Spin-resolved AREPS and Magnetism Beamline (S2 Beamline), jointly built by the Laboratory for Topological Physics and SSRF, enable precise electronic structure measurements of microscale quantum materials.
In this study, the team utilized Nano-ARPES at SSRF (S2 Beamline) and the Maestro Beamline at the Advanced Light Source in the United States to systematically characterize the electronic structure of twisted bilayer graphene (Figure 1a). For the first time, they observed novel flat band replica phenomena in the electronic spectrum of superconducting MATBG device, with a fixed energy gap (150 meV, Figure 1b). This phenomenon was absent in non-superconducting MATBG (aligned with underlying hBN substrate) or non-magic-angle graphene. Combining experimental results with theoretical calculations, the researchers attributed these flat band replica features to strong coupling between flat band electrons and a graphene K point phonon with an energy of 150 meV (Figure 1c). Further experimental data (Figure 1d) revealed that this electron-phonon coupling is strongly correlated with the superconductivity in twisted graphene. These findings shed light on the unique electronic structure of superconducting magic-angle graphene and pave the way for understanding its superconductivity and distinctive properties.
The Nano-ARPES technology applied in this research has broad applications in characterizing the electronic structures of nanoscale materials and devices. It provides an effective tool for exploring novel states and functions in these materials and devices, supporting the design and discovery of new quantum materials.
The research was a collaborative effort involving ShanghaiTech University, Oxford University, Princeton University, Lawrence Berkeley National Laboratory, and Emory University. ShanghaiTech University was the primary affiliation. Prof. Chen Cheng and Dr. Kevin P. Nuckolls from Princeton University are the co-first authors. Prof. Chen Yulin and Prof. Wang Yao from Emory University are the corresponding authors. Prof. Liu Zhongkai and Prof. Liu Jianpeng from the Topological Physics Laboratory also contributed to parts of the experimental and theoretical work.
Figure 1: (a) Schematic of Nano-ARPES measurement of magic-angle graphene devices. (b) Flat band replica phenomena in superconducting magic-angle graphene. (c) Inter-valley electron-phonon coupling mechanism in superconducting magic-angle graphene. (d) Correlation between superconductivity and flat band replicas (strong electron-phonon coupling) in twisted graphene.
Figure 2: Illustration of electron-phonon coupling in superconducting magic-angle graphene.
*This article is provided by Prof. Chen Cheng