A New Twist for Superconductivity in Bilayer Graphene

SCIENTIFIC ACHIEVEMENT

Using the Advanced Light Source (ALS) to study twisted bilayer graphene (TBG) systems, researchers found intriguing spectroscopic features in a superconducting “magic-angle” TBG—features that are absent in non-superconducting TBG.

SIGNIFICANCE AND IMPACT

The results provide crucial information on superconductivity in magic-angle TBG for next-gen electronics and advanced energy technologies.

On a dark purple-blue background, two layers of graphene (in ball-and-stick representation) are shown slightly offset from each other by a small angle. In the foreground, a magnifying glass shows a zoomed-in view of the twisted bilayer graphene, with ghosting effects (suggesting lattice vibrations) and two small white circles with comet-like tails (depicting electrons in motion). — Artistic depiction of a twisted bilayer graphene system. A very slight twist in the alignment of the two graphene layers creates a moiré pattern—a periodic modulation of the electronic environment that can give rise to exotic behaviors such as superconductivity. Advanced nanoscale probes provide important clues as to the connections between electronic structure and material properties.

Searching for the science behind the magic

Two-dimensional materials like graphene give scientists great flexibility in engineering electronic properties because they can be stacked like sheets of paper. Besides choosing what materials to stack and in what order, researchers can manipulate the electrical and optical properties of these stacks by controlling the twist angle between layers. Because of this versatility, two-dimensional materials provide an ideal platform for investigating the complex interplay between phenomena such as band topology, strong electron correlation, magnetism, and superconductivity, all of which are relevant to next-gen electronics and advanced energy technologies.

So-called “magic-angle” twisted bilayer graphene (MATBG) attracts broad research interest primarily because of its surprising and unusual superconducting properties, which resemble those of high-temperature superconductors. The superconductivity in MATBG devices is thought to arise from flat bands that form in a material’s electronic band structure when two layers of graphene are twisted at a “magic” angle of about 1.08 degrees. Flat bands indicate a high density of states, which significantly enhances electron interactions and the resulting potential for exotic phenomena. Despite intensive experimental efforts, the origin of MATBG superconductivity remains elusive.

Advanced micro-ARPES at the ALS

Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for probing the fine electronic structure of materials and 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 techniques, typically limited to a spatial resolution of hundreds of microns, cannot be directly applied. Persistent efforts have led to the development of advanced micro- and nano-ARPES techniques, extending ARPES research to sub-micrometer quantum materials and devices.

Here, researchers systematically characterized the electronic structure of several twisted bilayer graphene (TBG) devices using micro-ARPES at ALS Beamline 7.0.2 (MAESTRO), where a world-leading nano-ARPES system has also been developed. The high-quality moiré superlattice and superconductivity of these devices were characterized by a group at Princeton University.

(a) A schematic drawing of the sample device, with an x-ray beam coming in from a source in the upper right, and a photoelectron being emitted toward a hemispherical analyzer in the upper left. (b) Optical image of the sample device from above, with colors as described in caption; a scale bar is labeled “20 µm.” (c) A three-dimensional example of the ARPES data, showing multiple cone-shaped features. — (a) Schematic of the micro-ARPES experiment geometry. (b) Optical image showing the hexagonal boron nitride (hBN) substrate in blue, the silicon dioxide (SiO₂) substrate in purple, the gold contact in orange, and the MATBG region marked by white dashed lines. Inset: The intensity from the MATBG sample is selectively accumulated, making it distinguishable from the hBN and SiO₂ substrates. (c) The three-dimensional band structure of the MATBG, featuring hybridized Dirac cones, can be well captured using this technique.

A crucial clue in flat-band “echoes”

In addition to the observation of characteristic flat bands (initially reported in 2020, also with data from MAESTRO), this study further observed novel “echoes” (replicas) of the main flat band, specifically in superconducting MATBG devices. Strikingly, these replicas were absent in non-superconducting TBG devices, either when the MATBG was aligned to its substrate or when the TBG twist angle deviated slightly from the magic angle. The main flat bands, however—minus the replicas—were still seen in the non-superconducting devices.

Calculations, primarily by a group from Emory University, suggest that the formation of the flat-band replicas in superconducting MATBG can be attributed to strong coupling between flat-band electrons and an optical phonon mode (lattice vibrations), with the spacing between replicas determined by the phonon energy. The coupling is facilitated by the scattering of electrons between valleys (local minima or maxima) in momentum space.

The results indicate a strong relationship between electron–phonon coupling and superconductivity in TBG systems—crucial information for understanding the emergence of superconductivity in MATBG. The researchers expect that the micro- and nano-ARPES capabilities will be enhanced with the ALS upgrade (ALS-U) and will find widespread use in investigating strong correlation effects in moiré systems.

(a) Six panels of ARPES data, in two rows of three panels each. The top row shows raw ARPES data (black and white) for three different 2D cuts through the 3D data. The bottom row (white-to-orange intensity gradient) shows the first derivative of the respective top row panels. Small arrows indicate flat bands and replicas, which are more pronounced in the bottom row. (b) Two line graphs trace the spectral intensity in a region, marked by a green box in (a), that contains the flat-band replicas. Again, the top graph shows the raw data, and the bottom graph shows the second derivative. A second curve in the bottom graph shows a curve in which a constant background has been subtracted. All curves show three peaks, with the distance between the first and third peaks being 293 meV. — (a) ARPES data from superconducting hBN-unaligned MATBG and the associated second partial derivative plots. The red and green arrows highlight the flat band and its first and second replicas. Scale bar is 0.1 Å⁻¹. (b) Top: Integrated ARPES spectral intensity in the green dashed-line box in (a). Bottom: the orange curve represents the same data shown in the top panel with the removal of a smooth background. 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.

Contact: Cheng Chen

Researchers: C. Chen and Y. Chen (ShanghaiTech University and University of Oxford); K.P. Nuckolls, D. Wong, M. Oh, R.L. Lee, and A. Yazdani (Princeton University); S. Ding and H. Yan (Emory University); W. Miao (University of California, Santa Barbara); S. He and C. Peng (University of Oxford); D. Pei, H. Xiao, H. Gao, Q. Li, S. Zhang, J. Liu, and Z. Liu (ShanghaiTech University); Y. Li (Wuhan University); C. Hao and L. He (Beijing Normal University); K. Watanabe and T. Taniguchi (National Institute for Materials Science, Japan); C. Jozwiak, A. Bostwick, and E. Rotenberg (ALS); C. Li, X. Han, D. Pan, and X. Dai (Hong Kong University of Science and Technology); C. Liu (Pennsylvania State University); B.A. Bernevig (Princeton University, Donostia International Physics Center, Spain, and Basque Foundation for Science); and Y. Wang (Emory University and Clemson University).

Funding: Oxford–ShanghaiTech collaboration project; Shanghai Municipal Science and Technology Major Project; Gordon and Betty Moore Foundation; US Department of Energy, Office of Science, Basic Energy Sciences program (DOE BES); National Science Foundation; Army Research Office; Office of Naval Research; Simons Foundation; European Research Council; National Natural Science Foundation of China; National Key R&D program of China; and Research Grants Council of the Hong Kong Special Administrative Region, China. Operation of the ALS is supported by DOE BES.

Publication: C. Chen, K.P. Nuckolls, S. Ding, W. Miao, D. Wong, M. Oh, R.L. Lee, S. He, C. Peng, D. Pei, Y. Li, C. Hao, H. Yan, H. Xiao, H. Gao, Q. Li, S. Zhang, J. Liu, L. He, K. Watanabe, T. Taniguchi, C. Jozwiak, A. Bostwick, E. Rotenberg, C. Li, X. Han, D. Pan, Z. Liu, X. Dai, C. Liu, B.A. Bernevig, Y. Wang, A. Yazdani, and Y. Chen, “Strong electron-phonon coupling in magic-angle twisted bilayer graphene,” Nature 636, 342 (2024), doi:10.1038/s41586-024-08227-w.

ALS SCIENCE HIGHLIGHT #523