Flat band in millimeter-scale magic-angle twisted bilayer graphene

We fabricated millimeter-scale magic-angle twisted bilayer graphene, and observed the flat band, which is regarded as the origin of superconductivity.
Published in Materials
Flat band in millimeter-scale magic-angle twisted bilayer graphene
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Twisted bilayer graphene (TBG) is two graphene layers stacked with a certain rotation angle. Twist angle θ is a new degree of freedom in graphene to introduce a lot of new properties. A new technology made by twisting multilayer two-dimensional materials is called “twistronics”. As one of the significant properties, superconductivity was reported in TBG with a magic-angle θ = 1.1º.

Most of the research in TBG have been done using micrometer-scale sample, fabricated by the micromechanical cleaving technique. In order to perform various experiments and to realize twistronics, we should develop a new technique to obtain large-area TBG.

Based on this background, we succeeded in fabricating millimeter-scale TBG by transferring graphene epitaxially grown on the silicon carbide substrate. The procedure is shown in Figure 1. First, we make graphene by thermal decomposition of SiC, whose size was 5 x 5 mm2. It was transferred onto another graphene on SiC with some twist angle θ. Thus, we have successfully fabricated millimeter-scale TBG.

Figure 1. Fabrication procedure of large-area TBG. Gold was deposited on graphene/SiC1, and graphene and gold were exfoliated. Then, it was transferred onto graphene/SiC2 with the angle θ. Tape and gold were removed.

The origin of superconductivity in magic-angle TBG is thought to be the flat band at the Fermi energy, which results in the large density of states. In this study, we performed angle-resolved photoemission spectroscopy (ARPES) measurements of large-area magic-angle TBG. Figure 2 shows an optical microscope image and ARPES results of the magic-angle TBG sample. From the optical micrograph, the twist angle was estimated to be 0.6-1.0º. In the ARPES image, there are a few bands, which is denoted by blue and red arrows. The intensity was stronger at E-EF = -0.22, -0.37, and -0.51 eV, as shown by broken lines. In the spectral function calculated from the band structure of the 1.08º TBG, there is a flat band at -0.37 eV. On the right, the intensity profiles along kx = 0.016 Å-1 are shown. In these profiles, the positions of the intensity maxima are in good agreement. In particular, the red broken line corresponds to the flat band, and there is a peak at this energy in the experimental result. In other words, we have successfully observed the flat band in the magic-angle TBG. The fact that the flat band were present at -0.37 eV indicates that the TBG was electron-doped. It is the first time that we observed both the lower energy and the higher energy regions of the flat band. We have also obtained the results suggesting the existence of hidden symmetry which was not observed in previous experiments with TBG.

Figure 2. Optical micrograph and ARPES results of the magic-angle TBG sample. An ARPES image and the spectral function image based on the band structure of 1.08º TBG are shown. The right graphs are intensity profiles along the kx = 0.016 Å-1 line (black arrow) in the images.

For more information, please visit our paper.
https://www.nature.com/articles/s43246-021-00221-3

See also video:
https://youtu.be/KE2duY1nPv4

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