Discovery of 2D Anisotropic Dirac Cones
I. Matsuda, Sugino, and Komori Groups
As the fifth element in the periodic table, boron atoms form numerous bulk allotropes and provide a rich chemistry and physics. Boron atomic sheets, borophene, also have an enormous number of two-dimensional (2D) polymorphs. Recently, various borophene phases have been synthesized on metal substrates, such as the β12 sheet. Theoretical works have revealed numerous novel properties of these borophene phases, such as phonon-mediated superconductivity, but experimental examinations of their electronic structures are far from enough.
Fig. 1. Photoemission intensity plots measured with 25 eV p polarized photons at the X and X′ points. Red arrows and black arrows indicate positions of the Dirac cones of DC1 and DC2, respectively. Red broken circle show a region where one finds gap-opening for the DC1 band.
Fig. 2. Schematic drawing of the structure model of χ3 borophene and the anisotropic Dirac cones, Δ-Dirac ones.
In the present research, we report observation of 2D anisotropic Dirac cones, Dirac delta (Δ)-cone, in χ3 borophene by the first principles calculations and angle-resolved photoemission spectroscopy measurements using synchrotron radiation. The Dirac cones are centered at the X and X′ points of the Brillouin zone, as shown in Fig.1. The calculation reveals that these Dirac cones are mainly derived from the pz orbitals of boron, with negligible contributions from the silver orbitals of the substrate. This represents that the χ3 borophene sheet is the quasi-freestanding layer.
The atomic structure and the electronic structure of the Dirac Fermion are summarized in Fig. 2. Our results validate χ3 borophene as the first 2D material that hosts anisotropic Dirac cones. This may stimulate further research interest and enable the realization of high-performance boron devices.
References
- [1] B. Feng, J. Zhang, S. Ito, M. Arita, C. Cheng, L. Chen, K. Wu, F. Komori, O. Sugino, K. Miyamoto, T. Okuda, S. Meng, and I. Matsuda, Adv. Mater. 30, 1704025 (2018).
- [2] B. Feng, O. Sugino, R.-Y. Liu, J. Zhang, R. Yukawa, M. Kawamura, T. Iimori, H. Kim, Y. Hasegawa, H. Li, L. Chen, K. Wu, H. Kumigashira, F. Komori, T.-C. Chiang, S. Meng, and I. Matsuda, Phys. Rev. Lett. 118, 096401 (2017).