Two-dimensional (2D) crystals, such as graphene, black phosphorus, and 2H transition-metal dichalcogenides, have emerged as a class of materials that may impact our future electronics technologies. Controlling the band structure of these materials is important not only to study a new class of Dirac and Weyl fermions, but also to explore novel composite particles. In this talk, I will introduce our recent angle-resolved photoemission spectroscopy (ARPES) studies on black phosphorus and MoS2. The widely tunable band gap of black phosphorus [1] could be exploited to artificially create the 2D Dirac semimetal phase with a pair of Dirac points [2] protected by spacetime inversion symmetry [3]. Surface doping to MoS2 [4] could be used to discover a hitherto unobserved spectral function of Holstein polarons, a small quasiparticle that carries a cloud of self-induced phonons with it [5].

 References

1. J. Kim et al., Science 349, 723 (2015).
2. J. Kim et al., Phys. Rev. Lett. 119, 226801 (2017).
3. J. Ahn et al., Phys. Rev. Lett. 118, 156401 (2017).
4. M. Kang et al., Nano Lett 17, 1610 (2017).
5. M. Kang et al., Nature Mater. 17, 676 (2018).