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Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high energy physics. Although Lorentz invariance is required in high energy physics, it is not necessarily obeyed in condensed matter physics, and thus Lorentz-violating type-II Weyl/Dirac fermions could be realized in topological semimetals. The recent realization of type-II Weyl fermions raises the question whether their spin-degenerate counterpart—type-II Dirac fermions—can be experimentally realized too. Here, we report the experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe2 single crystal. Angle-resolved photoemission spectroscopy measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the Γ-A direction, confirming PtTe2 as a type-II Dirac semimetal. Our results provide opportunities for investigating novel quantum phenomena (e.g., anisotropic magneto-transport) and topological phase transition.

Graphene/hexagonal boron nitride (h-BN) has emerged as a model van der Waals heterostructure 1 as the superlattice potential, which is induced by lattice mismatch and crystal orientation, gives rise to various novel quantum phenomena, such as the self-similar Hofstadter butterfly states 2-5 . Although the newly generated second-generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, fundamental knowledge of SDCs, such as locations and dispersion, and the e ect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report direct experimental results on the dispersion of SDCs in 0 • -aligned graphene/h-BN heterostructures using angle-resolved photoemission spectroscopy. Our data unambiguously reveal SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of approximately 100 meV and approximately 160 meV are observed at the SDCs and the original graphene Dirac cone, respectively. Our work highlights the important role of a strong inversion-symmetry-breaking perturbation potential in the physics of graphene/h-BN, and fills critical knowledge gaps in the band structure engineering of Dirac fermions by a superlattice potential.Hexagonal boron nitride (h-BN) shares a similar honeycomb lattice structure to graphene, yet its lattice is stretched by 1.8%. Moreover, the breaking of the inversion symmetry by distinct boron and nitrogen sublattices leads to a large bandgap (5.97 eV) in the π band, which is in sharp contrast to the gapless Dirac cones in graphene. By stacking graphene atop h-BN to form a van der Waals heterostructure 1 , graphene/h-BN not only exhibits greatly improved properties for device applications, such as reduced ripples, suppressed charge inhomogeneities and higher mobility 6,7 , but also provides unique opportunities for band structure engineering of Dirac fermions by a periodic potential 8,9 . The superlattice potential induced by the lattice mismatch and crystal orientation can significantly modify the electronic properties of graphene and lead to various novel quantum phenomena, for example, the emergence of second-generation Dirac cones (SDCs), which are crucial for the realization of Hofstadter butterfly states under an applied magnetic field 2-5 , renormalization of the Fermi velocity 8,[10][11][12] , gap opening at the Dirac point 4,13-16 , topological currents 15 and gate-dependent pseudospin mixing 17 . Hence, understanding the effects of the superlattice potential on the band structure of graphene is crucial for advancing its device applications, and for gaining new knowledge about the fundamental physics of Dirac fermions in a periodic potential.Previously, the existence of SDCs has been deduced from scanning tunnelling spectroscopy, resistivity and capacitance measurements 2,5,18,19 . However, such measurements are not capable of mapping out the electronic dispersion with momentum-resolved informatio...

The interaction between magnetic impurities and the gapless surface state is of critical importance for realizing novel quantum phenomena and new functionalities in topological insulators. By combining angle-resolved photoemission spectroscopic experiments with density functional theory calculations, we show that surface deposition of Cr atoms on Bi 2 Se 3 does not lead to gap opening of the surface state at the Dirac point, indicating the absence of long-range out-of-plane ferromagnetism down to our measurement temperature of 15 K. This is in sharp contrast to bulk Cr doping, and the origin is attributed to different Cr occupation sites. These results highlight the importance of nanoscale configuration of doped magnetic impurities in determining the electronic and magnetic properties of topological insulators.Three dimensional (3D) topological insulator (TI), a novel class of materials, 1,2 is a topologically nontrivial bulk insulator with conducting surface state (SS). Protected by the time-reversal symmetry (TRS), the SS is spin-polarized, gapless and robust against nonmagnetic impurities. [3][4][5][6] Meanwhile, magnetic impurities may induce out-of-plane ferromagnetism and break the TRS in TI, resulting in a band gap at the Dirac point of the SS. 7 Such TRS breaking is essential for the realization of various novel quantum phenomena which have great application potential in future nano-electronic and spintronic devices, such as quantum anomalous Hall effect (QAHE), 8-11 topological magnetoelectric effect (TME) 12 and birefringent spin lens. 13 Thus, it is critical to understand the interaction between 3D TIs and magnetic impurities both for fundamental physics and nano-device applications.The interaction between 3D TI and doped magnetic impurities can modify the electronic and magnetic properties of 3D TI, and the results may strongly depend on the types of magnetic atoms, occupation sites of the magnetic impurities 14 and experimental conditions 2 under which they are introduced. For example, although doping Fe and Cr into the bulk Bi 2 Se 3 crystal or thin film during the growth process leads to a gap opening at the Dirac point suggesting TRS breaking, 7,15-17 the robustness of the gapless SS against Fe deposition on the surface has been debated 18,19 and surface deposition of Cr still remains to be investigated.Here we present a combined experimental and theoretical study of the electronic structures of Bi 2 Se 3 upon surface deposition of magnetic Cr atoms on freshly cleaved Bi 2 Se 3 single crystals. Angle-resolved photoemission spectroscopy (ARPES) data show that the SS is robust upon surface deposition of Cr, and the absence of gap opening at the Dirac point suggests that there is no long-range out-of-plane ferromagnetism down to the lowest measurement temperature of 15 K. In addition, Cr atoms dope electrons to the bulk states of Bi 2 Se 3 forming a two-dimensional electron gas with large Rashba-splitting on the surface. These While doping electrons to the SS with preserved TRS, Cr deposit...

In van der Waals heterostructures, the periodic potential from the Moiré superlattice can be used as a control knob to modulate the electronic structure of the constituent materials. Here we present a nanoscale angle-resolved photoemission spectroscopy (Nano- ARPES