Direct observation of type-II Dirac fermions

In recent years, a lot of novel quantum states (e.g. type-I Dirac/Weyl femions) has been observed in condensed matter physics. Since the Lorentz invariance is not necessarily obeyed in condensed matter physics, the low energy quasi-partical excitations such as type-II Dirac/Weyl fermions could be possibly observed in topological quantum materials. After the successful observation of type-II Weyl fermions in MoTe₂, the exploration of type-II Dirac fermions is still under way. Recently, using the ARPES endstation at NSRL, Prof. Shuyun Zhou group in Tsinghua University characterized the electronic structures of 1T PtTe₂ and for the first time observed type-II Dirac fermions. The results are published in Nature Communications (Nat. Commun. 8, 257 (2017)). This work provides opportunities for investigating novel quantum phenomena (e.g., anisotropic magneto-transport) and topological phase transition.

he in-plane and out-of-plane dispersions in PtTe₂