A Dirac fermion in a topological Dirac semimetal is a quadruple-degenerate quasi-particle state with a relativistic linear dispersion. Breaking either time-reversal or inversion symmetry turns this system into a Weyl semimetal that hosts double-degenerate Weyl fermion states with opposite chiralities. These two kinds of quasi-particles, although described by a relativistic Dirac equation, do not necessarily obey Lorentz invariance, allowing the existence of so-called type-II fermions. Recent theoretical discovery of type-II Weyl fermions evokes the prediction of type-II Dirac fermions in PtSe2-type transition metal dichalcogenides, expecting an experimental confirmation. Here, we report an experimental realization of type-II Dirac fermions in PdTe2 by angle-resolved photoemission spectroscopy combined with {\it ab-initio} band calculations. Our experimental finding makes the first example that has both superconductivity and type-II Dirac fermions, which turns the topological material research into a new phase.
@article{arxiv.1612.06946,
title = {Experimental Realization of Type-II Dirac Fermions in PdTe$_2$ Superconductor},
author = {Han-Jin Noh and Jinwon Jeong and En-Jin Cho and Kyoo Kim and B. I. Min and Byeong-Gyu Park},
journal= {arXiv preprint arXiv:1612.06946},
year = {2017}
}