Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe2

TLDR

Topological semimetals host Dirac and Weyl fermions with linear dispersions that obey Lorentz invariance, but Lorentz‑violating type‑II Dirac fermions with strongly tilted cones can also arise in such materials. The study reports the first experimental evidence of type‑II Dirac fermions in bulk stoichiometric PtTe₂ single crystals. ARPES measurements and first‑principles calculations reveal a pair of strongly tilted Dirac cones along the Γ‑A direction in PtTe₂, confirming it as a type‑II Dirac semimetal and indicating new exotic properties distinct from type‑I Dirac fermions.

Abstract

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. These fermions with linear dispersions near the Dirac or Weyl points obey Lorentz invariance, and the chiral anomaly leads to novel quantum phenomena such as negative magnetoresistance. The Lorentz invariance is, however, not necessarily respected in condensed matter physics, and thus Lorentz-violating type-II Dirac fermions with strongly tilted cones can be realized in topological semimetals. Here, we report the first experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe$_2$ single crystal. Angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the $\Gamma$-A direction under the symmetry protection, confirming PtTe$_2$ as a type-II Dirac semimetal. The realization of type-II Dirac fermions opens a new door for exotic physical properties distinguished from type-I Dirac fermions in condensed matter materials.