Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe$_2$ (1607.03643v1)

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.

• The paper identifies type-II Dirac fermions with Lorentz violation in PtTe2 using ARPES and first-principles calculations.
• It reveals strongly tilted Dirac cones where T(k) exceeds U(k), leading to intersecting electron and hole pockets and a finite density of states at the Fermi level.
• The findings advance topological semimetal research and open pathways for developing quantum devices with unique anisotropic properties.

Lorentz-Violating Type-II Dirac Fermions in PtTe$_2$

This paper presents a detailed investigation of Lorentz-violating type-II Dirac fermions within the transition metal dichalcogenide PtTe$_2$. Topological semimetals, such as Dirac and Weyl semimetals, which host fermions analogous to those in high-energy physics, have garnered significant interest due to their unique quantum phenomena and transport properties. However, the Lorentz invariance typically obeyed by these fermions in high-energy contexts is often violated in condensed matter systems. In this work, the authors identify type-II Dirac fermions characterized by strongly tilted Dirac cones within the bulk PtTe$_2$ single crystal—a structure distinct from their type-I counterparts due to this Lorentz violation.

The paper relies on both angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations to reveal the topological characteristics of PtTe$_2$. The experimental data from ARPES indicate the presence of a pair of type-II Dirac cones along the $\Gamma$-A direction. This discovery marks PtTe$_2$ as a type-II Dirac semimetal, protected by crystalline symmetries pertinent to its 3D Dirac electronic structure.

From a theoretical perspective, the authors use a Hamiltonian framework describing Dirac fermions, $$H(\vec{k})=\sum\limits_{i=x,y,z\atop j=0,x,y,z}k_iA_{ij}\sigma_j$$. The dispersion relation, $\epsilon_{\pm}(\vec{k})=T(\vec{k}) \pm U(\vec{k})$, distinguishes between type-I and type-II Dirac semimetals based on the comparative magnitudes of $T(\vec{k})$ and $U(\vec{k})$. For type-II Dirac semimetals, $T(\vec{k}) > U(\vec{k})$ along certain directions, leading to the characteristic tilted-cone dispersion observed.

Numerical results corroborate the ARPES data, validating the presence of Lorentz-violating Dirac fermions. Notable is the intersection between electron and hole pockets at the Dirac point, yielding a finite density of states at the Fermi energy. Such geometry markedly influences electronic and magnetic properties, deviating significantly from the more symmetric type-I Dirac cones, which exhibit zero density of states at the Dirac point.

The implications of identifying type-II Dirac fermions in PtTe$_2$ are multifaceted. Practically, this realization could lead to advancements in electronic devices leveraging their unique anisotropic properties. Theoretically, it enriches our understanding of topological materials, expanding the classification and potential applications of Dirac and Weyl semimetals. This work opens avenues to explore type-II Dirac materials further, with potential developments in the domain of quantum materials and topological quantum computing.

In conclusion, this paper marks a significant step in condensed matter physics, enhancing our understanding of Lorentz-violating topological phenomena in transition metal dichalcogenides. Future research may build on these findings to explore novel transport phenomena and integrate such materials into advanced technological applications.