Experimental Realization of Type-II Dirac Fermions in PdTe$_2$ Superconductor

TL;DR

This paper reports the experimental discovery of type-II Dirac fermions in the superconductor PdTe$_2$, combining topological physics with superconductivity, confirmed through spectroscopy and calculations, marking a new phase in topological material research.

Contribution

First experimental realization of type-II Dirac fermions in a superconductor, demonstrating coexistence with superconductivity in PdTe$_2$.

Findings

Type-II Dirac fermions observed in PdTe$_2$

Confirmed by angle-resolved photoemission spectroscopy and calculations

First example with both superconductivity and type-II Dirac fermions

Abstract

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 PtSe$_2$-type transition metal dichalcogenides, expecting an experimental confirmation. Here, we report an experimental realization of type-II Dirac fermions in PdTe$_2$ 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.