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

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.

Experimental Realization of Type-II Dirac Fermions in PdTe₂ Superconductor

The paper titled "Experimental Realization of Type-II Dirac Fermions in PdTe₂ Superconductor" presents a comprehensive experimental and theoretical investigation into type-II Dirac fermions within the superconducting material PdTe₂. It leverages angle-resolved photoemission spectroscopy (ARPES) in conjunction with first-principles band calculation methods, particularly density functional theory (DFT), to confirm the existence and characteristics of these fermionic states.

Overview of Key Findings

The study's central theme is the identification of massless type-II Dirac fermions within a superconducting host material. These findings mark PdTe₂ as the debut instance where both superconductivity and type-II Dirac fermions coexist. Notably, angle-resolved photoemission spectroscopy (ARPES) discerned the type-II Dirac point with a binding energy of about 0.5 eV below the Fermi level at specific points in the Brillouin zone. This pivotal discovery was substantiated by theoretical computations, aligning perfectly with experimental observations to affirm symmetry considerations and electronic dispersion characteristics specific to PdTe₂.

Methodology

The experimental setup is vital in this research, focusing on the high-resolution capabilities of ARPES to probe electronic structure. The authors utilized photon-energy dependent and circular dichroic ARPES, enabling detailed mapping of band dispersion relative to the Dirac point, as well as disentangling surface from bulk state contributions. Complementing these empirical analyses are rigorous {\it ab-initio} computations, employing full-potential linearized augmented plane wave methods with spin-orbit coupling to ascertain electronic behaviors aligned with symmetry-protected topological materials.

Strong Numerical Results

Several numerical insights emerge significantly from the study. The Fermi energy proximity to the observed massless Dirac fermions at $\varepsilon_{D-II} = -0.5$ eV solidifies PdTe₂'s role as a prominent representative in exploring type-II Dirac fermion behavior amidst a superconducting state. The Dirac fermions' unique dispersion characteristics—restricted along certain crystallographic axes—further underscore the distinct topology contrasting typical Lorentz-invariant systems.

Theoretical and Practical Implications

This elucidation of type-II Dirac fermions bears profound implications, enhancing understanding of low-energy excitations within superconductors and advancing the field of topological quantum materials. Practically, the coexistence of superconductivity and type-II Dirac states in PdTe₂ could serve as a testing ground for novel quantum phenomena, such as unconventional superconductive mechanisms when tuned near the Dirac point. Theoretically, study extension into the peculiar transport and electromagnetic response attributes of such systems opens new research avenues, especially in condensed matter physics, given the breach of Lorentz invariance typical to these fermionic types.

Future Directions

This research paves pathways for ensuing investigations into correlated electron phenomena in such systems, including potential application in quantum computing proximities and exotic electronic device functionalities. The next logical steps involve manipulating carrier concentration to smoothly regulate the Fermi level near type-II Dirac points, enhancing superconductive traits while observing ensuing alteration in quantum state dynamics.

In conclusion, the confluence of the PdTe₂ material characteristics with cutting-edge spectroscopic and computational techniques has significantly advanced the understanding of type-II Dirac fermions, setting a precedent for future explorations into complex material systems at the intersection of topological phenomena and superconductivity.