Two-dimensional type-II Dirac fermions in layered oxides

TL;DR

This paper reports the experimental discovery of two-dimensional type-II Dirac fermions in a high-temperature superconductor, revealing new topological features protected by crystal symmetry and suggesting similar phenomena in nickelate systems.

Contribution

It provides the first experimental observation of 2D type-II Dirac fermions in a layered oxide superconductor, linking band structure, symmetry protection, and topological properties.

Findings

Dirac point found approximately 1 eV below Fermi level

Dirac point protected by mirror symmetry

Spin-orbit coupling induces topologically non-trivial bands

Abstract

Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La$_{1.77}$Sr$_{0.23}$CuO$_4$. The Dirac point, constituting the crossing of $d_{x^2-y^2}$ and $d_{z^2}$ bands, is found approximately one electronvolt below the Fermi level ($E_\mathrm{F}$) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near $E_\mathrm{F}$.