Two-dimensional type-II Dirac fermions in layered oxides

M Horio,A M Cook,Y Sassa,J Chang,C E Matt,M Månsson,S M Hayden,M Shi,O J Lipscombe,T Neupert,N C Plumb,K Hauser,K Kramer,D Sutter

doi:10.1038/s41467-018-05715-2

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 La1.77Sr0.23CuO4. 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 (EF) 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 EF.

Highlights

Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear as low-energy quasi-particle excitations in electronic band structures
Both systems display a type-II Dirac cone that is protected by mirror symmetry preventing hybridisation between the dz[2] and dx2Ày2 bands along the Γ–M direction in the Brillouin zone[20,21]
Dirac fermions are classified by their dimensionality and the degree to which they break Lorentz invariance

Summary

Introduction

Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear as low-energy quasi-particle excitations in electronic band structures. The concept of topologically protected Dirac fermions has been applied to the band structure found in high-temperature iron-based superconductors[9,10]. Three-dimensional type-II Dirac fermions have been identified experimentally in PtTe214,15 and PdTe216.

Results

Conclusion