Type-II Dirac Nodal Lines in double-kagome-layered CsV$_8$Sb$_{12}$

TL;DR

This paper reports the experimental discovery of type-II Dirac nodal line semimetals in double-kagome-layered CsV$_8$Sb$_{12}$, revealing unique band topology and potential for novel transport phenomena.

Contribution

It provides the first experimental evidence of type-II Dirac nodal lines in CsV$_8$Sb$_{12}$, combining ARPES measurements with first-principles calculations to elucidate their properties.

Findings

Multiple type-II Dirac nodal lines observed near the Fermi level.

Spin-orbit coupling induces only a small gap, maintaining effective gapless spectra.

Identification of CsV$_8$Sb$_{12}$ as a platform for exploring novel topological transport effects.

Abstract

Lorentz-violating type-II Dirac nodal line semimetals (DNLSs), hosting curves of band degeneracy formed by two dispersion branches with the same sign of slope, represent a novel states of matter. While being studied extensively in theory, convincing experimental evidences of type-II DNLSs remain elusive. Recently, Vanadium-based kagome materials have emerged as a fertile ground to study the interplay between lattice symmetry and band topology. In this work, we study the low-energy band structure of double-kagome-layered CsV$_8$Sb$_{12}$ and identify it as a scarce type-II DNLS protected by mirror symmetry. We have observed multiple DNLs consisting of type-II Dirac cones close to or almost at the Fermi level via angle-resolved photoemission spectroscopy (ARPES). First-principle analyses show that spin-orbit coupling only opens a small gap, resulting effectively gapless ARPES spectra, yet generating large spin Berry curvature. These type-II DNLs, together with the interaction between a low-energy van Hove singularity and quasi-1D band as we observed in the same material, suggest CsV$_8$Sb$_{12}$ as an ideal platform for exploring novel transport properties such as chiral anomaly, the Klein tunneling and fractional quantum Hall effect.