Type-II Dirac Nodal Lines in a Double-Kagome-Layered Semimetal

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 state of matter. While being studied extensively in theory, convincing experimental evidence 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. This work studies the low-energy band structure of double-kagome-layered CsV8Sb12 and identifies it as a scarce type-II DNLS protected by mirror symmetry. This work observes multiple DNLs consisting of type-II Dirac cones close to or almost at the Fermi level via angle-resolved photoemission spectroscopy (ARPES), which provides an electronic explanation for the nonsaturating magnetoresistance effect as observed. First-principles theory analyses show that spin-orbit coupling only opens a small gap, resulting in 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-one-dimensional band as observed in the same material, suggest CsV8Sb12 as an ideal platform for exploring novel transport properties.