Dirac Cone Protected by Non-Symmorphic Symmetry and 3D Dirac Line Node in ZrSiS

Abstract

Materials harboring exotic quasiparticles, such as Dirac and Weyl fermions\cite{xu2015discovery,borisenko2015time,weng2015weyl,xu2015observation}, have garnered much attention from the physics and material science communities. These fermions are massless and, in some materials, have shown exceptional physical properties such as ultrahigh mobility and extremely large magnetoresistances \cite{liang2015ultrahigh,ali2014large,du2015unsaturated,shekhar2015large}. Recently, new materials have been predicted to exist which exhibit line nodes of Dirac cones \cite{PhysRevLett.115.036806,xie2015new,burkov2011topological,rhim2015landau}. Here, we show with angle resolved photoemission studies supported by \textit{ab initio} calculations that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones which form a Fermi surface with a diamond-shaped line of Dirac nodes. We also experimentally show, for the first time, that the square Si lattice in ZrSiS is an excellent template for realizing the new types of 2D Dirac cones recently predicted by Young and Kane \cite{young2015dirac} and image an unforseen surface state that arises close to the 2D Dirac cone. Finally, we find that the energy range of the linearly dispersed bands is as high as 2\,eV above and below the Fermi level; much larger than of any known Dirac material so far. This makes ZrSiS a very promising candidate to study the exotic behavior of Dirac electrons, or Weyl fermions if a magnetic field is applied, as well as the properties of lines of Dirac nodes