Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. 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 other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.

The long-range ordered surface alloy Bi=Ag 111 is found to exhibit a giant spin splitting of its surface electronic structure due to spin-orbit coupling, as is determined by angle-resolved photoelectron spectroscopy. First-principles electronic structure calculations fully confirm the experimental findings. The effect is brought about by a strong in-plane gradient of the crystal potential in the surface layer, in interplay with the structural asymmetry due to the surface-potential barrier. As a result, the spin polarization of the surface states is considerably rotated out of the surface plane. DOI: 10.1103/PhysRevLett.98.186807 PACS numbers: 73.20.At, 71.70.Ej, 79.60.ÿi In nonmagnetic solids, electronic states of opposite spin orientation are often implicitly taken to be degenerate (Kramers' degeneracy). However, spin degeneracy is a consequence of both time-reversal and inversion symmetry. If one of the latter is broken, the degeneracy can be lifted by, e.g., the spin-orbit (SO) interaction. This is, for example, the case in crystals that lack a center of inversion in the bulk (Dresselhaus effect) [1,2]. But also a structural inversion asymmetry, as it shows up at surfaces or interfaces, can lead to spin-split electronic states [RashbaBychkov (RB) effect] [3]. In particular, clean surfaces of noble metals show spin-split surface states, where the splitting increases with the strength of the atomic SO coupling (cf. Ag and Au in Table I). The splitting can be further enhanced by adsorption of adatoms [9][10][11][12]. Hence, using morphology and chemistry to tune the spin splitting of twodimensional electronic states is a promising path to create a new class of nanoscale structures suitable for spintronic devices. Doping GaAs by only a few percent with Bi atoms has been shown to strongly increase the spin-orbit splitting energy 0 [13]. However, a value for the Rashba-Bychkov type spin splitting has not been reported.The Au(111) L-gap surface state is the paradigm of a Rashba-Bychkov system with a spin splitting of a few tens of meV, that was investigated in detail by means of spinand angle-resolved photoelectron spectroscopy (ARPES) [14]. The nonrelativistic Hamilton operator of the spinorbit interaction,can be expressed for a two-dimensional gas of free electrons (in the xy plane) asin which the Rashba parameter R is essentially determined by the gradient of the potential V in z direction,and is the vector of Pauli matrices. This model reproduces remarkably well the very characteristic dispersion of the spin-split surface-state bands of Au(111). The spin polarizations P of the split and completely polarized (jPj 100%) electronic states lie axially symmetric within the surface plane (P ? k k ? e z ). Time-reversal symmetry requires P k k ÿP ÿk k and E k k E ÿk k . The two main contributions to the spin splitting are a strong atomic SO interaction and a potential gradient along the surface normal (z direction). By adsorption of noble gases and oxygen, the spin splitting was successfully enhanced by increasing ...

The application of graphene in nanoscale electronic devices requires the deliberate control of the density and character of its charge carriers. We show by angle-resolved photoemission spectroscopy that substantial hole doping in the conical band structure of epitaxial graphene monolayers can be achieved by the adsorption of bismuth, antimony, or gold. In the case of gold doping the Dirac point is shifted into the unoccupied states. Atomic doping presents excellent perspectives for large scale production.