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.

Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131-134]. Although experiments have shown an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to λ ≃ 0.58. On part of the graphene-derived π*-band Fermi surface, we then observe the opening of a Δ ≃ 0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with T c ≃ 5.9 K.graphene | superconductivity | ARPES

Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi(2)Se(3) as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi(2)Se(3) crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi(2)Se(3) band structure.

The application of graphene in electronic devices requires large scale epitaxial growth. The presence of the substrate, however, usually reduces the charge carrier mobility considerably. We show that it is possible to decouple the partially sp 3 -hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp 2 -hybridized graphene layer with improved electronic properties.Electrons in graphene -sp 2 -bonded carbon atoms arranged in a honeycomb lattice -behave like massless Dirac particles and exhibit an extremely high carrier mobility [1]. So far, the only feasible route towards large scale production of graphene is epitaxial growth on a substrate. The presence of the substrate will, however, influence the electronic properties of the graphene layer. To preserve its unique properties it is desirable to decouple the graphene layer from the substrate. Here we present a new approach for the growth of highly decoupled epitaxial graphene on a silicon carbide substrate. By decoupling the strongly interacting, partially sp 3hybridized first graphitic layer (commonly referred to as zero layer (ZL) [2]) from the SiC(0001) substrate by gold intercalation, we obtain a completely sp 2 -hybridized graphene layer with improved electronic properties as confirmed by angleresolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM) and Raman spectroscopy.There are essentially two ways for large scale epitaxial growth of graphene on a substrate: by cracking organic molecules on catalytic metal surfaces [3][4][5][6][7] or by thermal graphitization of SiC [2,[8][9][10][11]. Unfortunately, the presence of the substrate alters the electronic properties of the graphene layer on the surface and reduces the carrier mobility. Even though it has been shown that the graphene layer can be decoupled from a metallic substrate [6,[12][13][14] the system remains unsuitable for device applications. This problem can be solved by decoupling the graphene layer from a semiconducting SiC substrate [15].On both the silicon and the carbon terminated face of a SiC substrate, graphene is commonly grown by thermal graphitization in ultra high vacuum (UHV). When annealing the substrate at elevated temperatures Si atoms leave the surface whereas the C atoms remain and form carbon layers. On SiC(0001), the so-called C-face, the weak graphene-tosubstrate interaction results in the growth of rotationally disordered multilayer graphene and a precise thickness control becomes difficult [16]. On the other hand, the rotational disorder decouples the graphene layers so that the transport properties resemble those of isolated graphene sheets with room temperature mobilities in excess of 200,000 cm 2 /Vs [17].On SiC(0001), i. e. the Si-face, the comparatively strong graphene-to-substrate interaction results in uniform, long-range ordered layer-by-layer growth. The first carbon layer (=ZL) grown on the Si-face is partially sp 3 -hybridized to the substrate, wh...

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”

We demonstrate the induction of a giant Rashba-type spin-splitting on a semiconducting substrate by means of a Bi trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken so that the in-plane potential gradient induces a giant spin-splitting with a Rashba energy of about 140 meV, which is more than an order of magnitude larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations. We envision important implications for basic phenomena as well as for the semiconductor based technology.