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.

For the purpose of recovering the intriguing electronic properties of freestanding graphene at a solid surface, graphene self-organized on a Au monolayer on Ni(111) is prepared and characterized by scanning tunneling microscopy. Angle-resolved photoemission reveals a gapless linear pi-band dispersion near K[over] as a fingerprint of strictly monolayer graphene and a Dirac crossing energy equal to the Fermi energy (EF) within 25 meV meaning charge neutrality. Spin resolution shows a Rashba effect on the pi states with a large (approximately 13 meV) spin-orbit splitting up to EF which is independent of k.

1Recent discovery of both gapped and gapless topological phases in weakly correlated electron systems has introduced various relativistic particles and a number of exotic phenomena in condensed matter physics [1][2][3][4][5] . The Weyl fermion 6-8 is a prominent example of three dimensional (3D), gapless topological excitation, which has been experimentally identified in inversion symmetry breaking semimetals 4,5 . However, their realization in spontaneously time reversal symmetry (TRS) breaking magnetically ordered states of correlated materials has so far remained hypothetical 7, 9, 10 . Here, we report a set of experimental evidence for elusive magnetic Weyl fermions in Mn 3 Sn, a non-collinear antiferromagnet that exhibits a large anomalous Hall effect even at room temperature 11 . Detailed comparison between our angle resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations reveals significant bandwidth renormalization and damping effects due to the strong correlation among Mn 3d electrons. Moreover, our transport measurements have unveiled strong evidence for the chiral anomaly of Weyl fermions, namely, the emergence of positive magnetoconductance only in the presence of parallel electric and magnetic fields. The magnetic Weyl fermions of Mn 3 Sn have a significant technological potential, since a weak field (∼ 10 mT) is adequate for controlling the distribution of Weyl points and the large fictitious field (∼ a few 100 T) in the momentum space. Our discovery thus lays the foundation for a new field of science and technology involving the magnetic Weyl excitations of strongly correlated electron systems.Traditionally, topological properties have been considered for the systems supporting gapped bulk excitations 1 . However, over the past few years three dimensional gapless systems such asWeyl and Dirac semimetals have been discovered, which combine two seemingly disjoint notions 2 of gapless bulk excitations and band topology [2][3][4][5] . In 3D inversion or TRS breaking systems, two nondegenerate energy bands can linearly touch at pairs of isolated points in the momentum (k) space, giving rise to the Weyl quasiparticles. The touching points or Weyl nodes act as the unit strength (anti) monopoles of underlying Berry curvature [4][5][6][7] , leading to the protected zero energy surface states also known as the Fermi-arcs 4,5,7 , and many exotic bulk properties such as large anomalous Hall effect (AHE) 12 , optical gyrotropy 13 , and chiral anomaly 6,[14][15][16][17][18][19] . Interestingly, the Weyl fermions can describe low energy excitations of both weakly and strongly correlated electron systems. In weakly correlated, inversion symmetry breaking materials, where the symmetry breaking is entirely caused by the crystal structure rather than the collective properties of electrons, the ARPES has provided evidence for long-lived bulk Weyl fermions and the surface Fermi arcs 4, 5 .On the other hand, the magnetic Weyl fermions have been predicted for several...

Graphene in spintronics is predominantly considered for spin current leads of high performance due to weak intrinsic spin-orbit coupling of the graphene p electrons. Externally induced large spin-orbit coupling opens the possibility of using graphene in active elements of spintronic devices such as the Das-Datta spin field-effect transistor. Here we show that Au intercalation at the graphene-Ni interface creates a giant spin-orbit splitting (B100 meV) of the graphene Dirac cone up to the Fermi energy. Photoelectron spectroscopy reveals the hybridization with Au 5d states as the source for this giant splitting. An ab initio model of the system shows a Rashba-split spectrum around the Dirac point of graphene. A sharp graphene-Au interface at the equilibrium distance accounts for only B10 meV spin-orbit splitting and enhancement is due to the Au atoms in the hollow position that get closer to graphene and do not break the sublattice symmetry.

Surfaces and interfaces o er new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1-4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5-7 to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8-13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.Carrier concentration is a key parameter defining the ground state of correlated electron systems. At the LaAlO 3 /SrTiO 3 interface, the 2DEL density can be tailored by field-effect gating. As the system is depleted of carriers, its ground state evolves from a high-mobility 2DEL 4 into a two-dimensional superconductor 8-10 with pseudogap behaviour 11 and possible pairing above T c (ref. 12). An analogous 2DEL can be induced by doping the (001) surface of SrTiO 3 . As for the interface, the surface 2DEL is confined by a band-bending potential in SrTiO 3 and consists of an orbitally polarized ladder of quantum confined Ti t 2g electrons that are highly mobile in the surface plane [5][6][7]14 . Thus far, the surface 2DEL has been studied only at carrier densities around 2 × 10 14 cm −2 , approximately a factor of five higher than typically observed at the LaAlO 3 /SrTiO 3 interface [5][6][7] . In the following, we present ARPES data extending to lower carrier densities that are directly comparable to the LaAlO 3 /SrTiO 3 interface. We achieve this by preparing SrTiO 3 (001) wafers in situ, which results in well-ordered clean surfaces that can be studied by ARPES over extended timescales, as they are less susceptible to the ultraviolet-induced formation of charged oxygen vacancies reported for cleaved SrTiO 3 5,7,15,16 . Details of the sample preparation are given in Methods. Figure 1a shows an energy-momentum intensity map for a 2DEL with a carrier density of n 2D ≈ 2.9 × 10 13 cm −2 estimated from the Luttinger volume of the first light subband and the two equivalent heavy subbands (see Supplementary Section 2). The most striking features of this data are replica bands at higher binding energy following the dispersion of the primary quasiparticle (QP) bands. The replica bands are all separated by approximately 100 meV and progressively lose intensity, but can be visualized up to the third replica in the curvature plot shown in Fi...