A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.

The intense theoretical and experimental interest in topological insulators and semimetals has established band structure topology as a fundamental material property. Consequently, identifying band topologies has become an important, but often challenging problem, with no exhaustive solution at the present time. In this work we compile a series of techniques, some previously known, that allow for a solution to this problem for a large set of the possible band topologies. The method is based on tracking hybrid Wannier charge centers computed for relevant Bloch states, and it works at all levels of materials modeling: continuous k · p models, tight-binding models and ab initio calculations. We apply the method to compute and identify Chern, Z2 and crystalline topological insulators, as well as topological semimetal phases, using real material examples. Moreover, we provide a numerical implementation of this technique (the Z2Pack software package) that is ideally suited for high-throughput screening of materials databases for compounds with non-trivial topologies. We expect that our work will allow researchers to: (a) identify topological materials optimal for experimental probes, (b) classify existing compounds and (c) reveal materials that host novel, not yet described, topological states.

We observe a giant spin-orbit splitting in bulk and surface states of the non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics -a large, robust spin splitting and ambipolar conduction -are present in this material.

The recently discovered type-II Weyl points appear at the boundary between electron and hole pockets. Type-II Weyl semimetals that host such points are predicted to exhibit a new type of chiral anomaly and possess thermodynamic properties very different from their type-I counterparts. In this Letter, we describe the prediction of a type-II Weyl semimetal phase in the transition metal diphosphides MoP2 and WP2. These materials are characterized by relatively simple band structures with four pairs of type-II Weyl points. Neighboring Weyl points have the same chirality, which makes the predicted topological phase robust with respect to small perturbations of the crystalline lattice. In addition, this peculiar arrangement of the Weyl points results in long topological Fermi arcs, thus making them readily accessible in angle-resolved photoemission spectroscopy.Topological semimetals host band degeneracies in the vicinity of the Fermi level (E F ) that are associated with certain integer-valued topological invariants [1][2][3][4][5][6][7][8][9]. Since these invariants cannot continuously change their values, the associated degeneracies are protected against perturbations. One such semimetal phase -the Weyl semimetal (WSM) -hosts point-like linear band crossings of two bands with linear dispersion, the so-called Weyl points (WPs), in the vicinity of E F . These WPs represent sources or sinks of Berry curvature, and their associated topological invariant is the Chern number C = ±1 computed on a surface in momentum space that encloses the WP. Positive (negative) Chern numbers correspond to a source (sink) of the Berry curvature. A variety of topology-driven physical phenomena is predicted and observed in WSMs, ranging from the observation of open Fermi arcs in the surface spectrum [2,10] to the realization of the chiral anomaly of quantum field theory [11][12][13][14][15][16][17].It was recently shown [18] that unlike standard Lorentz-invariant field theory, condensed matter physics has two distinct types of Weyl fermions, and hence WSMs. While standard type-I Weyl fermions with closed Fermi surfaces were discovered in materials of the TaAs family [8,9,[19][20][21][22][23][24][25], the novel type-II Weyl fermions appear at the boundary between electron and hole pockets, leaving an open Fermi surface which results in the anisotropic chiral anomaly [18,26]. Two representatives of type-II WSMs materials considered to date are the orthorhombic low-temperature phases of WTe 2 and MoTe 2 [18,[27][28][29]. Type-II Weyl points were also recently predicted to exist in strained HgTe [30]. Eight (four) type-II WPs appear in WTe 2 (MoTe 2 ) formed by the valence and conduction bands. In WTe 2 some of the carrier pockets become topologically non-trivial, while in MoTe 2 they are all trivial, and the two materials represent very different Fermi arc arrangements [18,28]. In both cases, however, the band structure is very complicated and the arrangement of WPs is sensitive to small changes in the crystal structure, which, in turn, is se...

We report on the magnetic properties of single Co atoms on graphene on Pt(111). By means of scanning tunneling microscopy spin-excitation spectroscopy, we infer a magnetic anisotropy of K ¼ À8:1 meV with out-of-plane hard axis and a magnetic moment of 2:2 B . Co adsorbs on the sixfold graphene hollow site. Upon hydrogen adsorption, three differently hydrogenated species are identified. Their magnetic properties are very different from those of clean Co. Ab initio calculations support our results and reveal that the large magnetic anisotropy stems from strong ligand field effects due to the interaction between Co and graphene orbitals. DOI: 10.1103/PhysRevLett.111.236801 PACS numbers: 73.22.Pr, 32.10.Dk, 75.30.Gw, 75.70.Rf Graphene is a promising material for spintronics due to the possibility of realizing controllable spin transport [1], its perfect spin filtering [2], and spin-relaxation lengths of several micrometers at room temperature [3]. Doping graphene by magnetic impurities opens further possibilities [4][5][6]. In particular, the creation of extended magnetic phases [7], quantum critical Kondo anomalies [8,9], and the strong scattering of spin currents [10] have been predicted. Moreover, for 3d metal atoms on graphene, the calculated uniaxial magnetic anisotropies [11,12] are beyond the current record value for a surface-adsorbed atom [13]. Furthermore, hydrogen adsorption has been predicted to change the spin of the adatoms [14], underlining its potential to tailor the magnetic properties. However, the predicted anisotropies and moments are highly controversial and largely depend on how electron correlations are treated. For the prototypical system of Co atoms on graphene, spin moments between 1 and 3 B and anisotropies of different signs have been calculated [12,[15][16][17]. At present, there is only one experiment addressing the magnetic properties of transition-metal adatoms on graphene. It reports on-top adsorption, a high-spin ground state, and weak magnetic anisotropy for Co=graphene=SiCð0001Þ [18].Here we present the first local measurement of the magnetic moment and anisotropy of individual Co atoms on graphene on Pt(111). This substrate was chosen since graphene binds very weakly to it [19], thus approaching freestanding graphene. Using scanning tunneling microscopy (STM) spin-excitation spectroscopy [20], we find an exceptionally large magnetic anisotropy of K ¼ À8:1 AE 0:4 meV with out-of-plane hard axis and a magnetic moment of 2:2 AE 0:4 B . Fully relativistic density functional theory (DFT) calculations show the anisotropy to be mainly a hybridization effect. In addition to clean Co, we identify three hydrogenated species, CoH n , n 2 f1; 2; 3g, with very different magnetic behavior. The coexistence of clean and hydrogenated adatoms is expected to be a general feature of transition metal atoms on graphene that has to be considered in the interpretation of any ensemble measurement.The experiments were performed with a homebuilt STM operating at T ¼ 0:4 K and in magnetic fields up to ...