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 manipulation of the bandgap of graphene by various means has stirred great interest for potential applications. Here we show that treatment of graphene with xenon difluoride produces a partially fluorinated graphene (fluorographene) with covalent C-F bonding and local sp(3)-carbon hybridization. The material was characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, electron energy loss spectroscopy, photoluminescence spectroscopy, and near edge X-ray absorption spectroscopy. These results confirm the structural features of the fluorographane with a bandgap of 3.8 eV, close to that calculated for fluorinated single layer graphene, (CF)(n). The material luminesces broadly in the UV and visible light regions, and has optical properties resembling diamond, with both excitonic and direct optical absorption and emission features. These results suggest the use of fluorographane as a new, readily prepared material for electronic, optoelectronic applications, and energy harvesting applications.

Oxygen vacancies created in anatase TiO2 by UV photons (80 -130 eV) provide an effective electrondoping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.The anatase structural phase of titanium dioxide (TiO 2 ) can be the key element in novel applications. Whereas extensive work has been focused on its famous photocatalytic behavior [1-3], more and more proposed devices, such as memristors [4], spintronic devices [5], and photovoltaic cells [6][7][8], rely on its less well-known electronic properties. In particular, anatase has been recently suggested as a candidate for replacing the In-based technology for transparent conducting oxides [9] in a wide range of applications from solar cell elements, to light-emitting devices, to flat panels, to touch-screen controls [10]. The crucial quantity for the figure of merit in these devices is conductivity, and it is therefore of major interest to understand and control the electronic properties of pristine and doped anatase.Stoichiometric anatase is an insulator with a 3.2 eV band gap [11] but oxygen vacancies, typically present with concentrations in the 10 17 cm −3 range [12,13], create a shallow donor level ∼10 meV below the conduction band (CB) [14]. Since large single crystals became available for transport studies, a better insight has been gained on the influence of these donors on the electronic response of anatase. Above ∼60 K, the electrons thermally excited into the CB give rise to metallic-like transport. At lower temperatures, the anomalous increase of resistivity indicates that the charge carriers are not bare electrons but polarons [14], i.e., electrons coherently coupled to a lattice distorsion induced by the Coulomb interaction. Understanding the properties of such composite particles in anatase is important to better engineer the material for targeted applications, where the low electron mobility often represents the overall performance bottleneck. We will also demonstrate that, from the point of view of fundamental physics, anatase represents an excellent model compound to study the behavior of the "rare" large polaron quasiparticles (QPs), intermediate between localized small polarons and free electrons.We performed ARPES measurements on TiO 2 single crystals ( Fig. 1(a)) and thin films grown in situ on insulating LaAlO 3 and conducting Nb-doped SrTiO 3 substrates. Clean (001) surfaces were prepared as described in Suppl. Inf. The results presented have been obtained consistently both for single crystals and thin films, and therefore reflect intrinsic properties of the anatase phase, independent of the sample preparation method. While oxygen defects are always present to some extent after the surface preparation, we have found that exposure to UV photons...

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.

We employ angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structures of two rotational variants of epitaxial, single-layer graphene on Ir(111). As grown, the more-abundant R0 variant is nearly charge-neutral, with strong hybridization between graphene and Ir bands near the Fermi level. The graphene Fermi surface and its replicas exactly coincide with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry breaking introduces a small gap-inducing potential at the Dirac crossing, which is revealed by n-doping the graphene using K atoms. The energy gaps between main and replica bands (originating from the moiré interference pattern between graphene and Ir lattices) is shown to be non-uniform along the minizone boundary due to hybridization with Ir bands. An electronically mediated interaction is proposed to account for the stability of the R0 variant. The variant rotated 30° in-plane, R30, is p-doped as grown and K doping reveals no band gap at the Dirac crossing. No replica bands are found in ARPES measurements. Raman spectra from the R30 variant exhibit the characteristic phonon modes of graphene, while R0 spectra are featureless. These results show that the film/substrate interaction changes from chemisorption (R0) to physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has a work function lower than the clean substrate but higher than graphite.-2 -

Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport.

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low energy electron microscopy, low energy electron diffraction and angle resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of ∼ -0.3 eV and the opening of a ∼ 250 meV gap. Exposure of the samples to air resulted in intercalation of oxygen under the graphene on Cu(100), which formed a (superstructure. The effect of this intercalation on the graphene π bands is to increase the offset of the Dirac crossing (∼ -0.6 eV) and enlarge the gap (∼ 350 meV). No such effect is observed for the graphene on Cu (111) sample, with the surface state at Γ not showing the gap associated with a surface superstructure. The graphene film is found to protect the surface state from air exposure, with no change in the effective mass observed, as for 1 monolayer of Ag on Cu(111).

van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8–2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.