J. Osterwalder

Disentanglement of Surface and Bulk Rashba Spin Splittings in Noncentrosymmetric BiTeI

BiTeI has a layered and non-centrosymmetric structure where strong spin-orbit interaction leads to a giant spin splitting in the bulk bands. Here we present high-resolution angle-resolved photoemission (ARPES) data in the UV and soft x-ray regime that clearly disentangle the surface from the bulk electronic structure. Spin-resolved UV-ARPES measurements on opposite, nonequivalent surfaces show identical spin structures, thus clarifying the surface state character. Soft x-ray ARPES data clearly reveal the spindle-torus shape of the bulk Fermi surface, induced by the spin-orbit interaction. PACS numbers: 71.20.Nr, 71.70.Ej, 79.60.Bm 1 ar X iv :1 20 4. 21 96 v1 [ co nd -m at .m tr lsc i] 1 0 A pr 2 01 2 The breaking of inversion symmetry and its influence on the spin structure of surface states under action of spin–orbit interaction (SOI) has been extensively studied in recent years [1, 2]. The main finding is that the surface states become spin-split according to the Rashba model [3] resulting in two spin-polarized concentric Fermi contours. The lack of inversion symmetry in the bulk crystal structure is expected to induce a spin splitting with a more complex bandand spin-structure. Combined with strong SOI the Fermi surface can take the shape of a torus [4]. For non-centrosymmetric superconductors such as for example CePt3Si [5] this peculiar band structure is expected to result in topologically protected spin polarized edge states reminiscent of Majorana modes [6]. Recently, an ARPES and spin-resolved ARPES study by Ishizaka et al. [7] proposed that the semiconductor BiTeI features a very large spin-splitting, arising from the broken inversion symmetry in the crystal bulk and a strong SOI. Theoretical work based on the perturbative k ·p formalism linked the unusually large spin splitting in BiTeI to the negative crystal field splitting of the top valence bands [8]. Optical transition measurements [9] are in accordance with the giant bulk spin-splitting of the gap defining valence and conduction bands predicted by first principle calculations [7, 8]. In addition it was shown in recent theoretical work that BiTeI can become a topological insulator under action of hydrostatic pressure [10], and thus is closely related to non-centrosymmetric topological superconductors. The present study provides first band mapping of a system without bulk inversion symmetry and giant SOI by the example of BiTeI, featuring a three-dimensional Rashba splitting of the bulk bands. Further it is shown that the Rashba-split state observed for this material in the UV photon energy regime is not a quantum well state [7] but rather a surface state, using a simple symmetry argument based on spin-resolved ARPES (SARPES) measurements, which is confirmed by first principle calculations. All measurements were performed at the Swiss Light Source of the Paul-Scherrer-Institut. The SARPES data was measured with the Mott polarimeter at the COPHEE endstation [11] of the Surface and Interface Spectroscopy beamline at a photon energy of 24 eV. The spin-integrated data at photon energies 20-63 eV were taken at the high-resolution ARPES endstation at the same beamline. The soft x-ray ARPES data were taken at the SX-ARPES endstation of the ADRESS beamline at photon energies of 310-850 eV. All spin-integrated measurements were performed at a sample temperature of 11 K and a base pressure lower than 10−10 mbar, the SARPES data was taken at 20 K.

Cr-doped TiO2 anatase: A ferromagnetic insulator

Epitaxial ferromagnetic films of Cr-doped TiO2 anatase (CrxTi1−xO2−x∕2) were grown on LaAlO3(001) using oxygen-plasma-assisted molecular-beam epitaxy. CrK-shell x-ray absorption near-edge spectroscopy shows that the formal oxidation state of Cr is +3 throughout the films, with no evidence for either elemental Cr or half-metallic CrO2. Cr is found to substitute for Ti in the lattice, with uniform distribution throughout the doped region of the film. The Cr-doped anatase films exhibit room-temperature ferromagnetism aligned in-plane, with a saturation magnetization of ∼0.6μB/Cr atom.

X-ray photoelectron and Auger electron diffraction study of diamond and graphite surfaces

Abstract Natural diamond (001) and (111) surfaces as well as highly oriented pyrolithic graphite (HOPG) (0001) have been analyzed by X-ray induced photoelectron diffraction (XPD). The measured 2π patterns of C Is emission (964 and 1450 eV) and of KVV Auger emission (260 eV) are compared to single scattering cluster (SCC calculations, and excellent agreement is found. The comparisons show that photoelectron forward focusing is much less prominent in carbon solids than in all previously studied, heavier elements, and a direct interpretation of the data is therefore more difficult. The highly textured nature of HOPG presents itself as a circular pattern with concentric rings centered at the surface normal (c-axis). The aim of this work is to show the advantage of the XPD technique as a diagnostic tool for the investigation of epitaxial diamond growth with monolayer sensitivity.

Bulk and surface Rashba splitting in single termination BiTeCl

By angle-resolved photoemission spectroscopy (ARPES) we observe a giant Rashba-type spin splitting in the electronic bulk conduction and valence bands of the semiconductor BiTeCl. This material belongs to the group of bismuth tellurohalides BiTeX (Xxa0=xa0Cl,Br,I) which are layered non-centrosymmetric materials with strong spin–orbit interaction. By photon energy-dependent ARPES, we separate the bulk and surface contribution of the electronic structure and show that the tellurium-terminated (0001) crystal surface hosts spin-split two-dimensional surface states. On the chlorine-terminated surface at the opposite side of the crystal no surface states are observed due to photon-induced surface chemistry.

Holographic interpretation of photoelectron diffraction

Abstract Real space image formation in photoelectron holography is discussed using a reconstruction formalism based on the Helmholtz-Kirchhoff theorem in spherical coordinates. In this representation it is readily understood under which conditions reliable atomic images are formed and when artifacts prevail. In multilayer substrate emission, at energies of a few hundred eV and above where forward scattering amplitudes dominate the diffraction patterns, no atomic images are obtained and the structures observed in real space coordinates are formed due to the symmetry of the crystal lattice. This is illustrated by a comparison of holographic reconstructions of data from diamond (111) and Si(111) surfaces. On the other hand, forward scattering is absent in diffraction patterns obtained from adsorbate species situated on top of the surface. For c(2×2) Na/Al(001) we clearly observe images of first and second nearest in-plane neighbours.

Tuning of the Rashba effect in Pb quantum well states via a variable Schottky barrier

Spin-orbit interaction (SOI) in low-dimensional systems results in the fascinating property of spin-momentum locking. In a Rashba system the inversion symmetry normal to the plane of a two-dimensional (2D) electron gas is broken, generating a Fermi surface spin texture reminiscent of spin vortices of different radii which can be exploited in spin-based devices. Crucial for any application is the possibility to tune the momentum splitting through an external parameter. Here we show that in Pb quantum well states (QWS) the Rashba splitting depends on the Si substrate doping. Our results imply a doping dependence of the Schottky barrier which shifts the Si valence band relative to the QWS. A similar shift can be achieved by an external gate voltage or ultra-short laser pulses, opening up the possibility of terahertz spintronics.

Full hemispherical photoelectron diffraction and Fermi surface mapping

Abstract The routine measurement of full hemispherical photoemission intensity maps gives us the possibility for the combined investigation of structural and electronic phenomena at surfaces. As an example the growth of ultrathin films of Co on Cu(111) is studied as a function of film thickness. While X-ray photoelectron diffraction (XPD) shows the early appearance of stacking faults as a precursor of the hcp structure, Fermi surface maps reveal the very fast evolution of the Co Fermi surface that can be compared to measurements on a clean Co(0001) crystal. For the system O/Rh(111), XPD brings up important structural clues, relating changes in surface reactivity to small amounts of subsurface oxygen, which forces adjacent oxygen atoms to occupy new and more reactive adsorption sites. In the course of this last study we observed for the first time the weak backscattering signals in the angular pattern of adsorbate emission. These cone-like features are extremely sensitive to the adsorbate–substrate bond length.

Separating the bulk and surface n- to p-type transition in the topological insulator GeBi4-xSbxTe7

We identify the multilayered compound GeBi4Te7 to be a topological insulator with a Dirac point slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisfies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi4-xSbxTe7 we observe a transition from n to p type in bulk sensitive Seebeck coefficient measurements at a doping of x = 0.6. In surface sensitive ARPES measurements a rigid band shift is observed with Sb doping, accompanied by a movement of the Dirac point towards the Fermi level. Between x = 0.8 and x = 1 the Fermi level crosses the band gap, changing the surface transport regime. This difference of the n- to p-type transition between the surface region and the bulk is caused by band bending effects which are also responsible for a noncoexistence of insulating phases in the bulk and in the near surface region.

Unconventional Fermi surface spin textures in the BixPb1−x/Ag(111) surface alloy

The Fermi and Rashba energies of surface states in the

ALKALI-METAL ADSORPTION GEOMETRIES ON METAL SURFACES FROM PHOTOELECTRON-DIFFRACTION EXPERIMENTS

{text{Bi}}_{x}{text{Pb}}_{1ensuremath{-}x}/text{Ag}(111)