We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

Two-dimensional electron systems, as exploited for device applications, can lose their conducting properties because of local Coulomb repulsion, leading to a Mott-insulating state. In triangular geometries, any concomitant antiferromagnetic spin ordering can be prevented by geometric frustration, spurring speculations about 'melted' phases, known as spin liquid. Here we show that for a realization of a triangular electron system by epitaxial atom adsorption on a semiconductor, such spin disorder, however, does not appear. Our study compares the electron excitation spectra obtained from theoretical simulations of the correlated electron lattice with data from high-resolution photoemission. We find that an unusual row-wise antiferromagnetic spin alignment occurs that is reflected in the photoemission spectra as characteristic 'shadow bands' induced by the spin pattern. The magnetic order in a frustrated lattice of otherwise non-magnetic components emerges from longer-range electron hopping between the atoms. This finding can offer new ways of controlling magnetism on surfaces.

Atomic nanowires formed by Au on Ge(001) are scrutinized for the band topology of the conduction electron system by k-resolved photoemission. Two metallic electron pockets are observed. Their Fermi surface sheets form straight lines without undulations perpendicular to the chains within experimental uncertainty. The electrons hence emerge as strictly confined to one dimension. Moreover, the system is stable against a Peierls distortion down to 10 K, lending itself for studies of the spectral function. Indications for unusually low spectral weight at the chemical potential are discussed.

The spin texture of the metallic two-dimensional electron system (√3×√3)-Au/Ge(111) is revealed by fully three-dimensional spin-resolved photoemission, as well as by density functional calculations. The large hexagonal Fermi surface, generated by the Au atoms, shows a significant splitting due to spin-orbit interactions. The planar components of the spin exhibit helical character, accompanied by a strong out-of-plane spin component with alternating signs along the six Fermi surface sections. Moreover, in-plane spin rotations towards a radial direction are observed close to the hexagon corners. Such a threefold-symmetric spin pattern is not described by the conventional Rashba model. Instead, it reveals an interplay with Dresselhaus-like spin-orbit effects as a result of the crystalline anisotropies. Breaking the translational symmetry at the solidvacuum interface strongly affects electrons, including their spin properties. The resulting structure-inversion asymmetry can induce a splitting of the surface bands based on the spin-orbit interaction, known as the Rashba effect [1]. The resulting lift of spin degeneracy is the basis for the emerging and important field of spintronics. As a current and prominent research object, topological insulators with their characteristic Dirac surface states show a spin-momentum locking [2]. Additional examples are surfaces formed by, or decorated with heavy atoms. This can be probed by angle-resolved photoelectron spectroscopy (ARPES), preferably with spin detection (SARPES), as shown, e.g., for surfaces like Au(111), Bi(111) [3,4] or surface alloys such as Bi/Ag(111) [5,6].The realization of a strong Rashba effect in a metallic two-dimensional (2D) electron system at a semiconductor surface or interface would be particularly desirable, since it offers the perspective to manipulate spins electronically [7]. Different concepts to achieve spin filtering in Rashba systems, via ferromagnetic top electrodes [8] or by resonant tunneling [9], are intensively discussed. Studies in heterostructures show that the Rashba coupling strength can effectively be controlled by a gate field [10]. At semiconductor surfaces, it was demonstrated that heavy atoms may induce a particularly large Rashba splitting, as reported for the insulating bands formed by Bi and Tl reconstructions on Si(111) [11,12,13]. However, conducting spin-split states are needed to utilize spin control in electronic transport applications. To date, these have only been found in the β-phase of (√3×√3)-Pb/Ge(111) [14], where a Rashba situation was observed using SARPES with in-plane spin detection only. In this regard, the Au-induced (√3×√3)-reconstructed surface of Ge (111) with its spin-split metallic states represents a promising 2D electron system to be scrutinized in this study [15,16].Additional perspectives are provided by the exploitation of the three-dimensional (3D) orientation of the spin vector at the Fermi surface (FS). For Tl/Si(111) and in surface alloys indications have been found that spins rotate out of plane...

The quasiparticle dynamics of electrons in a magnetically ordered state is investigated by high-resolution angle-resolved photoemission of Ni(110) at 10 K. The self-energy is extracted for high binding energies reaching up to 500 meV, using a Gutzwiller calculation as a reference frame for correlated quasiparticles. Significant deviations exist in the 300 meV range, as identified on magnetic bulk bands for the first time. The discrepancy is strikingly well described by a self-energy model assuming interactions with spin excitations. Implications relating to different electron-electron correlation regimes are discussed. PACS numbers:The many-body ground state of condensed matter is reflected in its single-particle excitations, which in many cases are significantly modified by coupling to collective modes. Such interactions lead to a pronounced change in the quasiparticle (QP) band dispersion, a so-called kink. In metals, the kink from electron-phonon coupling is well-established [1]. Energy-renormalization is also found in the high-T c cuprates [2]. However, the nature of this feature is not yet completely clarified, albeit of primordial importance to the mechanism of superconductivity. It is being discussed whether the kink is derived from coupling to phonons or to spin fluctuations [3]. Their similar energy scales in the cuprates make it difficult to separate the contributions. Recent experiments on relationships with sample parameters [4,5] argue for the magnetic coupling model. Closer resemblance to magnetic metals is found in the newly discovered iron-based pnictides [6]. Their parent compounds are true metals with delocalized electrons forming an antiferromagnetic spin density wave. A pairing mechanism based on spin fluctuations has been suggested [7].Interestingly, a different explanation for kinks in strongly correlated electron systems was suggested recently, which does not require electron-boson coupling. Calculations based on pure electron-electron interaction found two well-separated regimes of QP renormalization [8]. Near the Fermi level, well-defined QPs exist according to Fermi liquid theory. Beyond a characteristic energy scale, the slope of the electronic self-energy changes abruptly, resulting in reduced QP lifetimes and energy renormalization. In the transition between these situations, a dispersion anomaly is expected to emerge [8].In order to gain access to these many-body interactions, a three-dimensional Fermi liquid in the ferromagnetic state seems a suitable model system. The energy scales for the lattice and spin wave excitations in typical ferromagnets such as Ni differ by approximately an order of magnitude, and hence will affect the QPs at different binding energies [9,10]. Furthermore, it is established that the valence band states are strongly correlated [11,12], which is proven by a concomitant photoemission satellite. This allows to directly adress the interplay of correlation physics and QP formation in the presence of distinct spin excitations.In this Letter, we present a high-r...