Ph. Hofmann

Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor

The coupling between spin, valley and layer degrees of freedom in transition-metal dichalcogenides is shown to give rise to spin-polarized electron states, providing opportunities to create and manipulate spin and valley polarizations in bulk solids. Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices1. This is generally accepted to require breaking global structural inversion symmetry1,2,3,4,5. In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe2. Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized6, we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides7,8, of key importance for using these compounds in proposed valleytronic devices.

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

Abstract A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 A. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

Emergent quantum confinement at topological insulator surfaces

Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z(2) topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement.

The local adsorption structure of acetylene on Cu(111)

Abstract A quantitative structural study of the adsorption of acetylene on Cu(111) has been carried out by scanned energy mode photoelectron diffraction from the C 1s core levels. The molecule is found to be adsorbed with the CC axis almost parallel to the surface with the carbon atoms occupying the two symmetrically distinct three-fold coordinated hollow sites on the surface. The C to top Cu atom layer spacing appears to be slightly smaller (1.38 ± 0.03 A ) above the “fcc” hollow site (above a third-layer Cu atom) than above the “hcp” hollow site (1.44 ± 0.03 A ) . The best fit to theory is obtained with the C atoms exactly in the three-fold symmetric sites of the substrate which constrains the CC bondlength to a value (1.48 ± 0.10 A ) substantially larger than that of the free molecule. This result is consistent with previously published vibrational spectroscopy from this adsorption system which indicates significant lowering of the CC bond order.

Structure determination of ammonia on Cu(110) — a low-symmetry adsorption site

Abstract The local adsorption structure of ammonia on Cu(110) has been determined in a quantitative fashion using N 1s scanned-energy mode photoelectron diffraction. While inversion of the photoelectron diffraction spectra using a direct method indicates that the adsorbed NH 3 molecules are near to atop sites, a fuller multiple scattering analysis shows that the molecule is actually displaced 0.37 A off the atop site in a 〈100〉 azimuth. The result is ascribed to adsorbate-adsorbate interactions (steric hindrance) similar to those found in (2 × 1)CO (pmg) structures seen on several fcc (110) surfaces, although in the case of ammonia, it occurs at coverages well below saturation, implying that adsorbate-adsorbate attraction also occurs. These general conclusions are entirely consistent with those of a recent ESDIAD study of this system.

The geometric structure of the surface methoxy species on Cu(111)

Abstract The adsorption geometry of the surface methoxy species (-OCH 3 ) on Cu(111) has been determined quantitatively using scanned energy mode photoelectron diffraction in a two-step approach. First, using direct methods it was established that the fragment adsorbs via the oxygen atom at the “fcc” three-fold adsorption site. Subsequently, the structural parameters were refined by a search in multi-parameter space using diffraction spectra obtained in different emission geometries. The oxygen atom is found to be situated 1.32(±0.05) A above the three nearest-neighbour copper atoms with the O-C axis essentially perpendicular to the surface. The O-C distance is 1.42(−0.03/+ 0.10) A; a relaxation as well as a rumple of the outermost Cu layer occur.

In-plane magnetic anisotropy of Fe atoms on Bi2Se3(111).

The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on the prototypical 3D topological insulator Bi(2)Se(3) using local low-temperature scanning tunneling spectroscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi(2)Se(3) surface, in the coverage range ≈ 1% of a monolayer, are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface plane, contrary to what was assumed in recent investigations on the supposed opening of a gap. Using ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.

Coverage-dependent changes in the adsorption geometry of benzene on Ni{111}

Abstract Photoelectron diffraction in the scanned energy mode has been used to determine the structure of benzene adsorbed on to an Ni{111} surface. In the disordered phase at low coverage the molecule is centered over a bridge site with the CC bonds oriented in the 〈211〉 directions, but in the ordered overlayer at saturation coverage the hcp threefold symmetric hollow site is occupied with the CC bonds oriented in the 〈110〉 directions. The molecular planes are situated 1.92(±0.05) A and 1.91(±0.04) A, respectively, above the Ni atoms which form each adsorption site. There is no significant distortion of the benzene ring. The results are compared with photoemission measurements and quantum chemical calculations.

Controllable Magnetic Doping of the Surface State of a Topological Insulator

A combined experimental and theoretical study of doping individual Fe atoms into Bi(2)Se(3) is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment, thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.

Surface-sensitive conductance measurements

Several approaches for surface-sensitive conductance measurements are reviewed. Particular emphasis is placed on nanoscale multi-point probe techniques. The results for two model systems, which have given rise to some dispute, are discussed in detail: Si(111)(7 × 7) and ([Formula: see text])Ag-Si(111). Other recent examples are also given, such as phase transitions in quasi-one-dimensional structures on semiconductor surfaces and the surface sheet conductivity of Bi(111), the surface of a semimetal.