Handedness-dependent quasiparticle interference in the two enantiomers of the topological chiral semimetal PdGa

It has recently been proposed that combining chirality with topological band theory results in a totally new class of fermions. Understanding how these unconventional quasiparticles propagate and interact remains largely unexplored so far. Here, we use scanning tunneling microscopy to visualize the electronic properties of the prototypical chiral topological semimetal PdGa. We reveal chiral quantum interference patterns of opposite spiraling directions for the two PdGa enantiomers, a direct manifestation of the change of sign of their Chern number. Additionally, we demonstrate that PdGa remains topologically non-trivial over a large energy range, experimentally detecting Fermi arcs in an energy window of more than 1.6eV that is symmetrically centered around the Fermi level. These results are a consequence of the deep connection between chirality in real and reciprocal space in this class of materials, and, thereby, establish PdGa as an ideal topological chiral semimetal. Direct visualization of chiral effects in topological chiral semimetals remains elusive. Here, Sessi et al. demonstrate that quasiparticle scattering at impurities in the two enantiomers of PdGa gives rise to handedness dependent quantum interference patterns.

Coverage-dependent self-assembly of rubrene molecules on noble metal surfaces observed by scanning tunneling microscopy

François Patthey, Marina Pivetta, Wolf-Dieter Schneider

Coverage-dependent self-assembly of rubrene molecules on different noble metal surfaces, Au(111) and Au(100), Ag(111) and Ag(100), is presented. On Au(111), the homochiral supramolecular assemblies evolve with increasing rubrene coverage from very small structures composed of a few molecules, to honeycomb islets, and to one-dimensional chains of supramolecular pentamers. At higher coverage, the racemic mixture of molecules forms close-packed islands. On Au(100), chains of pentamers and two different types of densely packed islands are formed. On the Ag surfaces, exclusively close-packed islands are created, independently of the rubrene coverage. Moreover, the role of the chiral nature of the molecules in the self-assembly process is discussed, as well as the existence of different molecular conformers depending on the supramolecular assembled phase. The observed differences and similarities reflect the influence of the electronic properties and the geometric structure of the various substrates on molecular self-assembly. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.

Wiley-Blackwell2010

Bonding and Charge Transfer in Metal-Organic Coordination Networks on Au(111) with Strong Acceptor Molecules

Nan Jiang, Klaus Kern, Alexander Langner, Sebastian Stepanow

The geometric and electronic structure of two structurally similar metal organic networks grown on the Au(111) surface is investigated by scanning tunnelling microscopy (STM) and spectroscopy (STS) combined with density functional theory (DFT) calculations. The networks are composed of (i) F4TCNQ (C12F4N4, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane) molecules and Au adatoms segregated from the pristine metal surface, and (ii) TCNQ (C12H4N4, 7,7,8,8-tetracyanoquinodimethane) and codeposited Mn atoms. In both cases, the strong electron acceptor character of the molecules results in metal to-ligand charge transfer to the lowest unoccupied molecular orbital (LUMO). The amount of electrons donated from the 4-fold coordinated Mn atoms to TCNQ is higher compared to the 2-fold coordinated Au adatoms to F4TCNQ This behavior is reflected in the appearance of distinct spectral features in STS data in the energy region close to the Fermi level resulting from the intricate interplay between surface states, adatom states, and molecular orbitals. These observations are consistent with a picture in which the LUMO of the TCNQ acceptor molecule hybridizes with Mn and Au substrate metal states becoming practically filled, while the LUMO of F4TCNQ is only partially filled despite being the stronger electron acceptor. Our results reveal the importance of the type of bonding between the strong acceptor and the metal center (Au or Mn) as well as its coordination in the determination of the charge transfer to the adlayer, which is important for its electronic properties.

Amer Chemical Soc2012

Electronic and magnetic interactions in single molecular and atomic contacts

Verena Katharina Schendel

This thesis encompasses an investigation of organic molecules on metallic substrates as well as a study of electronic and magnetic effects in atomic-sized palladium contacts. For this, a scanning tunneling microscope, operating at 6 K and ultra-high vacuum is used. The clean environment allows characterization of single molecules and atoms under well-defined conditions. Moreover, by examining at low-temperatures it is possible to study electronic and magnetic effects spectroscopically, which would otherwise be thermally obscured. The first part of this thesis focuses on studying organic molecules absorbed on metallic substrates. The molecules under investigation are thiophene derivatives of the archetypical organic semiconductor pentacene. The thiophene derivatives, tetracenothiophene (TCT) and anthradithiophene (ADT), contain either one or two thiophene groups at the terminal benzene rings. The dependence of the deposition temperature as well as the impact of different metallic substrates on their adsorption behavior is studied. When depositing the molecules on a Au substrate at room temperature, the molecules stay fully intact. In contrast, when deposited onto a Cu substrate, a breaking of the thiophene entity is observed. The breaking can be prevented by cooling the substrate to 200 K during deposition. Conductance measurements on the TCT molecules with an intact and broken thiophene entity reveal differences in their transport characteristics. The intact ones exhibit lower conductances than the broken ones. This can essentially be attributed to the fact that the latter ones bind covalently to the substrate, which facilitates transport. ADT deposited on Cu(111) can be reversibly and repeatedly interconverted between two distinct adsorption congurations associated with different conductances, and thus has been investigated as a molecular switch. The switching mechanism is related to a change between two different bonding configurations of the molecule to the substrate. The non-switched state ("off") can be ascribed to a strongly bound state and the switched state ("on") to a weaker bound configuration. The switching process is not only restricted to the location beneath the tip but also can be initiated within an extended area of about 100 nm in radius. This long-ranged addressing is enabled by means of surface state carriers of the Cu(111) surface. The non-local switching process is fully reversible, isomer selective and efficient over radial distances of about 100 nm. The second part of this work examines atomic-sized palladium (Pd) contacts. Pd exhibits outstanding properties owing to its high density of states (DOS) at the Fermi level. This renders Pd a "nearly ferromagnetic" material. Upon size confinement the DOS can be pushed to higher values and Pd can develop a net magnetization. Transport measurements were carried out for two distinct contact geometries; a tip-adatom and and a tip-surface contact geometry. Whereas for the tip-adatom contacts no magnetic state can be detected, measurements conducted for the tip-surface geometry might indicate the presence of a magnetic moment. The differences are attributed to the adsorption configuration of the single bridging atom in the junction. Spectra taken in point-contact with an adatom reveal a distinct inverted V-shaped feature. Presently we attribute this feature to paramagnons.

EPFL2015

Electronic and magnetic interactions in single molecular and atomic contacts

Verena Katharina Schendel

EPFL2015