Electronic and magnetic interactions in single molecular and atomic contacts

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.


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