Spectral electronic properties of noble metal nanowires with controlled architectures

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This project continues the fruitful investigations of the first period focused on the electronic properties of quasi-one-dimensional (quasi-1D) self-assembled atom chains hosted on high-index semiconductor terraces. Key model systems are given by the family of nanowires formed by the noble metal Au on Si (hhk) substrates, e.g., Si(553) and Si(775), which has received and important extension to Ge(hhk)-Au discovered in our group. The central idea is that variation of the crystal index (and hence the terrace width) allows to tune the inter-chain coupling and the phenomena associated with it. These materials thus open the door to witness exciting fundamental physics, such as the role of strong spin-orbit coupling (SOC) which splits the electron bands by their spin character, and ¿ based on the multiple Fermi surface nesting conditions ¿ the possibility of Peierls instabilities with metal-insulator transitions as a function of temperature. In addition, as recent development fostered by our studies in the first funding period, indications have emerged that the step edges of the silicon terraces are spin-polarized, with a long-range antiferromagnetic ordering, as seen in Si(553)-Au. We access these effects by scanning tunneling microscopy (STM) and spectroscopy (STS), which is complemented by angle-resolved photoemission (ARPES), all of which is performed down to low temperatures. Major new avenues for the current funding period are functional modifications to the systems, especially by doping, and, importantly, by change of substrate material (Si vs. Ge). Supported by a first successful demonstration, a change of band-filling induced by doping can indeed give rise to an energy gap in the Au chains ¿ which means that the wires can be switched ¿on¿ and ¿off¿ in a controlled manner. As a novel thrust for this period, replacing the Si(hhk) substrate with Ge(hhk) will imply an altered physical scenario, namely a change of lattice parameter, screening and hybridization. This awaits detailed study of the effects on both the Au chains as well as on the edge magnetism, respectively. For all materials, we will also carefully look at the effects of doping-induced charge transfer on the magnetic pattern. Likewise we want to explore the role of added magnetic impurities. ¿ While many of these phenomena can be accessed by our high-resolution spectroscopy methods, important aspects will be done in collaboration with other projects, including spin-sensitive STM, time-resolved electron diffraction, transport and optical signatures. These joint experiments will be accompanied by predictive modeling in the two theory projects.