SFB Colloquium

Low dimensional structures, e.g. quantum wires and films on surfaces, reveal fascinating phenomena of condensed matter physics. The electronic effects are intimately related to magnetic phases and complex spin textures if spin-orbit coupling is effective. The growth of these structures by self-assembly provides the possibility to control precisely the atomic structure, which is mandatory to observe such effects. In this talk I will focus on recent transport experiments on mesoscopic and electronically correlated 1D systems, i.e. epitaxially grown graphene nanoribbons and Au-induced atomic wires on Si templates.  

Recently, the Si(553)-Au system has attracted a lot of attention. The proposed spin ordering within the atomic wire ensemble resembles a spin-liquid phase that tends to utilize interactions of gapped spin states with metallic channels hosted in close proximity. A strong debate emerged about the origin of the atomic reconstructions in this system and their feedback to the electronic structure. While the x2 reconstruction along the Au double strands is assumed to be temperature independent, a x3 reconstruction along the wire emerges at low temperatures and is attributed to an ordered charge distribution with a frustrated spin texture of every third Si-edge atom. Our results clearly show that the surface states remain metallic below the phase transition temperature Tc=100 K for the x3 reconstruction along the Si-chain and rules out the formation of a spin-liquid triggered CDW. Instead, we found a sharp change in the surface conductivity at 65 K. Relying on the recent calculation that the ground state of the Si(553)-Au system is described by the D-model, the charge transfer should come along with a diamagnetic to spin-liquid transition.  

The second example is about ballistic transport in graphene nanoribbons. Besides the possibility to tune the band gap via electron confinement, the edges geometry give rise to robust edge states with fascinating electronic and magnetic properties. Thereby, the direct growth by sublimation of Si on pre-structured SiC(0001) is a promising alternative for the controlled fabrication of GNRs of exceptionally high quality and well-defined edges. In contrast to armchair GNRs, the hallmark for ribbons with zigzag edges is a probe-spacing and temperature independent conductance of 1*e²/h which strongly indicates fully spin-polarized ballistic transport. These robust signatures can be observed in a range of a few µm down to hundreds of nm at room temperature. Recently, we were able spatially resolving also the bulk channels on our epitaxial ribbons. Based on tight-binding calculations the transport findings can be modelled including asymmetric coupling of the ribbon edges resulting in transversal electric fields, which originate from distinct edge terminations on both sides of the GNR.