Festkörperkolloquium Wintersemester 2015/16
Donnerstag, 16-18 Uhr, Raum 3701-268 (Appelstraße 2)
|Rolf Haug||Spin-Coherent Dot-Cavity Electronics|
|Christoph Tegenkamp||Epitaxial Graphene on SiC studied by Electron Spectroscopy|
|Michael Oestreich||Efficient generation and on-chip routing of single photons from quantum dots|
|Christoph Tegenkamp||Atomic Friction Experiments|
(Max Planck Institut Göttingen)
|Rolf Haug||Waves in weakly scattering random media: from branched electron flows to the random focusing of tsunami waves|
15.10.2015 - Spin-Coherent Dot-Cavity Electronics
Eidgenössische Technische Hochschule Zürich
Quantum physics has profited enormously from combining optical cavities and atoms into a quantum engineering platform, where atoms mediate interactions among photons and photons communicate information between atoms. In mesoscopic physics, the constituents of the optical success story have been independently realized: (i) prototypes of electronic cavities were implemented through structuring of a two-dimensional electron gas (2DEG), yielding extended fermionic modes akin to quantum corrals on metal surfaces, and (ii) artificial atoms in the form of quantum dots have been studied, unraveling numerous interesting phenomena such as the Coulomb blockade and the Kondo effect. The combination of several dots into controlled quantum bits (qubits) has been demonstrated, thus promoting the next challenge of introducing coherent coupling between distant qubits without relying on nearest-neighbor exchange. Analogously to cavity quantum optics, such long-range coherent coupling could be provided by a suitably engineered cavity mode.
I report on the realization of an engineered cavity mode and on its coherent coupling to the spin of a localized electron. As spin coherence is conserved, such a cavity mode constitutes a tunable and purely electrical component that may serve as a quantum bus for coherently coupling spatially separated qubits.
05.11.2015 - Epitaxial Graphene on SiC studied by Electron Spectroscopy
Institut für Physik: Technische Physik - Technische Universität Chemnitz
The outstanding properties of graphene (high charge carrier mobility, exceptional mechanical strength, chemical resistance, etc.) suggest various applications in the areas of, e.g., high frequency electronic devices, MEMS, optoelectronics, plasmonics, sensors, or photovoltaics. In general, graphene will be in contact with other materials, be it in the form of electrical contacts, a substrate, or an encapsulation for the purpose of protection and conservation of the device. Thus, the influence of the surrounding materials on the properties of graphene needs to be studied and understood. In the present work, I will discuss two different cases, in which the properties of graphene are strongly modified by other materials. First I will discuss the doping of so-called quasi-freestanding graphene on H-saturated SiC(0001). I will show that the observed p-type doping is a consequence of the spontaneous polarization present in hexagonal SiC polytypes. Secondly, I will discuss the coupling of graphene plasmons to optically active modes on the substrate, a phenomenon that was observed to be very robust and present even in the case of quasi-freestanding graphene.
12.11.2015 - Efficient generation and on-chip routing of single photons from quantum dots
Institut für Halbleiteroptik und Funktionelle Grenzflächen, Universität Stuttgart
 Rengstl, U., Schwartz, M., Herzog, T., Hargart, F., Paul, M., Portalupi, S. L., ... & Michler, P. (2015). On-chip beamsplitter operation on single photons from quasi-resonantly excited quantum dots embedded in GaAs rib waveguides. Applied Physics Letters, 107(2), 021101.
 Jons, K. D., Atkinson, P., Muller, M., Heldmaier, M., Ulrich, S. M., Schmidt, O. G., & Michler, P. (2012). Triggered indistinguishable single photons with narrow line widths from site-controlled quantum dots. Nano letters, 13(1), 126-130.
03.12.2015 - Atomic Friction Experiments
Departement Physik, Universität Basel
Experiments, based on advanced force microscopy, give the opportunity to study friction on the nanometer scale. Novel phenomena, such as atomic stick-slip, are observed. The transition from atomic-scale stick slip to continuous sliding, also called the transition to superlubricity, is presented and applied to control friction. Therefore, atomic friction can be switched on and off. Force microscopy experiments in contact mode were often found to be limited by long-range force, which have to be compensated by short-range forces and thus lead to a multi-atom contact. Recent experiments on ionic crystals and molecular crystals show that the contact size can be minimized to diameters of a single atom or single molecule. It is found that the use of contact resonance force microscopy enhances the contrast. Contact resonance force microscopy shows atomic stick-slip and atomic-scale variations of the contact resonance frequency, which further characterizes the small contact . Friction force microscopy of layered molecular crystals show contrast variations as a function of the orientation of the molecules . An interesting extension of these experiments is to attach single molecules to the end of the probing tip and to use this single molecule as a contact to the substrate. Molecular wires can be picked up from a surface in a controlled manner to determine the detachment energies of submolecular units and to investigate the sliding of molecular chains on surfaces .
 P. Steiner, R. Roth, E. Gnecco, Th. Glatzel, A. Baratoff and E. Meyer, Modulation of contact resonance frequency accompanying atomic-scale stick-slip in friction microscopy, Nanotechnology, 20, (495701), (2009), 6
 G. Fessler, I. Zimmermann, T. Glatzel, E. Gnecco, P. Steiner, R. Roth, T. D. Keene, S.-X. Liu, S. Decurtins, and E. Meyer, Orientation dependent molecular friction on organic layer compound crystals, Appl. Phys. Lett., 98, (8), (2011), 083119
 S. Kawai, M. Koch, E. Gnecco, A. Sadeghi, R. Pawlak, T. Glatzel, J. Schwarz, S. Goedecker, S. Hecht, A. Baratoff, L. Grill, and E. Meyer, Quantifying the atomic-level mechanics of single long physisorbed molecular chains, Proc. Natl. Acad. Sci. USA, 111, (11), (2014), 3968–397
14.01.2016 - Waves in weakly scattering random media: from branched electron flows to the random focusing of tsunami waves
Max-Planck-Institut für Dynamik und Selbstorganisation, Göttingen
Wave propagation in random media: this might sound abstract but is in fact very tangible and almost omnipresent in science and everyday life. Examples are surface water waves, but also light, sound, electrons, tsunamis and even earth quakes are waves that in a natural environment typically propagate through a complex medium. Due to its complexity, the medium is often best described as random, with examples including the turbulent atmosphere, complex patterns of ocean currents or a semiconductor crystal sprinkled with impurities. In recent years it has become clear that even very small fluctuations in the random medium, if they are correlated, lead to focussing of the waves in pronounced branch-like spatial structures and to extreme wave intensities. This branching has been reported for electron, micro, sound, and water waves.
I will give an overview over the progress we made in the last few years in the understanding of branched flows and the statistic of extreme waves, and discuss how the same phenomena are observable in ballistic electron transport in semiconductors and the propagation of tsunamis waves.