Der nunmehr sichtbare Dritte: Bei der Reibung einzelner Polymermoleküle auf Festkörpern in flüssiger Umgebung existiert neben den erwarteten Mechanismen wie Gleit- und Haftreibung ein dritter, nanoskopischer Reibungsmechanismus, der unabhängig von Normalkraft, Geschwindigkeit und adsorbierter Polymerlänge ist. Basierend auf der Interpretation von Messungen mit verschiedenen Polymeren auf nanostrukturierten Oberflächen wird ein Modell für diesen Reibungsmechanismus vorgeschlagen. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor v = 2/3, 3/5, and 1/2, reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap. DOI: 10.1103/PhysRevB.87.041303

1 IMETUM und Physik Department, TUM, Boltzmannstrasse 11, 85748 Garching. 2 Ernst-Berl Institut fur Makromolekulare Chemie, TU Darmstadt, Petersenstrasse 22, 64287 Darmstadt, Germany. Fig. 1: Observed nanoscale friction mechanisms. A single polymer molecule at a solidliquid interface can respond in different ways to a lateral external force, here exerted by an AFM cantilever tip (depicted in yellow). (a) Previously theoretically described slip occurs when the polymer has a very high mobility and undetectably low friction. In this case, the vertical force (Fz) versus lateral extension (x) plot shows a plateau of constant force, before the polymer detaches from the surface and Fz becomes zero. (b) The most frequent motif is desorption stick. It is characteristically a decrease in the force Fz with increasing lateral pulling extension x. Here the static friction coefficient is high enough to favor desorption over slipping. While the polymer does not move laterally, it leaves the surface continuously with very low internal friction, that is, the polymer is confined in two dimensions. (c) In several cases the polymer is stuck to the surface owing to strong directional bonds (indicated as orange sticks); that is, confined in three dimensions (cooperative stick). A lateral external force elastically stretches the polymer. In the images, one sphere corresponds to at least 100 monomers [1]. Nanoscale Friction Mechanisms at Solid–Liquid Interfaces

We investigate the average propagation length of photogenerated nonequilibrium electrons in a two-dimensional electron gas using a quantum point contact as a local photocurrent detector. To this end, electrons are photogenerated both quasiresonantly and nonresonantly to the optical interband transition in the quantum well comprising the two-dimensional electron gas. The photocurrent is analyzed as a function of the distance between the excitation spot in the two-dimensional electron gas and the detector. We find that the determined propagation length depends nonmonotonically on the laser intensity. We interpret the observation by an interplay of an enlarged scattering phase space of the photogenerated electrons and the screening of sample specific scatterers.

We use GaAs-based quantum point contacts as mesoscopic detectors to analyze the flow of photogenerated electrons in a two-dimensional electron gas at a perpendicular magnetic field. Whereas charge transport experiments always measure the classical cyclotron radius, we show that this changes dramatically when detecting the photoinduced nonequilibrium current in magnetic fields. The radius of the photocurrent flow patterns surprisingly exceeds the classical cyclotron value by far, both in experiment and Monte Carlo simulations. We identify electron-electron scattering as the underlying reason.

We report on photoluminescence experiments on spatially indirect excitons in an InGaAs coupled double quantum well device in which semitransparent gates are employed to tune the in-plane potential landscape. We introduce a trapping configuration in which exciton generation is spatially separated from the excitonic trapping potential. Suitably biased gates control the flow of indirect dipolar excitons from the generation area to the electrostatically defined trap. Thus the trap is filled only with indirect excitons precooled to the lattice temperature. Using a confocal microscope at liquid helium temperatures we map the in-plane distribution of excitons at various gate voltages and illumination conditions. Our small and strongly confining traps with precooled excitons demonstrate interesting many-body effects which can be interpreted in terms of the electrostatic screening, the Coulomb binding, and excitonic flows. Gate voltage dependencies of PL energy in our samples are not monotonic and can be explained by considering the nonlinear exciton flows between the elements of our structure. At strong illumination hysteretic switching of the trapped exciton population reflects a nonlinear character of the self-consistent trapping potential. An unusual nonlinear increase of the emission of the trap is likely coming from the many-body interactions in a dense exciton gas in the presence of a disorder potential at high light intensity. The designs of electrostatic traps proposed and realized here allow for stronger confinements and lower temperatures and will be used to search for coherent phenomena in dense exciton gases.

GaAs-based quantum point contacts (QPCs) are exploited to spatially resolve and analyze the ballistic, nonequilibrium flow of photogenerated electrons in a nanoscale circuit. Electron-hole pairs are photogenerated in a two-dimensional electron gas (2DEG), and the resulting current through an adjacent QPC is measured as a function of the laser spot position. The transmission of photogenerated electrons through the QPC is governed by the energy dispersion and the quantized momentum values of the electron modes in the QPC.