Unidirectional Real-Time Photoswitching of Diarylethene Molecular Monolayer Junctions with Multilayer Graphene Electrodes
2019-03-27, Koo, Jeongmin, Jang, Yeonsik, Martin, León, Kim, Dongku, Jeong, Hyunhak, Scheer, Elke, Kabdulov, Mikhail, Huhn, Thomas, Pauly, Fabian, Lee, Takhee
We fabricate and characterize vertical molecular junctions consisting of self-assembled monolayers (SAMs) of diarylethene (DAE) contacted by a multilayer graphene (MLG) electrode on the top and gold on the bottom. The DAE molecular junctions show two stable electrical states, a closed state (high conductance) or an open state (low conductance), which are created upon illumination with UV or visible light, respectively. For the Au-DAE-MLG junction structure, we observe that the current levels between the two conductance states are separated by two orders of magnitude. However, in a real-time measurement, we observe only unidirectional switching behavior from the open to the closed state.
Coulomb Blockade and Multiple Andreev Reflection in a Superconducting Single-Electron Transistor
2018-06, Lorenz, Thomas, Sprenger, Susanne, Scheer, Elke
In superconducting quantum point contacts, multiple Andreev reflection (MAR), which describes the coherent transport of m quasiparticles each carrying an electron charge with m≥3, sets in at voltage thresholds eV=2Δ/m. In single-electron transistors, Coulomb blockade, however, suppresses the current at low voltage. The required voltage for charge transport increases with the square of the effective charge eV∝(me)2. Thus, studying the charge transport in all-superconducting single-electron transistors (SSETs) sets these two phenomena into competition. In this article, we present the fabrication as well as a measurement scheme and transport data for a SSET with one junction in which the transmission and thereby the MAR contributions can be continuously tuned. All regimes from weak to strong coupling are addressed. We extend the Orthodox theory by incorporating MAR processes to describe the observed data qualitatively. We detect a new transport process the nature of which is unclear at present. Furthermore, we observe a renormalization of the charging energy when approaching the strong coupling regime.
Energy scales and dynamics of electronic excitations in functionalized gold nanoparticles measured at the single particle level
2019-07-07, Basu, Tuhin Shuvra, Diesch, Simon, Obergfell, Manuel, Demsar, Jure, Scheer, Elke
The knowledge of the electronic structure and dynamics of nanoparticles is a prerequisite to develop miniaturized single-electron devices based on nanoparticles. Low-temperature transport measurements of individual stable metallic nanoparticles enable unravelling the system specific electronic structure while ultrafast optical spectroscopy gives access to the electron dynamics. In this work, we investigate bare and thiol-functionalized gold nanoparticles. For the latter, we employ a fast and low-cost fabrication technique which yields nanoparticles with narrow size distribution. Using relatively long thiol-ended alkane chains for the functionalization modifies the electronic density of states of the nanoparticles. The study of decay dynamics of surface-plasmon-related hot electrons reveals the presence of electronic states at the interface which serve as a fast decay channel for electronic relaxation. By low-temperature scanning tunnelling microscopy we precisely investigate the energy scales and electronic interactions relevant for the tunnel charge transport through this system. We observe that the interaction between the functional ligand and the substrate on which the nanoparticles reside also influences the electronic transport. The procedure that we employ can be easily adapted to other metallic nanoparticles. Our findings are therefore important for incorporating them into single-electron devices.
Tuning the magnetic anisotropy energy of atomic wires
2019, Prestel, Martin W., Ritter, Markus F., Di Bernardo, Angelo, Pietsch, Torsten, Scheer, Elke
In this article we present the fabrication of freestanding thin-film nanobridges of Ir. We perform magnetoconductance (MC) measurements of atomic contacts and monoatomic chains of Ir, realized by the mechanically controlled break-junction method. We observe continuous changes of the MC on the field scale of several tesla, as observed earlier for atomic-size contacts of two other strong paramagnets, Pd and Pt. The amplitude and the shape of the MC depend on the orientation of the magnetic field as well as on subtle details of the atomic arrangement, as confirmed by stretching studies of the contacts. Both positive dominant MC and negative dominant MC occur and are attributed to collinear or noncollinear alignment of the magnetic moments of the electrodes, respectively. By careful manipulation of the chain geometry we are able to study the transition between these two cases, which is hallmarked by a complex MC behavior. For special arrangements the MC almost vanishes. Our findings are in agreement with recent calculations of the geometry dependence of the magnetic anisotropy energy and open a route to tailor the MC behavior as required for particular applications.
Single-electron transport through stabilised silicon nanocrystals
2018-07-26, Basu, Tuhin Shuvra, Diesch, Simon, Scheer, Elke
We have fabricated organically capped stable luminescent silicon nanocrystals with narrow size distribution by a novel, high yield and easy to implement technique. We demonstrate transport measurements of individual silicon nanocrystals by scanning tunnelling microscopy at a low temperature in a double-barrier tunnel junction arrangement in which we observed pronounced single electron tunnelling effects. The tunnelling spectroscopy of these nanocrystals with different diameters reveals quantum confinement induced bandgap modifications. Furthermore, from the features in the tunnelling spectra, we differentiate several energy contributions arising from electronic interactions inside the nanocrystal. By applying a magnetic field, we have detected a variation in the differential conductance profile that we attribute to arising from higher order tunnelling processes. We have also systematically simulated our experimental data with the Orthodox theory, and the results show good agreement with the experiment. The study establishes a correlation between the nanocrystal size and quantum confinement induced band-structure modifications which will pave the way to devise tailored nanocrystals.
Spatial Modulation of Nonlinear Flexural Vibrations of Membrane Resonators
2019-04-19, Yang, Fan, Rochau, Felix, Ochs, Jana Simone, Brieussel, Alexandre, Rastelli, Gianluca, Weig, Eva M., Scheer, Elke
We study the vibrational motion of mechanical resonators under strong drive in the strongly nonlinear regime. By imaging the vibrational state of rectangular silicon nitride membrane resonators and by analyzing the frequency response using optical interferometry, we show that, upon increasing the driving strength, the membrane adopts a peculiar deflection pattern formed by concentric rings superimposed onto the drum head shape of the fundamental mode. Such a circular symmetry cannot be described as a superposition of a small number of excited linear eigenmodes. Furthermore, different parts of the membrane vibrate at different multiples of the drive frequency, an observation that we denominate as "localization of overtones." We introduce a phenomenological model that is based on the coupling of a small number of effective nonlinear resonators, representing the different parts of the membrane, and that describes the experimental observations correctly.
Creation of equal-spin triplet superconductivity at the Al/EuS interface
2018-12-07, Diesch, Simon, Machon, Peter, Wolz, Michael, Sürgers, Christoph, Beckmann, Detlef, Belzig, Wolfgang, Scheer, Elke
In conventional superconductors, electrons of opposite spins are bound into Cooper pairs. However, when the superconductor is in contact with a non-uniformly ordered ferromagnet, an exotic type of superconductivity can appear at the interface, with electrons bound into three possible spin-triplet states. Triplet pairs with equal spin play a vital role in low-dissipation spintronics. Despite the observation of supercurrents through ferromagnets, spectroscopic evidence for the existence of equal-spin triplet pairs is still missing. Here we show a theoretical model that reveals a characteristic gap structure in the quasiparticle density of states which provides a unique signature for the presence of equal-spin triplet pairs. By scanning tunnelling spectroscopy we measure the local density of states to reveal the spin configuration of triplet pairs. We demonstrate that the Al/EuS interface causes strong and tunable spin-mixing by virtue of its spin-dependent transmission.
Fast quantitative optical detection of heat dissipation by surface plasmon polaritons
2018-06-13, Möller, Thomas B., Ganser, Andreas, Kratt, Martina, Dickreuter, Simon, Waitz, Reimar, Scheer, Elke, Boneberg, Johannes, Leiderer, Paul
Heat management at the nanoscale is an issue of increasing importance. In optoelectronic devices the transport and decay of plasmons contribute to the dissipation of heat. By comparison of experimental data and simulations we demonstrate that it is possible to gain quantitative information about excitation, propagation and decay of surface plasmon polaritons (SPPs) in a thin gold stripe supported by a silicon membrane. The temperature-dependent optical transmissivity of the membrane is used to determine the temperature distribution around the metal stripe with high spatial and temporal resolution. This method is complementary to techniques where the propagation of SPPs is monitored optically, and provides additional information which is not readily accessible by other means. In particular, we demonstrate that the thermal conductivity of the membrane can also be derived from our analysis. The results presented here show the high potential of this tool for heat management studies in nanoscale devices.