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A single spin in a Josephson junction can reverse the flow of the supercurrent by changing the sign of the superconducting phase difference across it. At mesoscopic length scales, these pi-junctions are employed in various applications, such as finding the pairing symmetry of the underlying superconductor, as well as quantum computing. At the atomic scale, the counterpart of a single spin in a superconducting tunnel junction is known as a Yu-Shiba-Rusinov state. Observation of the supercurrent reversal in that setting has so far remained elusive. Here we demonstrate such a 0 to pi transition of a Josephson junction through a Yu-Shiba-Rusinov state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device, which provides our scanning tunnelling microscope with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with high resolution., Continuously changing the coupling between a magnetic impurity and a superconductor allows the observation of the reversal of supercurrent flow at the atomic scale.

We propose and experimentally demonstrate a self-calibrating detector of Cooper pair depairing in a superconductor based on a mesoscopic superconducting island coupled to normal metal leads. On average, exactly one electron passes through the device per broken Cooper pair, independent of the absorber volume, device or material parameters. The device operation is explained by a simple analytical model and verified with numerical simulations in quantitative agreement with experiment. In a proof-of-concept experiment, we use such a detector to measure the high-frequency phonons generated by another, electrically decoupled superconducting island, with a measurable signal resulting from less than 10 fW of dissipated power., 5 pages, 3 figures; supplementary material included

Majorana zero modes are leading candidates for topological quantum computation due to non-local qubit encoding and non-abelian exchange statistics. Spatially separated Majorana modes are expected to allow phase-coherent single-electron transport through a topological superconducting island via a mechanism referred to as teleportation. Here we experimentally investigate such a system by patterning an elongated epitaxial InAs-Al island embedded in an Aharonov-Bohm interferometer. With increasing parallel magnetic field, a discrete sub-gap state in the island is lowered to zero energy yielding persistent 1e-periodic Coulomb blockade conductance peaks (e is the elementary charge). In this condition, conductance through the interferometer is observed to oscillate in a perpendicular magnetic field with a flux period of h/e (h is Planck’s constant), indicating coherent transport of single electrons through the islands, a signature of electron teleportation via Majorana modes., Theories predict teleportation of phase-coherent single electrons through a topological superconducting island. Here, the authors report persistent Coulomb blockade conductance peaks due to coherent transport of single electrons through patterned InAs-Al islands embedded in an Aharonov-Bohm interferometer.

We report on the fabrication and measurements of platinum-self-aligned nanogap devices containing cubed iron (core)/iron oxide (shell) nanoparticles (NPs) with two average different sizes (13 and 17 nm). The nanoparticles are deposited by means of a cluster gun technique. Their trapping across the nanogap is demonstrated by comparing the current vs voltage characteristics (I-Vs) before and after the deposition. At low temperature, the I-Vs can be well fitted to the Korotkov and Nazarov Coulomb blockade model, which captures the coexistence of single-electron tunneling and tunnel barrier suppression upon a bias voltage increase. The measurements thus show that Coulombblockaded devices can be made with a nanoparticle cluster source, which extends the existing possibilities to fabricate such devices to those in which it is very challenging to reduce the usual NP agglomeration given by a solution method., This study was supported by the EU Horizon 2020 research and innovation program under the Marie-Sklodowska-Curie Grant Agreement No. 645658 (DAFNEOX Project), by two FONDECYT REGULAR Grant Nos. 1181080 and 1161775, and by two FONDEQUIP Grant Nos. EQM140055 and EQM180009. We thank the Spanish Ministry of Science, Innovation and Universities (Project Nos. MAT2015-71664-R and RTI2018-099960-B-I00) and the Serbian Ministry of Education, Science and Technological Development (Project No. III45018) for their support. A.P. and Z.K. thank Senzor-INFIZ (Serbia) for the cooperation provided during their respective secondments.

Frank W. J. Hekking performed his PhD work on "Aspects of Electron Transport in Semiconductor Nanostructures" at the TU Delft in 1992. He then worked as a postdoc at the University of Karlsruhe, the University of Minnesota, the Cavendish Laboratory at the University of Cambridge, and the Ruhr University at Bochum. In 1999 he joined the LPMMC (Laboratoire de Physique et Mod\' elisation des Milieux Condens\' es) in Grenoble and was appointed Professor at the Universit\' e Joseph Fourier and afterwards Universit\' e Grenoble Alpes. Frank Hekking was nominated as a member of the Institut Universitaire de France, for the periods 2002-2007 and 2012-2017. This review provides an overview of his scientific contributions to several fields of mesoscopic electron transport and superconductivity as well as atomic gases, and is organized along sections describing the different themes., We acknowledge the financial support of the Fondation Nanosciences Grenoble, the LabEx LANEF (ANR-10-LABX-51-01), the Center for Theoretical Physics in Grenoble, the University Grenoble Alpes. Funding information The recent activity of Frank Hekking was founded by Institut Universitaire de France, the ANR SuperRing (ANR-15-CE30-0012-02), the ANR QPS-NanoWires (ANR-15-CE30-0021-02).

Using the transversal vibration resonance of a suspended carbon nanotube as charge detector for its embedded quantum dot, we investigate the case of strong Kondo correlations between a quantum dot and its leads. We demonstrate that even when large Kondo conductance is carried at odd electron number, the charging behaviour remains similar between odd and even quantum dot occupation. While the Kondo conductance is caused by higher order processes, a sequential tunneling only model can describe the time-averaged charge. The gate potentials of maximum current and fastest charge increase display a characteristic relative shift, which is suppressed at increased temperature. These observations agree very well with models for Kondo-correlated quantum dots., Comment: 5 figures

This paper describes the fabrication of electrically addressable, high-aspect-ratio (>10000:1) nanowires of gold with square cross sections of 100 nm on each side that are separated by gaps of 1.7-2.2 nm which were defined using self-assembled monolayers (SAMs) as templates. We fabricated these nanowires and nanogaps without a clean room or any photo- or electron-beam lithographic processes by mechanically sectioning sandwich structures of gold separated by a SAM using an ultramicrotome. This process Is a form of edge lithography known as Nanoskiving. These wires can be manually positioned by transporting them on drops of water and are directly electrically addressable; no further lithography is required to connect them to an electrometer. Once a block has been prepared for Nanoskiving (which takes less than one day), hundreds of thousands of nanogaps can be generated, on demand, at a rate of about one nanogap per second. After ashing the organic components with oxygen plasma, we measured the width of a free-standing gap formed from a SAM of 16-mercaptodohexanoic acid (2.4 nm in length) of 2.6 +/- 0.5 nm by transmission electron microscopy. By fitting current-voltage plots of unashed gaps containing three alkanedithiolates of differing lengths to Simmons' approximation, we derived a value of beta = 0.75 angstrom(-1) (0.94 n(C)(-1)) at 500 mV. This value is in excellent agreement with literature values determined by a variety of methods, demonstrating that the gap-size can be controlled at resolutions as low as 2.5 angstrom (i.e., two carbon atoms).

In this paper, we report on the integration technique and fabrication of a scanning probe interrogating the location of charges and their tracks inside quantum devices. Our unique approach is to pattern the charged sensor into a high topography micromechanical structure. A single-electron transistor (SET) is directly integrated onto the microfabricated cantilever that extends out from the body of a scanning force microscope (SFM) probe of standard dimensions. In a novel tactic and by reversing their traditional roles, a tuning fork (TF) completes the probe to provide the self-actuating and self-sensing qualities necessary for an oscillatory force sensor. We show sharp edges on the Coulomb diamonds, indication that the SET fabrication step yields devices of high quality. We demonstrate topographical scans with this probe. All stages of the fabrication process are executed on batches of probes which is an essential step away from the time-consuming and individual preparation of other implementations. It opens the door to a more reproducible and large volume production.

It is understood that molecular conjugation plays an important role in charge transport through single-molecule junctions. Here, we investigate electron transport through an anthraquinone based single-molecule three-terminal device. With the use of an electric-field induced by a gate electrode, the molecule is reduced resulting into a ten-fold increase in the off-resonant differential conductance. Theoretical calculations link the change in differential conductance to a reduction-induced change in conjugation, thereby lifting destructive interference of transport pathways., Nano Letters (2015)

Quantum-effects will play an important role in both future CMOS and 'beyond CMOS' technologies. By comparing single-electron transistors formed in un-patterned, uniform-width silicon nanowire (SiNW) devices with core widths from ∼5-40 nm, and gated lengths of 1 μm and ∼50 nm, we show conditions under which these effects become significant. Coulomb blockade drain-source current-voltage characteristics, and single-electron current oscillations with gate voltage have been observed at room temperature. Detailed electrical characteristics have been measured from 8-300 K. We show that while shortening the nanowire gate length to 50 nm reduces the likelihood of quantum dots to only a few, it increases their influence on the electrical characteristics. This highlights explicitly both the significance of quantum effects for understanding the electrical performance of nominally 'classical' SiNW devices and also their potential for new quantum effect 'beyond CMOS' devices.

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International audience; Silicon-on-insulator nanowire transistors of very small dimensions exhibit electrostatic or quantum effects like Coulomb blockade or single-dopant transport at low temperature. The same process also yields excellent field-effect transistors (FETs) for larger dimensions, allowing us to design integrated circuits. Using the same process, we cointegrate a FET-based ring oscillator circuit operating at cryogenic temperature which generates a radio-frequency (rf) signal on the gate of a nanoscale device showing Coulomb oscillations. We observe rectification of the rf signal, in good agreement with modeling.

International audience; We report the observation of an atomic like behavior from T = 4.2 K up to room temperature in n- and p-type Omega-gate silicon nanowire (NW) transistors. For that purpose, we modified the design of a NW transistor and introduced long spacers between the source/drain and the channel in order to separate the channel from the electrodes. The channel was made extremely small (3.4 nm in diameter with 10 nm gate length) with a thick gate oxide (7 nm) in order to enhance the Coulomb repulsion between carriers, which can be as large as 200 meV when surface roughness promotes charge confinement. Parasitic stochastic Coulomb blockade effect can be eliminated in our devices by choosing proper control voltages. Moreover, the quantum dot can be tuned so that the resonant current at T = 4.2 K exceeds that at room temperature.

The efficiency of the future devices for quantum information processing will be limited mostly by the finite decoherence rates of the individual qubits and quantum gates. Recently, substantial progress was achieved in enhancing the time within which a solid-state qubit demonstrates coherent dynamics. This progress is based mostly on a successful isolation of the qubits from external decoherence sources obtained by clever engineering. Under these conditions, the material-inherent sources of noise start to play a crucial role. In most cases, quantum devices are affected by noise decreasing with frequency, f, approximately as 1/f. According to the present point of view, such noise is due to material- and device-specific microscopic degrees of freedom interacting with quantum variables of the nanodevice. The simplest picture is that the environment that destroys the phase coherence of the device can be thought of as a system of two-state fluctuators, which experience random hops between their states. If the hopping times are distributed in a exponentially broad domain, the resulting fluctuations have a spectrum close to 1/f in a large frequency range. In this paper we review the current state of the theory of decoherence due to degrees of freedom producing 1/f noise. We discuss basic mechanisms of such noises in various nanodevices and then review several models describing the interaction of the noise sources with quantum devices. The main focus of the review is to analyze how the 1/f noise destroys their coherent operation. We start from individual qubits concentrating mostly on the devices based on superconductor circuits, and then discuss some special issues related to more complicated architectures. Finally, we consider several strategies for minimizing the noise-induced decoherence., Comment: Revised manuscript accepted for publication in Rev. Mod. Phys. 63 pages, 44 figures

Electron transport through a quantum dot or single molecule coupled to a quantum oscillator is studied by the Keldysh nonequilibrium Green's function formalism to obtain insight into the quantum dynamics of the electronic and oscillator degrees of freedom. We tune the electronic level of the quantum dot by a gate voltage, where the leads are kept at zero temperature. Due to the nonequilibrium distribution of the electrons in the quantum dot, the spectral function becomes a function of the gate voltage. Novel spectral features are identified for the ground and excited states of nanomechanical oscillators that can be used to enhance the measurement sensitivity.

Tendency to bifurcate can often be utilized to improve performance characteristics of amplifiers or even to build detectors. Bloch oscillating transistor is such a device. Here we show that bistable behaviour can be approached by tuning the base current and that the critical value depends on the Josephson coupling energy $E_J$ of the device. We demonstrate record-large current gains for device operation near the bifurcation point at small $E_J$. From our results for the current gains at various $E_J$, we determine the bifurcation threshold on the $E_J$ - base current plane. The bifurcation threshold curve can be understood using the interplay of inter- and intra-band tunneling events., 24 pages, 8 figures

Measuring the temperature of a two-dimensional electron gas at temperatures of a few mK is a challenging issue, which standard thermometry schemes may fail to tackle. We propose and analyze a nongalvanic thermometer, based on a quantum point contact and quantum dot, which delivers virtually no power to the electron system to be measured., Comment: 5 pages, 3 figures

Das Verständnis von Transportprozessen wechselwirkender Teilchen in eingeschränkter Geometrie ist für die Grundlagenforschung von fundamentalem Interesse, da vielerlei Vorgänge in der Natur auf dem Austausch wechselwirkender Teilchen durch enge, oftmals zweidimensionale Strukturen basieren. In biologischen und physikochemischen Systemen treten solche Prozesse z.B. in Poren, Ionenkanälen oder Zeolithe auf.Bei der Untersuchung solcher Vorgänge stößt die Wissenschaft jedoch häufig an ihre Grenzen, da solche Systeme oft experimentell schlecht zugänglich sind. Aus diesem Grund wird daher auf sogenannte Modellsysteme ausgewichen, welche eine einfachere Überprüfung fundamentaler physikalischer Modellvorstellungen ermöglichen.Die hier vorliegende Arbeit widmet sich der experimentellen Realisation eines solchen Modellsystems zur Untersuchung grundlegender physikalischer Modelle beim Transport wechselwirkender Teilchen. Dies geschieht mit Hilfe von superparamagnetischen Kolloiden, welche innerhalb eines Mikrokanals angeordnet werden. Die dabei erhaltenen Ergebnisse lassen sich in vier Bereiche einteilen: Die Untersuchung der Wechselwirkung zwischen den Teilchen mit Hilfe eines Videomikroskops, die Mobilität der Partikel in Abhängigkeit der im Kanal ausgebildeten Teilchen-Anordnung, der dynamische Transport der Teilchen in Abhängigkeit von energetischen Hindernissen im Kanal, sowie der zeitlichen Korrelation von Teilchen beim Transport durch einen von zwei Barrieren separierten Bereich im Kanal. Zusätzlich wurden alle Experimente durch ausführliche numerische Simulationen ergänzt.

Electrical transport of a highly doped disordered conducting polymer, viz. poly-3,4-ethylenedioxythiophene stabilized with poly-4-styrenesulphonic acid, is investigated as a function of bias and temperature. The transport shows universal power-law scaling with both bias and temperature. All measurements constitute a single universal curve, and the complete J(V,T) characteristics are described by a single equation. We relate this scaling to dissipative tunneling processes, such as Coulomb blockade. © 2010 The American Physical Society.

Es wurde gezeigt, dass durch die Vorpositionierung von Quantenpunkten, diese mit einem gezielten Abstand im Bereich von einigen 100 nm zueinander und daher mit einer definierten Dichte in Speicherbauelemente eingebracht werden können. Es wurde bei tiefen Temperaturen wohldefinierte Coulombblockade demonstriert. Durch die Analyse der Coulomb-Rauten war es möglich, auf die Größe und Ladeenergie von Quantenpunkten im Kanal zu schliessen. Es wurde gezeigt, dass vorpositionierte Quantenpunkte sehr gut als Floating Gate eingesetzt werden können. Die Speichereigenschaften dieser Quantenpunkte wurden im Hinblick auf die Hysteresebreite DeltaVth in Abhängigkeit der Kanalbreite, der Drainspannung und der Temperatur untersucht und diskutiert. Hierbei konnte eine deutliche Abhängigkeit der Thresholdspannung von der Kanalbreite der Struktur ermittelt werden. Für Strukturen mit einem breiten Kanal wurde festgestellt, dass der Stromfluss bereits bei negativen Gatespannungen einsetzt, während für schmale Strukturen positive Gatespannungen nötig sind, um einen Ladungstransport hervorzurufen. Zur Bestimmung der Temperaturstabilität der Ladezustände wurde sowohl die Thresholdspannung als auch die Hysteresebreite als Funktion der Probentemperatur im Bereich von 4.2K bis Raumtemperatur bei verschiedenen Drainspannungen bestimmt. Hierbei wurde festgestellt, dass die Hysteresebreite bis zu einer kritischen Temperatur stufenförmig abnimmt und danach wieder leicht ansteigt. Bei der Untersuchung der Threshold- Spannung wurde ein Unterschied Vth,zu und Vth,auf festgestellt. Erstmals konnte ein lateral und vertikal positionierter InAs Quantenpunkt als Speicher für den Betrieb bei Raumtemperatur demonstriert werden. Ferner wurde die Wirkung eines Gate-Leckstromes auf den gemessenen Drain- Strom eines monolithischen Drei-Kontakt-Struktur untersucht und diskutiert. Die untersuchten Proben basieren auf einem neuen Parallel-Design, in welchem das Gate nicht wie üblich zwischen Source und Drain positioniert wurde, sondern in serieller Verbindung mit dem Drain- oder Sourcekontakt, d.h. mit einem zentralen Drain zwischen Source und Gate, gesetzt wurde. Hierdurch konnte eine merkliche Reduzierung des Probeninnenwiderstandes erreicht werde. Zu Beginn wurden zur Charakterisierung der Probe Transportmessungen bei Raumtemperatur durchführt. Hierbei konnte verglichen mit herkömmlichen Quantendrahttranistoren realisiert auf demselbenWafer, zum einen eine deutlich höhere Transconductance durch das parallele Design erreicht werden. Zum anderen zeigte die ermittelte Transconductance nicht den erwarteten linearen Verlauf in Abhängigkeit der Drainspannung, sondern einen quadratischen. Die Messungen zeigten außerdem einen Abfall des Drain-Stromes ab einer kritischen Größe des Gate-Leckstromwertes, welcher auf ein dynamisches Gate, hervorgerufen durch die Ladungsträger aus dem Gate, zurückgeführt wird. Diese zusätzliche virtuelle Kapazität addiert sich in paralleler Anordnung zum geometrischen Gate-Kondensator und verbessert die Transistoreigenschaften. Zum Abschluss der Arbeit wurden Hochfrequenzmessungen zur Ermittlung einer Leistungsverstärkung von Drei-Kontakt-Strukturen bei Raumtemperatur für unterschiedliche Gate- und Drainspannungen durchgeführt. Um die Hochfrequenzeigenschaften der untersuchten Probe zu erhöhen, wurde hierfür ein Design gewählt, in welchem die Goldkontakte zur Kontaktierung sehr nahe an die aktive Region heranragen. Für diese Spannungskombination konnte für eine Frequenz im Gigaherz-Bereich eine positive Spannungsverstärkung > 1 dB gemessen werden. Höhere Spannungen führen zu einem Sättigungswert in der Leistungsverstärkung. Dies wird zurückgeführt auf den maximal zur Verfügung stehenden Strom in der aktiven Region zwischen den nahen Goldkontakten. Zudem wurde eine Lösung vorgestellt, um das fundamentale Problem der Impedanzfehlanpassung für Hochfrequenzmessungen von nanoelektronischen Bauelementen mit einem hohen Innerwiderstand zu lösen. Eine Anpassung der unterschiedlichen Impedanzen zwischen Bauelement und Messapparatur ist unbedingt notwendig, um Reflexionen bei der Übertragung zu vermeiden und somit die Gewinnoptimierung zu erhöhen. Zur Behebung der Fehlanpassung wurde im Rahmen dieser Arbeit ein Impedanz-Anpassungs-Netzwerk auf einer PCB-Platine realisiert, welches mit der Probe verbunden wurde. Die Anpassung wurde durch eingebaute Strichleitungen in das Layout des Anpassungsboards vorgenommen. Durchgeführte Simulationen der Probe in Verbindung mit dem Anpassungs-Netzwerk bestätigten die experimentellen Ergebnisse. Durch die Anpassung konnte der simulierte Reflexionskoeffizient deutlich reduziert werden, bei gleichzeitiger Erhöhung des Transmissionskoeffizienten. Ebenfalls zeigten die Messungen an einer Drei-Kontakt-Struktur mit Anpassungs-Board eine signifikante Verbesserung der Leistungsverstärkung., Dynamical Charging and Discharging of laterally aligned quantum dot structures We can demonstrate that the direct positioning enables us to embed quantum dots with given periods to each other of only a few 100 nm and therefore with a defined density into the memory-structures. For low temperatures, well defined Coulombblockade can be observed. The analysis of the measured diamond patterns allows the determination of the dimension and the charging energy of the embedded quantum dots in the channel. The memory properties of these quantum dots were analyzed and discussed in terms of the hysteresis width DeltaVth which depends on the channel width, the applied drain voltage and the device temperature. The measurements reveal a dependence of the threshold voltage on the channel width of the structure. For devices with a wide channel the current transport sets in with negative applied gate voltages, in contrast to structures with narrow channels, requiring positive gate voltages to cause a current flow through the channel. To explain these results we assume that in large channels a higher negative voltage is necessary to deplete the charges out of the channel due to the higher charge density. To analyze the temperature stability of the charge states the threshold voltage as well as the hysteresis width is detected as a function of the temperature for different drain voltages in the range of 4.2K up to room temperature. It is determined that the hysteresis width decreases to a critical temperature before it rises again. For the investigation of the threshold voltage a difference between Vth,up and Vth,down is demonstrated. We assume that this difference is caused by the different charging behavior for increasing charge energies. In this work, lateral and vertical positioned InAs quantum dots could be demonstrated as a memory device operated at room temperature for the first time. Improved transistor functionality caused by gate leakage currents in nanoscaled Three Terminal Structures Further we investigate the role of gate leakage on the drain current in a monolithic, unipolar GaAs/AlGaAs heterostructure based on three leaky coupled contacts. Two in-plane barriers, defined by rows of etched holes in a two-dimensional electron gas, separate the leaky gate from the central drain and the drain from the source. Because of this the internal resistance of the structure can be appreciably decreased. It should be noted that the observed differential voltage amplification in the gate leakage regime of the studied structure is by far larger compared to the voltage amplification of any in-plane wire transistor fabricated from the same wafer, which were controlled by two non-leaking in-plane gates. The calculated transconductance increases quadratically and not in a non-linear manner, as expected. A pronounced reduction of the drain current sets in when the gate starts to leak, pointing at a large parallel gate capacitor. We associate the gate-leakage current induced gating with a virtual floating gate induced by the space charge injected from the gate. The space charge can hereby be described by a parallel gate capacitor that can control a low dimensional channel lying nearby. High frequency measurements on Three Terminal Structures High frequency measurements for determination of the power gain in Three Terminal Structures are carried out at room temperature. To improve the high frequency properties of the investigated structures a special design was chosen, where the gold contacts for contacting the sample approach very closely the active switching region. The measurements show that negative gate voltages are much more efficient to the power gain than positive ones. For these voltage combinations a power gain > 1 dB for frequencies in the GHz range is detected, whereas the power gain saturates for higher voltages. This is interpreted in terms of the maximum number of charges in the active region between the gold contacts. Furthermore an answer to the fundamental obstacle of the impedance mismatch for high frequency measurements on nanoelectronic structures with high internal resistance is given. Such a matching between the device and the measurement setup is necessary to reduce signal reflections and therefore increase the gain. To match the impedances, an impedancematching- network on a PCB-plate (printed circuit board) via integrated stubs was realized. Simulation data of the sample in connection with the matching-network is in very good agreement with the experimental data. Using the network reduces the simulated reflection coefficient and simultaneously raises the transmission coefficient. The measurements also show a significant improvement of the power gain behaviour.

Das Verhalten von ultrakleinen Tunnelkontakten mit Kapazitäten im Femtofaradbereich kann bei tiefen Temperaturen wesentlich durch Ladungseffekte bestimmt sein. Diese äußern sich beispielsweise in einer Unterdrückung des Tunnelstroms, der sogenannten Coulombblockade, die zu einer Coulomblücke bei kleinen Spannungen und einer Verschiebung der Strom-Spannungs-Charakteristik bei großen Spannungen führt. Das Auftreten von Ladungseffekten hängt jedoch nicht nur von den Eigenschaften des Tunnelkontakts ab, sondern wird auch maßgeblich durch dessen Umgebung beeinflusst. Tunnelraten für Kontakte mit großem Tunnelwiderstand, die über eine äußere Impedanz mit einer Spannungsquelle verbunden sind, lassen sich störungstheoretisch berechnen. Man findet, dass die Raten von der Wahrscheinlichkeit für den Energieaustausch zwischen der tunnelnden Ladung und der Umgebung abhängen. Diese Wahrscheinlichkeit ist durch die Impedanz der Umgebung und damit durch deren Anregungsspektrum bestimmt, das sich in Ableitungen der Strom-Spannungs-Charakteristik deutlich widerspiegelt. Eine Analyse des Einflusses der Umgebung ergibt, dass Ladungseffekte für kleine Impedanzen durch Quantenfluktuationen unterdrückt werden. Eine Coulomblücke in der Strom-Spannungs-Charakteristik tritt nur auf, wenn die Impedanz größer als das Widerstandsquant h/e² ist. Für große Spannungen findet man jedoch immer eine durch Ladungseffekte verschobene Strom-Spannungs-Charakteristik. In Josephsonkontakten, in denen die Josephsonkopplung klein gegenüber der Ladungsenergie ist, wird der Cooperpaarstrom direkt durch die Wahrscheinlichkeit des Energieaustauschs eines tunnelnden Cooperpaars mit der Umgebung bestimmt. Ladungseffekte werden hier nicht so stark unterdrückt, da die relevante Widerstandsskala durch h/4e² gegeben ist. Das Tunneln von Quasiteilchen in ultrakleinen Josephsonkontakten lässt sich analog zum Tunneln in normalleitenden Kontakten behandeln, wenn man die Energieabhängigkeit der Zustandsdichte in der Nähe der Energielücke berücksichtigt. In Mehrkontaktsystemen treten Ladungseffekte wegen der Existenz diskreter Inselladungen unabhängig von der Impedanz der Umgebung auf. Dabei führt die Anwesenheit weiterer Kontakte zu einer verminderten effektiven Impedanz, so dass die Umgebung häufig vernachlässigt werden kann. Methoden der Netzwerkanalyse haben sich als sehr geeignet erwiesen, um den Einfluss komplizierter Umgebungen auf das Tunneln in Mehrkontaktsystemen zu untersuchen. Für Anwendungen ist die Möglichkeit von Interesse, die Inselladungen durch an die Inseln angelegte Spannungen zu verschieben und damit den Strom durch die Tunnelkontakte zu kontrollieren. Auf diese Weise lassen sich hochempfindliche Elektrometer und eventuell auch neue Stromstandards realisieren.

We report a study of spin dependent transport in a system composed of a quantum dot coupled to a normal metal lead and a ferromagnetic lead (NM-QD-FM). We use the master equation approach to calculate the spin-resolved currents in the presence of an external bias and an intra-dot Coulomb interaction. We find that for a range of positive external biases (current flow from the normal metal to the ferromagnet) the current polarization $\wp=(I_\uparrow-I_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to zero, while for the corresponding negative biases (current flow from the ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The system thus operates as a rectifier for spin--current polarization. This effect follows from an interplay between Coulomb interaction and nonequilibrium spin accumulation in the dot. In the parameter range considered, we also show that the above results can be obtained via nonequilibrium Green functions within a Hartree-Fock type approximation.

This thesis presents an experimental study of the thermoelectrical properties of semiconductor quantum dots (QD). The measurements give information about the interplay between first order tunneling and macroscopic quantum tunneling transport effects in the presence of thermal gradients by the direct comparison of the thermoelectric response and the energy spectrum of the QD. The aim of the thesis is to contribute to the understanding of the charge and spin transport in few-electron quantum dots with respect to potential applications in future quantum computing devices. It also gives new insight into the field of low temperature thermoelectricity. The investigated QDs were defined electrostatically in a two dimensional electron gas (2DEG) formed with a GaAs/(Al,Ga)As heterostructure by means of metallic gate electrodes on top of the heterostructure. Negative voltages with respect to the potential of the 2DEG applied to the gate electrodes were used to deplete the electron gas below them and to form an isolated island of electron gas in the 2DEG which contains a few ten electrons. This QD was electrically connected to the 2DEG via two tunneling barriers. A special electron heating technique was used to create a temperature difference between the two connecting reservoirs across the QD. The resulting thermoelectric voltage was used to study the charge and spin transport processes with respect to the discrete energy spectrum and the magnetic properties of the QD. Such a two dimensional island usually exhibits a discrete energy spectrum, which is comparable to that of atoms. At temperatures below a few degrees Kelvin, the electrostatic charging energy of the QDs exceeds the thermal activation energy of the electrons in the leads, and the transport of electrons through the QD is dominated by electron-electron interaction effects. The measurements clarify the overall line shape of thermopower oscillations and the observed fine structure as well as additional spin effects in the thermoelectrical transport. The observations demonstrate that it is possible to control and optimize the strength and direction of the electronic heat flow on the scale of a single impurity and create spin-correlated thermoelectric transport in nanostructures, where the experimenter has a close control of the exact transport conditions. The results support the assumption that the performance of thermoelectric devices can be enhanced by the adjustment of the QD energy levels and by exploiting the properties of the spin-correlated charge transport via localized, spin-degenerate impurity states. Within this context, spin entropy has been identified as a driving force for the thermoelectric transport in the spin-correlated transport regime in addition to the kinetic contributions. Fundamental considerations, which are based on simple model assumptions, suggest that spin entropy plays an important role in the presence of charge valence fluctuations in the QD. The presented model gives an adequate starting point for future quantitative analysis of the thermoelectricity in the spin-correlated transport regime. These future studies might cover the physics in the limit of single electron QDs or the physics of more complex structures such as QD molecules as well as QD chains. In particular, it should be noted that the experimental investigations of the thermopower of few-electron QDs address questions concerning the entropy transport and entropy production with respect to single-bit information processing operations. These questions are of fundamental physical interest due to their close connection to the problem of minimal energy requirements in communication, and thus ultimately to the so called "Maxwell's demon" with respect to the second law of thermodynamics., Diese Dissertation präsentiert eine experimentelle Studie über die thermoelektrischen Eigenschaften von Halbleiterquantenpunkten. Das thermoelektrische Verhalten der Quantenpunkte wird unter besonderer Berücksichtigung ihrer jeweiligen Energiespektren und magnetischen bzw Spin-Eigenschaften diskutiert. Die durchgeführten Messungen geben Aufschluss über das Zusammenspiel von Einzelelektronentunnelprozessen erster und höherer Ordnung unter dem Einfluss thermischer Gradienten. Somit trägt diese Dissertation zum Verständnis des Ladungs- und Spintransports in potentiellen, zukünftigen Bausteinen für die Quanteninformationsverarbeitung bei und ermöglicht neue Einblicke in das Themengebiet der Thermoelektrizität bei sehr tiefen Temperaturen. Die untersuchten Quantenpunkte wurden in einem zweidimensionalen Elektronengas (2DEG) mittels nanostrukturierter, metallischer "gates" erzeugt, die auf der Oberfläche einer GaAs/AlGaAs Heterostrukturoberfläche aufgebracht wurden. Durch das Anlegen negativer Spannungen in Bezug auf das Potential des 2DEGs, wurde das Elektronengas unter den gates verdrängt, so dass eine isolierte Insel entstand, die bis zu ca. 30 Elektronen zählte. Zwei Tunnelbarrieren dienten als elektrische Verbindung dieses Quantenpunkts zu den Zuleitungen. Unter Verwendung einer speziellen Stromheizungstechnik wurde eine Temperaturdifferenz zwischen den zwei Zuleitungsreservoirs über dem Quantenpunkt erzeugt. Die Untersuchung von Ladungs- und Spintransportprozessen erfolgte über den direkten Vergleich der resultierenden thermoelektrischen Spannung mit den jeweiligen Energiespektren der Quantenpunkte. Im Allgemeinen weist eine solche zweidimensionale Insel ein diskretes Energiespektrum auf, das vergleichbar mit dem einzelner Atome ist. Unterhalb einer Temperatur von wenigen Grad Kelvin, ist die elektrostatische Aufladungsenergie des Quantenpunkts größer als die thermische Anregungsenergie der Elektronen in den Zuleitungen. Als Folge bestimmen Elektron-Elektron-Wechselwirkungseffekte den Transport von Elektronen durch den Quantenpunkt. Die durchgeführten Messungen erklären den Verlauf der Thermokraft als Funktion des Quantenpunktpotentials einschließlich der aufgeprägten Feinstruktur sowie zusätzliche thermoelektrische Effekte, die von den Spin-Eigenschaften des Quantenpunkts hervorgerufen werden. Die Beobachtungen beweisen, dass es möglich ist Stärke und Richtung des elektronischen Wärmeflusses auf der Größenskala einzelner Verunreinigungen zu kontrollieren und gegebenenfalls zu optimieren sowie Spin-korrelierten thermoelektrischen Transport in künstlich hergestellten Nanostrukturen zu verwirklichen, welche eine gezielte Kontrolle der Transportbedingungen erlauben. Die Ergebnisse untermauern die Annahmen einer möglichen Verbesserung der Effizienz thermoelektrisch aktiver Materialien durch die Anpassung der energetischen Lage entsprechender Quantenpunktzustände und durch die Ausnutzung der thermoelektrischen Effekte im Spin-korrelierten Ladungstransport durch energetisch entartete, lokalisierte Zustände. In diesem Rahmen wurde erläutert, dass Spinentropie neben den kinetischen Beiträgen eine weitere treibende Kraft des thermoelektrischen Transports durch Quantenpunkte darstellt. Grundlegende Überlegungen, die auf einfachen Modellannahmen beruhen, lassen erwarten, dass die Beiträge der Spinentropie zum thermoelektischen Transport bei vorhandenen Fluktuationen der Anzahl der Ladungen auf dem Quantenpunkt eine signifikante Rolle spielen. Das vorgestellte Modell bietet hierzu einen geeigneten Ausgangspunkt für weitere quantitative Analysen der Thermoelektrizität im Spin-korrelierten Transportregime. Insbesondere sei darauf hingewiesen, dass die experimentelle Untersuchung der Thermokraft von Quantenpunktstrukturen, wie sie hier verwendet wurden, den Entropietransport und die Entropieerzeugung in Bezug zu Ein-Bit-Rechenoperationen setzen. Fragestellungen dieser Art sind von fundamentalem physikalischen Interesse aufgrund ihrer engen Verknüpfung mit der Frage nach dem minimalen Energieaufwand, der eine Kommunikation ermöglicht. Dieses Problem wird häufig mittels des so genannten Maxwell'schen Dämon diskutiert und hinterfragt in ihrem Ursprung den zweiten Hauptsatz der Thermodynamik.

In this paper we review several aspects of mesoscopic hybrid superconducting systems. In particular we consider charge and heat transport properties in hybrid superconducting-metal structures and the effect of charging energy in superconducting nanostructures., Comment: 27 pages, 8 figures

We derive a master equation for the electron transport through molecular wires in the limit of strong Coulomb repulsion. This approach is applied to two typical situations: First, we study transport through an open conduction channel for which we find that the current exhibits an ohmic-like behaviour. Second, we explore the transport properties of a bridged molecular wire, where the current decays exponentially as a function of the wire length. For both situations, we discuss the differences to the case of non-interacting electrons.

We develop methods for calculating the zero-frequency noise for quantum shuttles, i.e. nanoelectromechanical devices where the mechanical motion is quantized. As a model system we consider a three-dot array, where the internal electronic coherence both complicates and enriches the physics. Two different formulations are presented: (i) quantum regression theorem, and (ii) the counting variable approach. It is demonstrated, both analytically and numerically, that the two formulations yield identical results, when the conditions of their respective applicability are fulfilled. We describe the results of extensive numerical calculations for current and current noise (Fano factor), based on a solution of a Markovian generalized master equation. The results for the current and noise are further analyzed in terms of Wigner functions, which help to distinguish different transport regimes (in particular, shuttling vs. cotunneling). In the case of weak inter-dot coupling, the electron transport proceeds via sequential tunneling between neighboring dots. A simple rate equation with the rates calculated analytically from the P(E)-theory is developed and shown to agree with the full numerics., 22 two-column pages, 9 figures

This letter presents experimental evidence that the electrical conductance of a single molecule can be altered by a chemical binding event. Self-assembled monolayers of electron donor tetramethyl xylyl dithiol (TMXYL) have been synthesized and chemically switched to a conducting state by reaction with an electron acceptor tetracyanoethylene (TCNE). Low bias conductance measurements obtained by scanning tunneling spectroscopy under ultrahigh vacuum conditions show a change from insulating to ohmic behavior as a result of the electron donor/acceptor interaction.

Magnetotransport experiments on a semiconductor quantum ring in the Coulomb blockade regime are described. The measurements allow to extract the discrete energy levels of a realistic ring, which are found to agree well with theoretical expectations. The interaction effects important for our ring structure are analysed in detail. The experimentally observed charging energy can be quantitatively understood within the Hartree approximation including a strong screening contribution due to the top gate. The strong screening effect is the reason for the frequent occurrence of spin-pairs in the addition spectra. The relation of the observed addition spectra to persistent currents is established and the magnitude of the current is extracted from experimental data.

A small quantum dot coupled to two external leads is considered. Different signs of the dot-lead coupling matrix elements give rise to qualitatively different behavior of physical observables such as the conductance, the phase of the transmission amplitude, and the differential capacitance of the dot. For certain relative signs the conductance may vanish at values of the gate potential, where the spectral density is maximal. Zeroes of the conductance are robust against increasing the dot-lead coupling. They are associated with abrupt phase lapses in the transmission phase whose width vanishes as the square of the temperature. We carefully distinguish between phase lapses of $\ensuremath{-}\ensuremath{\pi}$ and phase antilapses of $\ensuremath{\pi}.$