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The isolation of the two-dimensional carbon allotrope graphene in 2004 and the subsequent experimental proof of its remarkable electronic properties have refueled the research interest in low dimensional materials significantly. By now various two-dimensional materials have been discovered, and more recently the possibility of stacking different two-dimensional allotropes to create low dimensional few-layer systems has been discussed. These van-der-Waals coupled multilayers are very promising for future technological applications since (i) the weak interlayer interaction implies rich stacking possibilities for the few layer system, e.g., rotation or translation of layers and (ii) the electronic and transport properties of the multilayer depend sensitively on the stacking order. However, the theoretical treatment of arbitrarily stacked van-der-Waals coupled multilayers presents a significant theoretical challenge. A small rotation of one layer or a network of partial dislocations can lead to systems of essentially mesoscopic size, that fall outside the standard methods of electronic structure theory. Furthermore, such stacking faults can fundamentally change the physical properties of the material, and thus carefully accounting for their presence is essential to the correct treatment of low dimensional systems. In this thesis we describe an effective electronic structure theory, based on a general -- and exact -- mapping of the Slater-Koster tight-binding Hamiltonian to an effective continuum Hamiltonian H(r,p), that we show can successfully treat such systems. While our electronic structure theory can be applied to arbitrary multilayer or single layer structures, in this work we focus on one particular prototype: the twisted bilayer graphene. This material, consisting of two mutually rotated graphene monolayers, is simple in structure, yet captures the rich physics introduced by the stacking degree of freedom to full extent. We deploy an effective Hamiltonian approach that enables us to investigate the electronic structure for, essentially, any twist angles. Above a critical angle of approximately 0.02 degree the low energy electronic spectrum changes significantly as a function of the twist angle, while below this angle it is largely unchanged and resembles a superposition of all possible stacking types of the bilayer. Fermi surface calculations as a function of angle reveal that from a large angle limit in which the two Dirac cones of each layer are preserved there emerges a strong coupling low angle regime possessing a remarkably baroque Fermiology. At intermediate angles a number of isolated topological changes in the Fermi surface can be identified as a function of doping. In order to investigate the impact of this rich electronic structure on the transport properties of twisted bilayer graphene, we consider the scattering of electrons due to phonons within the Boltzmann transport formalism and calculate the resulting in-plane electric conductivity as a function of rotation angle, doping, and temperature. The low-energy (quasi-)acoustic phonon spectrum is derived by treating the two layers of the twisted bilayer as weakly coupled elastic planes. We find that the conductivity depends very sensitively on the twist angle of the layers, as well as on doping of the bilayer system. At large angles (more than 8 degree), the conductivity is -- within a certain energy range -- almost independent of doping, however the electronic decoupling of the layers that is found in the ground state only persists at low enough temperatures T < 10 K. At higher temperatures the layers are re-coupled due to the excitation of phonons. In the small angle regime (less than 2 degree) the conductivity is reduced by two orders of magnitude and varies significantly with doping, reflecting the complexity of the electronic ground state. Finally, we investigate the effect of a layer-perpendicular electric field and find that it strongly affects the transport properties at low temperatures whereas this effect is washed out at room temperature.

Graphene is a two-dimensional layer of sp2 -hybridized carbon atoms. It shows unique material properties resulting in marked relevance in both fundamental research and potential applications. One strategy to produce high-quality, large-scale graphene is the epitaxial growth of graphene on SiC(0001). The graphene-substrate interaction induces an ‘intrinsic’ majority charge carrier density in the graphene sheet, resulting in a conductive film that allows for applications in graphene electronics. In the present thesis, the DC and THz response behavior of the electrical contact region in hybrid MLG/QFBLG devices, comprising n-type monolayer graphene (MLG) and p-type quasi-freestanding bilayer graphene (QFBLG) side-by-side on a single chip, is investigated by temperature-dependent quantum transport and THz transmission in the graphene devices. In the first part, a device fabrication strategy is presented that is developed for MLG and QFBLG regions side-by-side and in electrical contact on a single chip. The conversion process from n-type MLG to p-type QFBLG via spatially controlled hydrogen intercalation is characterized by electrical probing and surface science techniques, namely scanning electron microscopy, scanning tunneling microscopy and photoelectron spectroscopy. It is shown that the conversion interface, i.e. the border line between MLG and QFBLG, is sharp at the nm scale and aligns with respect to the orientation of the buffer layer, which is a SiC surface reconstruction forming during the growth of epitaxial graphene. On a large scale, the presented fabrication strategy allows to pattern the charge carrier density in large-area devices without using metallic gates, which would show spurious influence in many experiments. The quantum- and magnetotransport measurements in hybrid MLG/QFBLG devices result in identification of concurrent transport mechanisms in the graphene constituents. While the temperature-dependent zero-magnetic- field resistivity is largely determined by the device-substrate interaction, quantum transport contributions at cryogenic temperatures are decoupled and quantified by studying temperature-dependent magnetotransport. The presented findings can be strikingly well allocated to the respective graphene constituents. Furthermore, the investigation of the contact region, formed between the differently charged MLG and QFBLG, via current-voltage characterization yields a linear, i.e. ohmic, response behavior with a distinct contribution of a non-symmetric zero-bias anomaly that can be allocated to the EEI-induced tunneling junction. Due to unequal manifestation of the chirality quantum number in the MLG and QFBLG, the potential violation of time-reversal symmetry in hybrid MLG/QFBLG devices is investigated. However, the comparison between the experimental investigations and Onsager’s reciprocal relations demonstrates no sign for this violation. The THz response of patterned epitaxial graphene is investigated experimentally in transmission geometry. It is shown that spatial-periodic modulation of the graphene’s sheet conductivity is required in order to resonantly excite surface plasmon-polaritons (SPPs), bound to the graphene sheet, by incident THz radiation. The occurrence of additional graphene contributions at quasi-periodically aligned SiC step edges during graphene growth already causes finite THz response of the investigated graphene devices. Excellent agreement between the experimental findings presented in this work and tailored, full wave electrodynamics simulation is demonstrated for lithographically patterned devices comprising a periodic array of isolated graphene stripes (MLG and QFBLG) with a clear signature for resonant excitation of SPPs in the THz spectral range. It is remarkable that the atomically thin and almost fully transparent graphene can reduce the polarization-dependent THz transmission by 50% when a periodic array of stripes is patterned in analogy to a wire grid polarizer in the case of epitaxial graphene. Finally, the investigation of a continuous-sheet graphene device comprising an array of alternating MLG and QFBLG stripes demonstrates the resonant excitation of SPPs in agreement with the full wave electrodynamics simulation based on the previously determined material properties. This indicates minor impact of the MLG-to-QFBLG junction onto the THz response of the hybrid device. It should be emphasized that the subtle contrast in conductivity, induced via the hybrid stripe patterning, can result in a relative polarization contrast in transmission of 20%, although the device comprises a continuous graphene sheet. Graphen ist eine zweidimensionale Lage bestehend aus sp2 -hybridisierten Kohlenstoffatomen. Es zeichnet sich durch einzigartige Materialeigenschaften aus, welche die erhebliche Relevanz dieses Materialsystems sowohl für die Grundlagenforschung als auch für potentielle Anwendungen begründen. Das epitaktische Wachstum von Graphen auf SiC(0001) stellt hierbei eine Möglichkeit dar, großflächig Graphen von hoher Qualität herzustellen. Aufgrund der Graphen-Substrat Wechselwirkung wird eine ‘intrinsische’ Majoritätsladungsträgerdichte in der Graphen-Schicht induziert. Die hieraus resultierende, elektrisch leitfähige Schicht ermöglicht vielfältige Anwendungen im Bereich der sogenannten Graphen-Elektronik. In der vorliegenden Arbeit wird das DC sowie das THz Verhalten der elektrischen Kontaktregion in hybriden MLG/QFBLG Bauelementen, welche aus n-typ Monolagen-Graphen (MLG) Seite an Seite mit p -typ quasi-freistehendem Bilagen-Graphen (QFBLG) auf demselben Chip bestehen, mittels temperaturabhängiger Quantentransport- und THz Transmissionsexperimente untersucht. Im ersten Teil wird das Herstellungsverfahren erläutert, welches entwickelt wurde, um MLG und QFBLG Bereiche Seite an Seite und im elektrischen Kontakt auf demselben Chip herzustellen. Hierzu wird die Konversion von n -typ MLG zu p -typ QFBLG mittels der räumlich kontrollierten Interkalation von Wasserstoff anhand elektrischer Messverfahren und Methoden der Oberflächenanalytik untersucht. Letztere schließen die Rasterelektronenmikroskopie, die Rastertunnelmikroskopie als auch die Photoelektronenspektroskopie ein. Es wird gezeigt, dass der Übergangsbereich zwischen MLG und QFBLG auf der nm-Skala scharf ist. Zudem richtet sich dieser Graphen-Übergangsbereich entlang der durch die sogenannte ‘buffer layer’ vorgegebenen Periodizität im Realraum aus. Die buffer layer ist eine während des Graphen-Wachstumsprozesses entstehende Oberflächenrekonstruktion des SiC. Das angesprochene Herstellungsverfahren ermöglicht die zielgerichtete Modulation der Ladungsträgerdichte in großflächigen Graphen-Bauteilen. Herauszustellen ist hierbei, dass die Verwendung der für diesen Zweck häufig üblichen metallischen Gate-Strukturen, welche einen störenden Einfluss auf eine Vielzahl von Experimenten ausüben, vermieden wird. Untersuchungen des Quantentransports in hybriden MLG/QFBLG Bauelementen erlauben die Identifikation der in den beteiligten Graphen-Komponenten gleichzeitig auftretenden Transportmechanismen. So kann gezeigt werden, dass das Hochtemperaturverhalten des spezifischen Widerstands ohne äußeres Magnetfeld mehrheitlich durch die Bauelement-Substrat Wechselwirkung bestimmt wird. Zudem können führende Beiträge zum Quantentransport mittels Untersuchung des temperaturabhängigen Magnetotransports bei kryogenen Temperaturen isoliert und quantifiziert werden. Die aufgezeigten Erkenntnisse können hervorragend den jeweiligen Graphen-Komponenten der untersuchten Bauelemente zugeordnet werden. Im Weiteren ergibt die Strom-Spannungs-Charakterisierung der elektrischen Kontaktregion zwischen den unterschiedlich geladenen MLG und QFBLG Bereichen ein lineares, d.h. ohmsches Antwortverhalten, welche jedoch einen merklichen Beitrag einer nichtsymmetrischen Altshuler-Aronov Anomalie (oft auch: zero-bias anomaly) aufweist. Aufgrund der unterschiedlichen Manifestierung der Chiralitätsquantenzahl in beiden Graphen-Komponenten wird abschließend die potentielle Verletzung des Prinzips der Zeitumkehrinvarianz bei Ladungstransport über diese Kontaktregion untersucht. Der Vergleich zwischen den Ergebnissen der angestellten, experimentellen Untersuchungen und den Onsagerschen Reziprozitätsbeziehungen belegt die Gültigkeit des grundlegenden Prinzips der Zeitumkehrinvarianz. Im letzten Teil dieser Arbeit wird das Antwortverhalten strukturierten, epitaktischen Graphens im THz Spektralbereich in Transmissionsgeometrie experimentell untersucht. Es wird aufgezeigt, dass eine räumlich periodische Modulation der elektrischen Leitfähigkeit der Graphen-Schicht notwendig ist, um resonant Oberflächenplasmon-Polaritonen (surface plasmon-polaritons, SPPs), welche evaneszent an das Graphen gebunden sind, durch einfallende THz Strahlung anzuregen. Das Entstehen zusätzlicher Graphen-Lagen entlang der quasi-periodisch angeordneten Stufenkanten des SiC während des Graphen-Wachstumsprozesses kann bereits eine endliche Systemantwort der untersuchten Graphen-Bauelemente zur Folge haben. Darüber hinaus kann die exzellente Übereinstimmung der experimentellen Erkenntnisse für lithographisch strukturierte Bauelemente, die aus der periodischen Anordnung isolierter Graphen-Streifen (MLG und QFBLG) bestehen, und speziell entwickelten Vollwellen-Elektrodynamik-Simulationen gezeigt werden. Hierbei tritt eine deutliche Signatur der resonanten Anregung von SPPs im THz Spektralbereich zu Tage. Es ist bemerkenswert, dass das atomar-dünne und annähernd vollständig transparente Graphen die polarisationsabhängige THz Transmission im Falle des epitaktischen Graphens um bis zu 50% reduzieren kann, wenn eine periodische Streifenanordnung in Analogie eines Drahtgitterpolarisators gewählt wird. Die abschließende Untersuchung eines Bauelements mit zusammenhängender Graphen-Schicht bestehend aus einer alternierenden Anordnung von MLG- und QFBLG-Streifen zeigt die resonante Anregung von SPPs in guter Übereinstimmung mit der respektiven Vollwellen-Elektrodynamik-Simulation, welche auf den eingangs ermittelten Materialeigenschaften basiert. Dies lässt den geringfügigen Einfluss des MLG/QFBLG Kontaktbereiches auf das THz Antwortverhalten des hybriden Bauelements schließen. Bemerkenswert ist, dass der subtile Leitfähigkeitskontrast, der durch die hybride Strukturierung der Graphen-Schicht erreicht wird, einen relativen Polarisationskontrast von bis zu 20% in der gemessenen THz Transmission hervorruft, obschon das Bauelement aus einer atomar dünnen Graphen-Schicht besteht.

The quasiclassical theory of superconductivity provides the most successful description of diffusive heterostructures comprising superconducting elements, namely, the Usadel equations for isotropic Green's functions. Since the quasiclassical and isotropic approximations break down close to interfaces, the Usadel equations have to be supplemented with boundary conditions for isotropic Green's functions (BCIGF), which are not derivable within the quasiclassical description. For a long time, the BCIGF were available only for spin-degenerate tunnel contacts, which posed a serious limitation on the applicability of the Usadel description to modern structures containing ferromagnetic elements. In this article, we close this gap and derive spin-dependent BCIGF for a contact encompassing superconducting and ferromagnetic correlations. This finally justifies several simplified versions of the spin-dependent BCIGF, which have been used in the literature so far. In the general case, our BCIGF are valid as soon as the quasiclassical isotropic approximation can be performed. However, their use require the knowledge of the full scattering matrix of the contact, an information usually not available for realistic interfaces. In the case of a weakly polarized tunnel interface, the BCIGF can be expressed in terms of a few parameters, i.e. the tunnel conductance of the interface and five conductance-like parameters accounting for the spin-dependence of the interface scattering amplitudes. In the case of a contact with a ferromagnetic insulator, it is possible to find explicit BCIGF also for stronger polarizations. The BCIGF derived in this article are sufficienly general to describe a variety of physical situations and may serve as a basis for modelling realistic nanostructures., Comment: This paper presents an improvement of arXiv:cond-mat/0204116. The present version takes into account corrections from the erratum Phys. Rev. B 83, 139901 (2011)

We study the coherent transport of heavy holes through a one-dimensional ring in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic in the in-plane components of momentum, gives rise to an angular momentum dependent spin texture of the eigenstates and influences transport. We analyze the dependence of the resulting differential conductance of the ring on hole polarization of the leads and the signature of the textures in the Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field. We find that the polarization-resolved conductance reveals whether the dominant spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic spin-orbit coupling can be distinguished from the conventional linear coupling by observing the four-peak structure in the Aharonov-Bohm oscillations., Comment: 12 pages, 11 figures

Im Rahmen dieser Arbeit wurde eine Meßapparatur zur Bestimmung des Leitwerts einzelner C60-Moleküle aufgebaut. Dabei werden mittels der Bruchkontakttechnik atomare Kontaktspitzen erzeugt und die Moleküle thermisch in situ auf die Kontakte verdampft.Es konnten dadurch Vibrationsmoden des C60 zwischen Goldelektroden nachgewiesen werden. Bei Messungen mit Aluminiumelektroden und C60 war mit großer Wahrscheinlichkeit der Kondoeffekt zu sehen. Dabei fungiert das C60 als Insel zwischen den Elektroden.Zusätzlich konnte im Rahmen der Charaktersierung der Elektroden Phononmoden des Aluminiums am Einatomkontakt nachgewiesen werden. Diese zeichnen sich durch eine Änderung ihrer Energie mit zunehmender Streckung der Elektroden aus.

Conductance histograms are a valuable tool to study the intrinsic conduction properties of metallic atomic-sized contacts. These histograms show a peak structure, which is characteristic of the type of metal under investigation. Despite the enormous progress in the understanding of the electronic transport in metallic nanowires, the origin of this peak structure is still a basic open problem. In the present work we tackle this issue, extending our theoretical analysis of Au conductance histograms to different types of metals, namely, Ag, Pt and ferromagnetic Ni. We combine classical molecular dynamics simulations of the breaking of nanocontacts with conductance calculations based on a tight-binding model. This combination gives us access to crucial information such as contact geometries, strain forces, minimum cross-sections, the conductance, transmissions of the individual conduction channels, and, in the case of Ni, the spin polarization of the current. We shall also briefly discuss investigations of Al atomic-sized contacts. From our analysis we conclude that the differences in the histograms of these metals are due to (i) the very different electronic structures, which means different atomic orbitals contributing to the transport and (ii) the different mechanical properties, which in a case such as Pt lead to the formation of special structures, namely, monoatomic chains. Of particular interest are results for Ni that indicate the absence of any conductance quantization, and show how the current polarization evolves (including large fluctuations) from negative values in thick contacts to even positive values in the tunneling regime after rupture of the contact. Finally, we also present a detailed analysis of the breaking forces of these metallic contacts, which are compared to the forces predicted from bulk considerations.

We study the interplay of Klein tunneling (= interband tunneling) between n-doped and p-doped regions in graphene and Andreev reflection (= electron-hole conversion) at a superconducting electrode. The tunneling conductance of an n-p-n junction initially increases upon lowering the temperature, while the coherence time of the electron-hole pairs is still less than their lifetime, but then drops back again when the coherence time exceeds the lifetime. This reentrance effect, known from diffusive conductors and ballistic quantum dots, provides a method to detect phase coherent Klein tunneling of electron-hole pairs., Comment: 4 pages, 3 figures

We show that the statistics of electron transfer in a coherent quantum point contact driven by an arbitrary time-dependent voltage is composed of elementary events of two kinds: unidirectional one-electron transfers determining the average current and bidirectional two-electron processes contributing to the noise only. This result pertains at vanishing temperature while the extended Keldysh-Green's function formalism in use also enables the systematic calculation of the higher-order current correlators at finite temperatures., 4 pages, 2 figures; to appear in PRL. Version 1 contains additional results for noise and the third cumulant at finite temperatures [Eqs. (19) and (20)]

We analyze the charge transport between a one-dimensional weakly interacting electron gas and a superconductor within the scaling approach in the basis of scattering states. We derive the renormalization group equations, which fully account for the intrinsic energy dependence due to Andreev reflection. A strong renormalization of the corresponding reflection phase is predicted even for a perfectly transparent metal-superconductor interface. The interaction-induced suppression of the Andreev conductance is shown to be highly sensitive to the normal state resistance, providing a possible explanation of experiments with carbon-nanotube/superconductor junctions by Morpurgo et al. [Science 286, 263 (2001)]., Comment: 4 pages, 2 figures

We investigate electrical transport through a single-electron transistor coupled to a nanomechanical oscillator. Using a combination of a master-equation approach and a numerical Monte Carlo method, we calculate the average current and the current noise in the strong-coupling regime, studying deviations from previously derived analytic results valid in the limit of weak-coupling. After generalizing the weak-coupling theory to enable the calculation of higher cumulants of the current, we use our numerical approach to study how the third cumulant is affected in the strong-coupling regime. In this case, we find an interesting crossover between a weak-coupling transport regime where the third cumulant heavily depends on the frequency of the oscillator to one where it becomes practically independent of this parameter. Finally, we study the spectrum of the transport noise and show that the two peaks found in the weak-coupling limit merge on increasing the coupling strength. Our calculation of the frequency-dependence of the noise also allows to describe how transport-induced damping of the mechanical oscillations is affected in the strong-coupling regime., 11 pages, 9 figures

We consider transport in a three terminal device attached to one superconducting and two normal metal terminals, using the circuit theory of mesoscopic superconductivity. We compute the nonlocal conductance of the current out of the first normal metal terminal in response to a bias voltage between the second normal metal terminal and the superconducting terminal. The nonlocal conductance is given by competing contributions from crossed Andreev reflection and electron cotunneling, and we determine the contribution from each process. The nonlocal conductance vanishes when there is no resistance between the superconducting terminal and the device, in agreement with previous theoretical work. Electron cotunneling dominates when there is a finite resistance between the device and the superconducting reservoir. Decoherence is taken into account, and the characteristic timescale is the particle dwell time. This gives rise to an effective Thouless energy. Both the conductance due to crossed Andreev reflection and electron cotunneling depend strongly on the Thouless energy. We suggest to experimentally determine independently the conductance due to crossed Andreev reflection and electron cotunneling in measurement of both energy and charge flow into one normal metal terminal in response to a bias voltage between the other normal metal terminal and the superconductor., Comment: v2: Published version with minor changes, 12 pages and 9 figures

We find the distribution of transmission eigenvalues in a series of identical junctions between chaotic cavities using the circuit theory of mesoscopic transport. This distribution rapidly approaches the diffusive wire limit as the number of junctions increases, independent of the specific scattering properties of a single junction. The cumulant generating function and the first three cumulants of the charge transfer through the system are obtained both in the normal and in the superconducting state., 7 pages, 3 figures

We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching approx. 60-70 %. The present device can be seen as a quasi-ballistic S-N beam-splitter junction., full paper including figures can be retrieved from: http://www.unibas.ch/phys-meso/Research/Papers/2004/Shot-Noise-Yjunction.pdf

We reconsider the problem of the superconducting proximity effect in a diffusive ferromagnet bounded by tunneling interfaces, using spin-dependent boundary conditions. This introduces for each interface a phase-shifting conductance Gphi which results from the spin dependence of the phase shifts acquired by the electrons upon scattering on the interface. We show that Gphi strongly affects the density of states and supercurrents predicted for superconducting/ferromagnetic hybrid circuits. We show the relevance of this effect by identifying clear signatures of Gphi in the data of T. Kontos et al [Phys. Rev. Lett. 86, 304 (2001), ibid. 89, 137007 (2002)]., submitted to Phys. Rev. Lett

We examine the full counting statistics of quantum dots, which display super-Poissonian shot noise. By an extension to a generic situation with many excited states we identify the underlying transport process. The statistics is a sum of independent Poissonian processes of bunches of different sizes, which leads to the enhanced noise. The obtained results could be useful to determine transport characteristics in molecules and large quantum dots, since the noise (and higher cumulants) allow to identify the internal level structure, which is not visible in the average current., 4 pages, 1 figure, submitted to PRB

Slow electrons dispersed on a superfluid helium surface complete a plane capacitor due to the substrate and the dielectric fluid. They built up a horizontal layer of charges, which freely move in about 10 nm height. This pure two-dimensional system allows experiments like in semiconductors. The conventional experimental procedure for mobility and transport studies is the capacitive coupling measured in the ac-method. As an alternative method serves the direct determination of the electron flow above the liquid helium surface. This dc-method has recently been realized for the first time, and has the main advantage of the insensitivity to stray capacitances, which influences the ac-signal. In combination with the dc-method an electrostatic confinement can be used to create a quasi 1-dimensional electron channel. If the helium covered substrate is an arrangement of in-plane electrodes hold at ascending potentials, the electrons move along the x-direction above the helium film. A 2D charge transport in analogy to a FET was realized with our configuration of source, gate, and drain electrode. The electrons are directly measured by an additional pick-up-electrode. The system is ditto suited to study the transport properties by changing the dimension. A negative gate potential reduces the effective width of the split gate. The thinner the helium film, the higher is the influence of the substrate roughness, i.e., pinning effects make the electron current strongly dependent on the film thickness. By measuring the decay time on different substrates we can conclude about the mobility. The mobility can change when the thickness of the helium film decreases from bulk to a thin film of a few hundred Å. There is a pronounced dip at a certain film thickness which varies with the roughness of the substrate covered by the helium film. In this context we presented both new theoretical investigations (in collaboration with V. Shikin, Chernogolovka, Russia) and experimental data. This data support the scenario of a discontinuity in the chemical potential of the electrons which results in a dip-like behaviour in the measured electron current which is directly proportional to the mobility.

We extend our previous work on shot noise for entangled and spin polarized electrons in a beam-splitter geometry with spin-orbit (\textit{s-o}) interaction in one of the incoming leads (lead 1). Besides accounting for both the Dresselhaus and the Rashba spin-orbit terms, we present general formulas for the shot noise of singlet and triplets states derived within the scattering approach. We determine the full scattering matrix of the system for the case of leads with \textit{two} orbital channels coupled via weak \textit{s-o} interactions inducing channel anticrossings. We show that this interband coupling coherently transfers electrons between the channels and gives rise to an additional modulation angle -- dependent on both the Rashba and Dresselhaus interaction strengths -- which allows for further independent coherent control of the electrons traversing the incoming leads. We derive explicit shot noise formulas for a variety of correlated pairs (e.g., Bell states) and lead spin polarizations. Interestingly, the singlet and \textit{each} of the triplets defined along the quantization axis perpendicular to lead 1 (with the local \textit{s-o} interaction) and in the plane of the beam splitter display distinctive shot noise for injection energies near the channel anticrossings; hence, one can tell apart all the triplets, in addition to the singlet, through noise measurements. We also find that spin-orbit induced backscattering within lead 1 reduces the visibility of the noise oscillations, due to the additional partition noise in this lead. Finally, we consider injection of two-particle wavepackets into leads with multiple discrete states and find that two-particle entanglement can still be observed via noise bunching and antibunching., 30 two-column pages and 7 figures

We study the charge transport statistics in coherent two-terminal double junctions within the framework of the circuit theory of mesoscopic transport. We obtain the general solution of the circuit-theory matrix equations for the Green's function of a chaotic cavity between arbitrary contacts. As an example we discuss the full counting statistics and the first three cumulants for an open asymmetric cavity between a superconductor and a normal-metal lead at temperatures and voltages below the superconducting gap. The third cumulant shows a characteristic sign change as a function of the asymmetry of the two quantum point contacts, which is related to the properties of the Andreev reflection eigenvalue distribution., 8 pages, 4 figures

We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire., Comment: 5 pages, 3 figures. to be published in Europhysics Letters

We present a comprehensive theoretical analysis of the dc transport properties of superconducting point contacts. We determine the full counting statistics for these junctions, which allows us to calculate not only the current or the noise, but all the cumulants of the current distribution. We show how the knowledge of the statistics of charge transfer provides an unprecedented level of understanding of the different transport properties for a great variety of situations. We illustrate our results with the analysis of junctions between BCS superconductors, contacts between superconductors with pair-breaking mechanisms and short diffusive bridges. We also discuss the temperature dependence of the different cumulants and show the differences with normal contacts., revtex4, 20 pages, 15 figures

We study shot noise and cross correlations in a four terminal spin-valve geometry using a Boltzmann-Langevin approach. The Fano factor (shot noise to current ratio) depends on the magnetic configuration of the leads and the spin-flip processes in the normal metal. In a four-terminal geometry, spin-flip processes are particular prominent in the cross correlations between terminals with opposite magnetization., 4 pages, 3 figures

We investigate current fluctuations in a three-terminal quantum dot in the sequential tunneling regime. Dynamical spin blockade can be induced when the spin-degeneracy of the dot states is lifted by a magnetic field. This results in super-Poissonian shot noise and positive zero-frequency cross-correlations. Our proposed setup can be realized with semiconductor quantum dots., 7 pages, 3 figures, final version, to appear in Europhys. Lett

We investigate current fluctuations in a three-terminal quantum dot in the sequential tunneling regime. In the voltage-bias configuration chosen here, the circuit is operated like a beam splitter, i.e. one lead is used as an input and the other two as outputs. In the limit where a double occupancy of the dot is not possible, a super-Poissonian Fano factor of the current in the input lead and positive cross-correlations between the current fluctuations in the two output leads can be obtained, due to dynamical channel-blockade. When a single orbital of the dot transports current, this effect can be obtained by lifting the spin-degeneracy of the circuit with ferromagnetic leads or with a magnetic field. When several orbitals participate in the electronic conduction, lifting spin-degeneracy is not necessary. In all cases, we show that a super-Poissonian Fano factor for the input current is not equivalent to positive cross-correlations between the outputs. We identify the conditions for obtaining these two effects and discuss possible experimental realizations., 18 pages, 20 Figures, submitted to Phys. rev. B

We derive the full distribution of transmitted particles through a superconducting point contact of arbitrary transparency under voltage bias. The charge transport is dominated by multiple Andreev reflections. The counting statistics is a multinomial distribution of processes, in which multiple charges ne (n=1,2,3,...) are transferred through the contact. For zero temperature we obtain analytical expressions for the probabilities of the multiple Andreev reflections. The current, shot noise and high current cumulants in a variety of situations can be obtained from our result., revtex4, 5 pages, 3 figures

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by a novel dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads for instance to positive zero-frequency cross-correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates., 4 pages, 4 figures, to appear in Phys. Rev. Lett

We study the full counting statistics of heterostructures consisting of normal metal parts connected to a superconducting terminal. Assuming that coherent superconducting correlations are suppressed in the normal metals we show, using Keldysh-Nambu Green's functions, that the system can be mapped onto a purely normal system with twice the number of elements. For a superconducting beam splitter with several normal terminals we obtain general results for the counting statistics., 7 pages, submitted to Europhys. Lett

We investigate nonequilibrium noise in a diffusive Andreev interferometer, in which currents emerging from two Normal metal/Superconductor (N-S) interfaces can interfere. We observe a modulation of the shot noise when the phase difference between the two N-S interfaces is varied by a magnetic flux. This is the signature of phase-sensitive fluctuations in the normal metal. The effective charge inferred from the shot noise measurement is close to q_eff=2e but shows phase-dependent deviations from 2e at finite energy, which we interprete as due to pair correlations. Experimental data are in good agreement with theoretical predictions based on an extended Keldysh Green's function approach., Comment: 4 pages, 3 figures

We study shot noise for spin-polarized currents and entangled electron pairs in a four-probe (beam splitter) geometry with a local Rashba spin-orbit (s-o) interaction in the incoming leads. Within the scattering formalism we find that shot noise exhibits Rashba-induced oscillations with continuous bunching and antibunching. We show that entangled states and triplet states can be identified via their Rashba phase in noise measurements. For two-channel leads we find an additional spin rotation due to s-o induced interband coupling which provides enhanced spin control. We show that the s-o interaction determines the Fano factor which provides a direct way to measure the Rashba coupling constant via noise., 4 pages, 3 eps figures. Minor changes to text (typos/misprints). Slight change to Fig. 2

We study the statistics of charge transport in a mesoscopic three-terminal device with one superconducting terminal and two normal-metal terminals. We calculate the full distribution of transmitted charges into the two symmetrically biased normal terminals. In a wide parameter range, we find large positive crosscorrelations between the currents in the two normal arms. We also determine the third cumulant that provides additional information on the statistics not contained in the current noise., Comment: 5 pages, 2 figures, revtex4

We consider transport in a diffusive cross-shaped Andreev interferometer geometry, both theoretically and experimentally. A strong and unexpected modulation of the conductance with the superconducting phase is found. In particular, a reversed resistance vs. phase difference is predicted, where the resistance is decreased by a phase gradient. A comparison of our quasiclassical calculation with experimental data shows quantitative agreement if we account for diamagnetic screening., Comment: 4 pages, 4 figures, revtex4

We study the current statistics in normal diffusive conductors in contact with a superconductor. Using an extension of the Keldysh Green's function method we are able to find the full distribution of charge transfers for all temperatures and voltages. For the non-Gaussian regime, we show that the equilibrium current fluctuations are enhanced by the presence of the superconductor. We predict an enhancement of the nonequilibrium current noise for temperatures below and voltages of the order of the Thouless energy E_Th=D/L^2. Our calculation fully accounts for the proximity effect in the normal metal and agrees with experimental data. We demonstrate that the calculation of the full current statistics is in fact simpler than a concrete calculation of the noise., 4 pages, 2 figures (included)

The phase of Aharonov-Bohm oscillations in mesoscopic diffusive metal rings in the presence of a magnetic field, can be modulated by the application of a dc-bias current ${I}_{\mathrm{dc}}.$ We address the question of how a variation of ${I}_{\mathrm{dc}}$ and hence of the microscopic phases of the electronic wave functions results in the observable ``global'' phase of the conductance oscillations. Whereas the first one can be varied continuously, the latter has to be quantized for a ring in a two-wire configuration by virtue of the Onsager symmetry relations. We observe a correlation between a phase flip by $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ and the amplitude of the oscillations.

We present an extension of the Keldysh-Green's function method, which allows to calculate the full distribution of transmitted particles through a mesoscopic superconductor. The method is applied to the statistics of supercurrent in short contacts. If the current is carried by Andreev bound states the distribution corresponds to switching between long trains of electrons going in opposite directions. For weak (gapless) superconductors or tunnel junctions we find that at low temperatures the distribution has negative `probabilities'. Accounting for the quantum mechanical nature of the measuring device shows that these negative values can indeed be measured., 5 pages, revtex, 1 figure, includes some remarks omitted in the published version due to length limitation