Temperature induced crossover between 0 and pi states in S/F/S junctions

Ferromagnetic Josephson junctions can show at equilibrium a pi phase difference between the superconducting electrodes. We explain this pi state in an original way by a modified spectrum of Andreev bound states shifted by the exchange energy. A simplified expression for the spectral supercurrent density is calculated and the non-monotonic temperature dependence of the critical current is discussed. This model accounts for the cancellation of the critical current with temperature observed in a small range of barrier thickness in our Nb/Cu52Ni48/Nb junctions. This cancellation corresponds to an inversion of the supercurrent and to a ground state crossover from a 0 state to a pi state. This transition is caused both by the thermal distribution of quasi-particles and by the temperature dependence of the exchange energy. The experimental curves are well reproduced by our theoretical expression except for the very small amplitude of the supercurrent attributed to a large spin-flip scattering

On the imaging of electron transport in semiconductor quantum structures by scanning-gate microscopy: successes and limitations

This paper presents a brief review of scanning-gate microscopy applied to the imaging of electron transport in buried semiconductor quantum structures. After an introduction to the technique and to some of its practical issues, we summarise a selection of its successful achievements found in the literature, including our own research. The latter focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. Based on our own experience, we then discuss in detail some of the limitations of scanning-gate microscopy. These include possible tip induced artefacts, effects of a large bias applied to the scanning tip, as well as consequences of unwanted charge traps on the conductance maps. We emphasize how special care must be paid in interpreting these scanning-gate images.Comment: Special issue on (nano)characterization of semiconductor materials and structure

A new transport phenomenon in nanostructures: A mesoscopic analog of the Braess paradox encountered in road networks

The Braess paradox, known for traffic and other classical networks, lies in the fact that adding a new route to a congested network in an attempt to relieve congestion can counter-intuitively degrade the overall network performance. Recently, we have extended the concept of Braess paradox to semiconductor mesoscopic networks, whose transport properties are governed by quantum physics. In this paper, we demonstrate theoretically that, alike in classical systems, congestion plays a key role in the occurrence of a Braess paradox in mesoscopic networks.Comment: Invited talk at Int. Conf. on Superlattices, Nanostructures, and Nanodevices (ICSNN2012), Dresden, July 2012; submitted to Nanoscale Res. Let

Scanning-gate microscopy of semiconductor nanostructures: an overview

This paper presents an overview of scanning-gate microscopy applied to the imaging of electron transport through buried semiconductor nanostructures. After a brief description of the technique and of its possible artifacts, we give a summary of some of its most instructive achievements found in the literature and we present an updated review of our own research. It focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the given examples, we emphasize how a local-probe approach is able to shed new, or complementary, light on transport phenomena which are usually studied by means of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201

Half-integer Shapiro steps at the 0-pi crossover of a ferromagnetic Josephson junction

We investigate the current-phase relation of S/F/S junctions near the crossover between the 0 and the pi ground states. We use Nb/CuNi/Nb junctions where this crossover is driven both by thickness and temperature. For a certain thickness a non-zero minimum of critical current is observed at the crossover temperature. We analyze this residual supercurrent by applying a high frequency excitation and observe the formation of half-integer Shapiro steps. We attribute these fractional steps to a doubling of the Josephson frequency due to a sin(2*phi) current-phase relation. This phase dependence is explained by the splitting of the energy levels in the ferromagnetic exchange field.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let

Low Magnetic Field Regime of a Gate-Defined Constriction in High-Mobility Graphene

We report on the evolution of the coherent electronic transport through a gate-defined constriction in a high-mobility graphene device from ballistic transport to quantum Hall regime upon increasing the magnetic field. At low field, the conductance exhibits Fabry-P\'erot resonances resulting from the npn cavities formed beneath the top-gated regions. Above a critical field $B^*$ corresponding to the cyclotron radius equal to the npn cavity length, Fabry-P\'erot resonances vanish and snake trajectories are guided through the constriction with a characteristic set of conductance oscillations. Increasing further the magnetic field allows us to probe the Landau level spectrum in the constriction, with distortions due to the combination of confinement and de-confinement of Landau levels in a saddle potential. These observations are confirmed by numerical calculations

Coulomb-mediated antibunching of an electron pair surfing on sound

Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the controlled interaction of flying electrons at the single particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realise a controlled-phase gate for in-flight quantum manipulations

Scanning gate investigations of quantum transport phenomena

The manuscript describes my research activities on quantum transport in electronic devices using an experimental technique called Scanning Gate Microscopy (SGM). This technique provides spatial information on electron density and electron flow in buried two-dimensional electron gases (2DEG) or to modify these quantities at specific positions in the 2DEG. We investigate III/V semiconductor heterostructures, such as GaAs/AlGaAs or InGaAs/InAlAs, which host 2DEGs with high electron mobility. At low temperature, the elastic mean free path is several microns and the electron transport is fully ballistic in nano-scale devices patterned by electron beam lithography. Since the transport is also phase coherent and the electron wavelength is large, quantum effects are observable and can be studied in various nanostructures forming constrictions, rings, cavities, etc... These effects are usually probed by transport or optical spectroscopy which give mainly access to the energy levels. The spatial distribution of the quantum states, however, cannot be observed directly in these buried nanostructures, as opposed to surface electronic systems where scanning tunneling microscopy can measure the local density of states. The aim of the SGM technique is thus to probe indirectly this spatial distribution by recording the effect of a local electrostatic perturbation on the transport properties of the nanostructure. The sharp tip of an atomic force microscope is used as a local gate which is scanned above the surface. The voltage applied on the tip with respect to the 2DEG induces a local change of the electrostatic potential in the 2DEG, affecting the electron states, and thus the device conductance if these states are involved in the conduction path. With this technique, the dream would be to image directly the wave function in any kind of nanostructure, but the resolution is unfortunately limited by the relatively large distance between the tip and the 2DEG, which is buried several tens of nanometers below the surface. On the other hand, the SGM technique provides very detailed images of conductance changes, with very fine features, because the quantum states are very sensitive to the potential landscape and thus to very small displacements of the tip.In my habilitation thesis, I first describe in details the SGM technique, then review the various experiments that we have done, and finally present a few research projects that are currently developed, or will be studied in the future, in particular on the Kondo effect in quantum dots and on the negative refraction in graphene

Scanning gate investigations of quantum transport phenomena

The manuscript describes my research activities on quantum transport in electronic devices using an experimental technique called Scanning Gate Microscopy (SGM). This technique provides spatial information on electron density and electron flow in buried two-dimensional electron gases (2DEG) or to modify these quantities at specific positions in the 2DEG. We investigate III/V semiconductor heterostructures, such as GaAs/AlGaAs or InGaAs/InAlAs, which host 2DEGs with high electron mobility. At low temperature, the elastic mean free path is several microns and the electron transport is fully ballistic in nano-scale devices patterned by electron beam lithography. Since the transport is also phase coherent and the electron wavelength is large, quantum effects are observable and can be studied in various nanostructures forming constrictions, rings, cavities, etc... These effects are usually probed by transport or optical spectroscopy which give mainly access to the energy levels. The spatial distribution of the quantum states, however, cannot be observed directly in these buried nanostructures, as opposed to surface electronic systems where scanning tunneling microscopy can measure the local density of states. The aim of the SGM technique is thus to probe indirectly this spatial distribution by recording the effect of a local electrostatic perturbation on the transport properties of the nanostructure. The sharp tip of an atomic force microscope is used as a local gate which is scanned above the surface. The voltage applied on the tip with respect to the 2DEG induces a local change of the electrostatic potential in the 2DEG, affecting the electron states, and thus the device conductance if these states are involved in the conduction path. With this technique, the dream would be to image directly the wave function in any kind of nanostructure, but the resolution is unfortunately limited by the relatively large distance between the tip and the 2DEG, which is buried several tens of nanometers below the surface. On the other hand, the SGM technique provides very detailed images of conductance changes, with very fine features, because the quantum states are very sensitive to the potential landscape and thus to very small displacements of the tip.In my habilitation thesis, I first describe in details the SGM technique, then review the various experiments that we have done, and finally present a few research projects that are currently developed, or will be studied in the future, in particular on the Kondo effect in quantum dots and on the negative refraction in graphene