Andreev spectroscopy of doped HgTe quantum wells

We investigate the Andreev reflection process in high-mobility HgTe/CdTe quantum wells. We find that Andreev conductance probes the dynamics of massive 2+1 Dirac fermions, and that both specular Andreev reflection and retroreflection can be realized even in presence of a large mismatch between the Fermi wavelengths at the two sides of the normal/superconducting junction.Comment: 7 pages, 6 figure

Long ranged singlet proximity effect in ferromagnetic nanowires

Recently a long ranged superconductor/ferromagnet (S/F) proximity effect has been reported in Co crystalline nanowires [1, Nature, 6 389 (2010)]. Since the authors of [1] take care to avoid the existence of magnetic domains, the triplet character of the long ranged proximity effect is improbable. Here we demonstrate that in the one-dimensional ballistic regime the standard singlet S/F proximity effect becomes long ranged. We provide an exact solution for the decay of the superconducting correlations near critical temperature ($T_{c}$) and for arbitrary impurities concentration. In particular we find a specific regime, between the diffusive and ballistic ones, where the decay length is simply the electronic mean-free path. Finally possible experiments which could permit to elucidate the nature of the observed long ranged proximity effect in Co nanowires are discussed.Comment: 4 page

Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions

We study the Josephson current in a junction comprising two superconductors linked by a strong ferromagnet in presence of impurities. We focus on a regime where the electron (and hole) motion is ballistic over the exchange length and diffusive on the scale of the weak link length. The current-phase relation is obtained for both two- and three dimensional ferromagnetic weak links. In the clean limit, the possibility of temperature-induced 0- transitions is demonstrated while the corresponding critical current versus temperature dependences are also studied.Comment: 10 pages, 7 figure

Klein tunneling in graphene: optics with massless electrons

This article provides a pedagogical review on Klein tunneling in graphene, i.e. the peculiar tunneling properties of two-dimensional massless Dirac electrons. We consider two simple situations in detail: a massless Dirac electron incident either on a potential step or on a potential barrier and use elementary quantum wave mechanics to obtain the transmission probability. We emphasize the connection to related phenomena in optics, such as the Snell-Descartes law of refraction, total internal reflection, Fabry-P\'erot resonances, negative refraction index materials (the so called meta-materials), etc. We also stress that Klein tunneling is not a genuine quantum tunneling effect as it does not necessarily involve passing through a classically forbidden region via evanescent waves. A crucial role in Klein tunneling is played by the conservation of (sublattice) pseudo-spin, which is discussed in detail. A major consequence is the absence of backscattering at normal incidence, of which we give a new shorten proof. The current experimental status is also thoroughly reviewed. The appendix contains the discussion of a one-dimensional toy model that clearly illustrates the difference in Klein tunneling between mono- and bi-layer graphene.Comment: short review article, 18 pages, 14 figures; v3: references added, several figures slightly modifie

Semiconductor Spintronics

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure

Interplay between superconductivity and spin-dependent fields in nanowire-based systems

The interplay between superconductivity, spin-orbit coupling, and Zeeman or exchange field, is studied theoretically in two different setups: a single wire in which all these fields coexist, and a double wire system in which superconducting pairing and the spin-dependent fields are spatially separated. We first explore a magnetoelectric effect, namely the appearance of anomalous charge supercurrents. We determine the conditions under which such currents are allowed by symmetry and express them in terms of the SU(2) electric and magnetic fields. In leading order in the strength of the fields we find that in the single wire setup such currents may appear only when the Zeeman field has both, a longitudinal and transverse component with respect to the spin-orbit field. In contrast, in the two wire setup a parallel component to the SOC can generate the anomalous current, which is allowed by symmetry. We confirm these findings by calculating explicitly the current in both setups together with the self-consistent superconducting order parameter. The latter shows in the ground-state a spatial modulation of the phase that leads to currents that compensate the anomalous current, such that in both cases the ground state corresponds to a total zero-current state. However, in the two wire setup this zero-current state consists of two finite currents flowing in each of the wires in opposite direction.Comment: 9 pages, 2 figure

Topological superconductivity in the one-dimensional interacting Creutz model

We consider one-dimensional topological insulators characterized by zero energy end states. In presence of proximity induced pairing, those end states can become Majorana states. We study here the fate of those various end states when Hubbard electron-electron repulsive interactions are added, using a combination of mean-field theory and density matrix renormalization group techniques.Transport électronique dans les isolants topologiquesEtats de Majorana et d'Andreev dans des circuits hybrides combinant des matériaux magnétiques et supraconducteur

Generation of a superconducting vortex via Néel skyrmions

We consider a type-II superconducting thin film in contact with a Néel skyrmion. The skyrmion induces spontaneous currents in the superconducting layer, which under the right condition generates a superconducting vortex in the absence of an external magnetic field. We compute the magnetic field and current distributions in the superconducting layer in the presence of Néel skyrmion.Control de courants supraconducteurs via des effets de spin et de champ: fondements pour une électronique non conventionnelleManipulation optique de quanta de flux individuels dans les supraconducteurs et applicationsNanoscale Coherent Hybrid Devices for Superconducting Quantum Technologie

Enhancing domain wall motion in magnetic wires by ion irradiation

The influence of low-energy He ion irradiation on the dynamics of a single Bloch domain wall was studied in magnetic wires based on Pt/Co/Pt trilayers exhibiting perpendicular anisotropy. The domain wall velocity is highly enhanced (up to three orders of magnitude) after irradiation at moderate fluence. A study in the thermally activated regime shows that this is consistent with a reduction of the density of pinning centers and of the pinning force. Uniform ion irradiation significantly improves domain wall motion, as required for future magnetic device