We study heterostructures of singlet superconductors and strongly spin-polarized ferromagnets and show that a relative phase arises between the superconducting proximity amplitudes in the two ferromagnetic spin bands. We find a tunable pure spin supercurrent in a spin-polarized ferromagnet contacted with only one superconductor electrode. We show that Josephson junctions are most effective for a spin polarization P approximately 0.3, and that critical currents for positive and negative bias differ for a high transmission Josephson junction, due to a relative phase between single and double pair transmission.

In a recent series of scanning probe experiments, it became possible to visualize local electron flow in a two-dimensional electron gas. In this paper, a Green's function technique is presented that enables efficient calculation of the quantity measured in such experiments. Efficient means that the computational effort scales like M 3 N (M is the width of the tight-binding lattice used, and N is its length), which is a factor M N better than the standard recursive technique for the same problem. Moreover, within our numerical framework it is also possible to calculate (with the same computational effort M 3 N ) the local density of states, the electron density, and the current distribution in the sample, which are not accessible with the standard recursive method. Furthermore, an imaging method is discussed where the scanning tip can be used to measure the local chemical potential. The numerical technique is used to study electron flow through a quantum point contact. All features seen in experiments on this system are reproduced and a new interference effect is observed resulting from the crossing of coherent beams of electron flow.

No abstract

We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors(SCs) and strongly spin-polarized ferromagnets(FMs) using recently derived boundary conditions for the Quasiclassical Theory of Superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin-filtering and spin-mixing and is non-perturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the possible magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized sub-gap Andreev bound states in SC/sFM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach.

Recently, a chirality-driven contribution to the anomalous Hall effect has been found that is induced by the Berry phase and does not directly involve spin-orbit coupling. In this paper, we will investigate this effect numerically in a two-dimensional electron gas with a simple magnetic texture model. Both the adiabatic and non-adiabatic regimes are studied, including the effect of disorder. By studying the transition between both regimes the discussion about the correct adiabaticity criterium in the diffusive limit is clarified.

Quantum spin Hall insulator/metal interfaces are formed in graphene ribbons with intrinsic spinorbit coupling by selectively doping two regions creating a potential step. For a clean graphene ribbon, the transmission of the topological edge states through a n-n or p-p junction is perfect irrespective of the ribbon termination, width, and potential step parameters due to the orthogonality of incoming and outgoing edge channels. This is shown numerically for an arbitrary crystallographic orientation of the ribbon and proven analytically for zigzag and metallic armchair boundary conditions. In disordered ribbons, the orthogonality between left-and right-movers is in general destroyed and backscattering sets in. However, transmission approaches one by increasing the ribbon's width, even in the presence of strong edge roughness.

We study the transition from ballistic to diffusive and localized transport in graphene nanoribbons in the presence of binary disorder, which can be generated by chemical adsorbates or substitutional doping. We show that the interplay between the induced average doping ͑arising from the nonzero average of the disorder͒ and impurity scattering modifies the traditional picture of phase-coherent transport. Close to the Dirac point, intrinsic evanescent modes produced by the impurities dominate transport at short lengths giving rise to a regime analogous to pseudodiffusive transport in clean graphene, but without the requirement of heavily doped contacts. This intrinsic pseudodiffusive regime precedes the traditional ballistic, diffusive, and localized regimes. The last two regimes exhibit a strongly modified effective number of propagating modes and a mean free path which becomes anomalously large close to the Dirac point.

The Casimir effect results from alterations of the zero-point electromagnetic energy introduced by boundary-conditions. For ferromagnetic layers separated by vacuum (or a dielectric) such boundaryconditions are influenced by the magneto-optical Kerr effect. We will show that this gives rise to a long-range magnetic interaction and discuss the effect for two different configurations (magnetization parallel and perpendicular to the layers). Analytical expressions are derived for two models and compared to numerical calculations. Numerical calculations of the effect for Fe are also presented and the possibility of an experimental observation of the Casimir magnetic interaction is discussed.