Transport properties of an electron-hole bilayer/superconductor hybrid junction

We investigate the transport properties of a junction consisting of an electron-hole bilayer in contact with normal and superconducting leads. The electron-hole bilayer is considered as a semi-metal with two electronic bands. We assume that in the region between the contacts the system hosts an exciton condensate described by a BCS-like model with a gap $\Gamma$ in the quasiparticle density of states. We first discuss how the subgap electronic transport through the junction is mainly governed by the interplay between two kinds of reflection processes at the interfaces: The standard Andreev reflection at the interface between the superconductor and the exciton condensate, and a coherent crossed reflection at the semi-metal/exciton-condensate interface that converts electrons from one layer into the other. We show that the differential conductance of the junction shows a minimum at voltages of the order of $\Gamma/e$. Such a minimum can be seen as a direct hallmark of the existence of the gapped excitonic state

Local density of states in superconductor-strong ferromagnet structures

We study the dependence of the local density of states (LDOS) on coordinates for a superconductor-ferromagnet (S/F) bilayer and a S/F/S structure assuming that the exchange energy h in the ferromagnet is sufficiently large: $>>1,$ where $\tau$ is the elastic relaxation time. This limit cannot be described by the Usadel equation and we solve the more general Eilenberger equation. We demonstrate that, in the main approximation in the parameter $(h\tau)^{-1}$, the proximity effect does not lead to a modification of the LDOS in the S/F system and a non-trivial dependence on coordinates shows up in next orders in $(h\tau) ^{-1}.$ In the S/F/S sandwich the correction to the LDOS is nonzero in the main approximation and depends on the phase difference between the superconductors. We also calculate the superconducting critical temperature $T_{c}$ for the bilayered system and show that it does not depend on the exchange energy of the ferromagnet in the limit of large h and a thick F layer.Comment: 9 pages, 5 figure

Quantum Spin Fluctuations as a Source of Long-Range Proximity Effects in Diffusive Ferromagnet-Superconductor Structures

We show that quantum spin fluctuations in inhomogeneous ferromagnets drastically affect the Andreev reflection of electrons and holes at a ferromagnet-superconductor interface. As a result a strong long-range proximity effect appears, associated with electron-hole spin triplet correlations and persisting on a lenght scale typical for non-magnetic materials, but anomalously large for ferromagnets.Comment: 4 pages, 2 figure

Feasibility of study magnetic proximity effects in bilayer "superconductor/ferromagnet" using waveguide-enhanced Polarized Neutron Reflectometry

A resonant enhancement of the neutron standing waves is proposed to use in order to increase the magnetic neutron scattering from a "superconductor/ferromagnet"(S/F) bilayer. The model calculations show that usage of this effect allows to increase the magnetic scattering intensity by factor of hundreds. Aspects related to the growth procedure (order of deposition, roughness of the layers etc) as well as experimental conditions (resolution, polarization of the neutron beam, background etc) are also discussed. Collected experimental data for the S/F heterostructure Cu(32nm)/V(40nm)/Fe(1nm)/MgO confirmed the presence of a resonant 60-fold amplification of the magnetic scattering.Comment: The manuscript of the article submitted to Crysstalography Reports. 23 pages, 5 figure

Full Scale Proton Beam Impact Testing of new CERN Collimators and Validation of a Numerical Approach for Future Operation

New collimators are being produced at CERN in the framework of a large particle accelerator upgrade project to protect beam lines against stray particles. Their movable jaws hold low density absorbers with tight geometric requirements, while being able to withstand direct proton beam impacts. Such events induce considerable thermo-mechanical loads, leading to complex structural responses, which make the numerical analysis challenging. Hence, an experiment has been developed to validate the jaw design under representative conditions and to acquire online results to enhance the numerical models. Two jaws have been impacted by high-intensity proton beams in a dedicated facility at CERN and have recreated the worst possible scenario in future operation. The analysis of online results coupled to post-irradiation examinations have demonstrated that the jaw response remains in the elastic domain. However, they have also highlighted how sensitive the jaw geometry is to its mounting support inside the collimator. Proton beam impacts, as well as handling activities, may alter the jaw flatness tolerance value by $\pm$ 70 ${\mu}$m, whereas the flatness tolerance requirement is 200 ${\mu}$m. In spite of having validated the jaw design for this application, the study points out numerical limitations caused by the difficulties in describing complex geometries and boundary conditions with such unprecedented requirements.Comment: 22 pages, 17 figures, Prepared for submission to JINS

Gap inversion in one-dimensional Andreev crystals

We study a periodic arrangement of magnetic regions in a one-dimensional superconducting wire. Due to the local exchange field, each region supports Andreev bound states that hybridize forming Bloch bands in the subgap spectrum of what we call the Andreev crystal (AC). As an illustration, ACs with ferromagnetic and antiferromagnetic alignment of the magnetic regions are considered. We relate the spectral asymmetry index of a spin-resolved Hamiltonian to the spin polarization and identify it as the observable that quantifies the closing and reopening of the excitation gap. In particular, antiferromagnetic ACs exhibit a sequence of gapped phases separated by gapless Dirac phase boundaries. Heterojunctions between antiferromagnetic ACs in neighboring phases support spin-polarized bound states at the interface. In a close analogy to the charge fractionalization in Dirac systems with a mass inversion, we find a fractionalization of the interface spin.Comment: 6 pages, 4 figure

Self-consistent microscopic calculations for non-local transport through nanoscale superconductors

We implement self-consistent microscopic calculations in order to describe out-of-equilibrium non-local transport in normal metal-superconductor-normal metal hybrid structures in the presence of a magnetic field and for arbitrary interface transparencies. A four terminal setup simulating usual experimental situations is described by means of a tight-binding model. We present results for the self-consistent order parameter and current profiles within the sample. These profiles illustrate a crossover from a quasi-equilibrium to a strong non-equilibrium situation when increasing the interface transparencies and the applied voltages. We analyze in detail the behavior of the non-local conductance in these two different regimes. While in quasi-equilibrium conditions this can be expressed as the difference between elastic cotunneling and crossed Andreev transmission coefficients, in a general situation additional contributions due to the voltage dependence of the self-consistent order parameter have to be taken into account. The present results provide a first step towards a self-consistent theory of non-local transport including non-equilibrium effects and describe qualitatively a recent experiment [Phys. Rev. Lett. 97, 237003 (2006)].Comment: 12 pages, 14 figures, 2 figures correcte

Supercurrent and Andreev bound state dynamics in superconducting quantum point contacts under microwave irradiation

We present here an extensive theoretical analysis of the supercurrent of a superconducting point contact of arbitrary transparency in the presence of a microwave field. Our study is mainly based on two different approaches: a two-level model that describes the dynamics of the Andreev bound states in these systems and a fully microscopic method based on the Keldysh-Green function technique. This combination provides both a deep insight into the physics of irradiated Josephson junctions and quantitative predictions for arbitrary range of parameters. The main predictions of our analysis are: (i) for weak fields and low temperatures, the microwaves can induce transitions between the Andreev states leading to a large suppression of the supercurrent at certain values of the phase, (ii) at strong fields, the current-phase relation is strongly distorted and the corresponding critical current does not follow a simple Bessel-function-like behavior, and (iii) at finite temperature, the microwave field can enhance the critical current by means of transitions connecting the continuum of states outside the gap region and the Andreev states inside the gap. Our study is of relevance for a large variety of superconducting weak links as well as for the proposals of using the Andreev bound states of a point contact for quantum computing applications.Comment: 16 pages, 11 figures, submitted to Phys. Rev.