We study spin transport in a superconducting nanowire using a set of closely spaced magnetic tunnel contacts. We observe a giant enhancement of the spin accumulation of up to 5 orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements, combined with our theoretical model, allow us to distinguish the individual spin-flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.

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. Dephasing is taken into account, and the characteristic time scale 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 experimental determination of 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.

We employ the Keldysh formalism in the quasiclassical approximation to study transport in a diffusive superconductor. The resulting 4 ϫ 4 transport equations describe the flow of charge and energy as well as the corresponding flow of spin and spin energy. Spin-flip scattering due to magnetic impurities is included. We find that the spin-flip length is renormalized in the superconducting case and propose an experimental system to measure the spin accumulation in a superconductor.

The injection of pure spin current into superconductors by the dynamics of a ferromagnetic contact is studied theoretically. Taking into account suppression of the order parameter at the interfaces (inverse proximity effect) and the energy-dependence of spin-flip scattering, we determine the temperature-dependent ferromagnetic resonance linewidth broadening. Our results agree with recent experiments in Nb|permalloy bilayers [C. Bell et al., arXiv:cond-mat/0702461].PACS numbers: 74.25. Fy, 74.78.Na, Cooper pairs in conventional superconductors are spinsinglet states and therefore cannot carry a spin current. Some aspects of the resilience of the superconducting state against spin-current injection have been experimentally demonstrated in hybrid ferromagnetsuperconductor spin valves [1], switches [2], and π-junctions [3]. In these experiments, the spin current flow in the superconducting state can only be inferred via charge current measurements. This complicates the understanding of the spin current flow in superconductors.Injection of a pure spin current into a superconductor has recently been demonstrated by Bell et al. [4] in ferromagnet|superconductor structures under ferromagnetic resonance (FMR) conditions, in which the precessing magnetization acts as a "spin pump" [5]. The spin angular momentum lost by the ferromagnet can be observed directly in terms of an increased broadening of the FMR spectrum. In this Letter we demonstrate theoretically that the spin transport thus measured as a function of temperature and device/material parameters offers direct insight into spin-flip relaxation and the inverse proximity effect in superconductors. Our theory agrees well with the recent experimental results [4], and we provide suggestions and predictions for future experiments.The theoretical challenge of spin-pumping into superconductors as compared to normal conductors is the strong energy dependence of quasiparticle transport properties around the superconducting energy gap [6]. Also, the energy dependent spin-flip scattering rates caused by spin-orbit coupling or magnetic impurities differ. Experiments that directly probe spin transport, such as Ref. 4, therefore provide unique information about the spin-flip scattering mechanism. A complicating factor is the inverse proximity effect [7] that suppresses the superconducting order parameter close to a metallic interface with ferromagnets like Ni, Co, and Fe. The resulting spatial dependence of the superconducting gap requires solution of the full transport equations in the entire superconducting layer. The spin currents measured at such interfaces therefore serve as probes of superconducting correlations in magnetic heterostructures, and the temperature dependence of the FMR linewidth near and below the critical temperature can provide a wealth of information about spin-flip processes and superconducting proximity physics, with potential implications for different areas of mesoscopic physics.The ferromagnet|superconductor|spin reservoir (F|S|R) structure. Precessi...