Session H1: Charge and Spin Transport in Josephson and Proximity Devices

Non-sinusoidal current-phase relations in SFS pi-Josephson junctions

We report the direct observation of a sin(2$\phi )$ component in the current-phase relation (CPR) of Superconductor-Ferromagnet-Superconductor (SFS) Josephson junctions. The deviation from a sinusoidal CPR is most evident near the crossover between the 0-junction to $\pi $-junction states reached by tuning the thickness of the ferromagnet barrier and the temperature. We measure the CPR in Nb-CuNi-Nb junctions using a phase-sensitive Josephson interferometer technique in which the junctions are incorporated into a superconducting loop coupled to a dc SQUID. We correlate the CPR data with measurements of subharmonic Shapiro steps and anomalous critical current diffraction patterns that have previously been cited as evidence for higher-order Josephson tunneling components. We will discuss possible origins and implications for the non-sinusoidal component. In collaboration with M.J.A. Stoutimore (University of Illinois at Urbana-Champaign) and A.Yu. Rusanov, V.A. Oboznov, V.V. Bolginov, A.N. Rossolenko, and V.V. Ryazanov (Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia).   

Crossed Cooper Pair Transmission and Pure Spin Supercurrents through Strongly Spin-polarized Ferromagnets

Interfaces between solids with different ordering phenomena have become a focus of research in recent years. One reason is that new and unexpected phases that are not stable in either of the adjacent materials can appear in the interface regions. The mechanism for creating such phases is due to induced symmetry breaking, as opposed to spontaneous symmetry breaking in the bulk materials. As a prominent example I discuss interface-induced exotic superconductivity in heterostructures composed of conventional singlet superconductors and strongly spin-polarized ferromagnets. I present new intriguing effects, such as a tunable pure spin-supercurrent in a strongly spin-polarized ferromagnet contacted with only one superconducting electrode, and a difference in the critical currents for positive and negative bias in a high transmission ferromagnetic Josephson junction [1]. The latter, rather surprising effect has a physical explanation in terms of a new ``crossed Cooper pair transmission'' process. In this process two singlet Cooper pairs are coherently decomposed into two equal-spin triplet pairs, which are respectively transmitted via different spin bands in the ferromagnet, after which they again recombine into two singlet pairs. This effect is analogous to the well-known crossed Andreev reflection process, which however is strongly suppressed in this particular case. Furthermore, I discuss how the manipulation of interface spins can be used to pump triplet pairs. This opens an avenue for new types of superconducting quantum devices and new ways to test properties of exotic superconducting phases in experiment. \par \medskip \noindent [1] R. Grein, M. Eschrig, G. Metalidis, and G. Sch\"on, Phys. Rev. Lett. {\bf 102}, 227005 (2009).   

Spin-transport and spin-transfer torque in SF nanostructures

The electronic spin degree of freedom develops interesting dynamics in Superconductor-Ferromagnet nanostructures driven out of equilibrium. Their unique spin transport properties may be exploited to electrically control the state of nanomagnets with the help of superconductors. We present an anatomy of microscopic scattering events (Andreev reflections, spin filtering, spin mixing etc.) in SFS and SFNFS junctions, and elucidate their roles in determining the spin and charge supercurrent in equilibrium, as well as the nonequilibrium spin current and spin- transfer torque under a bias voltage. In particular, we will focus on the nonlinear voltage dependence of spin current in SFS junctions, and the appearance of a new component of the spin-transfer torque that is perpendicular to the plane spanned by the two ferromagnetic moments in SFNFS structures. The latest theoretical developments that made these calculations tractable will be outlined. E. Zhao and J. A. Sauls, Phys. Rev. B 78 174511 (2008); Phys. Rev. Lett. 98 206601 (2007).   

Cooper Pair Mediated Coherence Between Normal Metals

Two electrons bound in a singlet state have long provided a conceptual and pedagogical framework for understanding the nonlocal nature of entangled quantum objects. As bound singlet electrons separated by a superconducting coherence length of up to several hundred nanometers occur naturally in conventional BCS superconductors in the form of Cooper pairs, recent investigations have focused on whether electrons in spatially separated probes placed within a coherence length of each other on a superconductor can be quantum mechanically coupled by the singlet pairs. We present experimental evidence for this phase-coherent, nonlocal coupling between electrons in normal metals probes linked by a superconductor. By embedding one normal metal probe in a hybrid normal-superconducting loop, the phase of the electrons in this probe can be tuned by an externally applied magnetic flux. Under appropriate non-equilibrium conditions the change in phase results in a corresponding change in voltage on the embedded normal metal probe. This phase-dependent change can also be observed on a second normal probe, even though this probe is not embedded in any loop or intersecting any current path, but rather is coherently coupled to the first probe through a superconducting section shorter than the coherence length.   

Tunable non-local entanglement of electrons probed by noise cross-correlation measurement

Nonlocal entanglement is crucial for quantum information processes. While nonlocal entanglement has been realized for photons, it is much more difficult to demonstrate for electrons. One approach that has been proposed is to use hybrid superconducting/normal-metal devices. When the distance between two normal-metal electrodes connected to a superconductor is comparable to the superconducting coherence length, theory predicts that two electrons in the normal-metal electrodes with opposite spin are entangled by Cooper pairs, leading to non-local entanglement of electrons. Such entanglement can be understood by a non-local process called crossed Andreev reflection (CAR), in which a Cooper pair splits into two coherent electrons with one in each normal-metal electrode, generating instantaneous current of the same sign, and inducing a positive current correlation. Experimentally, CAR is indicated by a negative non-local resistance. However, another non-local process, elastic cotunneling (EC), in which one electron tunnels through the superconductor from one normal-metal electrode to the other, contributes to a positive non-local resistance that cancels the contribution due to CAR, preventing us from measuring and control of the CAR component. Fortunately, EC leads to a negative current correlation with bias dependence different from that of CAR. Thus, noise correlation measurement is expected to be able to distinguish these two non-local processes. By cross-correlation measurements as well as measurements of the local and nonlocal resistance, we present here experimental evidence showing that by independently controlling the energy of electrons at the superconductor/normal-metal interfaces, nonlocal Andreev reflection, the signature of spin-entanglement, can be maximized, qualitatively in agreement with theoretical predication.