Superconductivity and ferromagnetism are competing long-range orders, which favor antiparallel or parallel alignment of the electron spin, respectively. In hybrid structures, their competition leads to many effects with potential applications in spintronics and quantum information processing.
 Schematic view of crossed Andreev reflection |
Crossed Andreev Reflection We concentrate on the investigation of structures where several ferromagnetic leads make contact to one superconductor. In these structures, crossed Andreev reflection (CAR) occurs, a process where an electron entering the superconductor from one of the leads gets reflected into another as a hole, creating a Cooper pair in the superconductor (see scheme). |
 Scanning electron image of an aluminum-copper hybrid structure |
The metallic structures we investigate are fabricated on an oxidized silicon substrate by e-beam lithography and shadow evaporation in a UHV system. Thereby, contact distances of a few hundred nanometers are realized. We use aluminum, which is superconducting at temperatures below 1.2 K and has a large coherence length. The normal metal leads are made of non-magnetic copper or ferromagnetic iron. When using ferromagnetic electrodes, their magnetization can be switched by an externally applied magnetic field to be either parallel or antiparallel, thereby acting as spin filters. According to the singlet structure of the Cooper pairs in aluminum, CAR is either favored (antiparallel) or suppressed (parallel). |
 Schematic view of Andreev bound states at spin-active interfaces |
Spin-active Interfaces At interfaces between superconductors and ferromagnets spin-dependent phase shifts of the electron wave functions may be present. These lead to spin-triplet superconductivity and Andreev bound states. The latter can be observed as resonances in the conductance of superconductor-ferromagnet nanocontacts. |
Publications
- M. J. Wolf, F. Hübler, S. Kolenda, H. v. Löhneysen, and D. Beckmann
Spin injection from a normal metal into a mesoscopic superconductor,
Phys. Rev. B 87, 024517 (2013) - F. Hübler, M. J. Wolf, D. Beckmann, and H. v. Löhneysen
Long-Range Spin-Polarized Quasiparticle Transport in Mesoscopic Al Superconductors with a Zeeman Splitting,
Phys. Rev. Lett. 109, 207001 (2012) - T.E. Golikova, F. Hübler, D. Beckmann, N.V. Klenov, S.V. Bakurskiy, M.Yu. Kupriyanov, I.E. Batov, V.V. Ryazanov,
Critical current in planar SNS Josephson junctions,
JETP Lett. 96, 668 (2012) - T. E. Golikova, F. Hübler, D. Beckmann, I. E. Batov, T. Yu. Karminskaya, M. Yu. Kupriyanov, A. A. Golubov, and V. V. Ryazanov
Double proximity effect in hybrid planar superconductor-(normal metal/ferromagnet)-superconductor structures,
Phys. Rev. B 86, 064416 (2012) - F. Hübler, M. J. Wolf, T. Scherer, D. Wang, D. Beckmann, and H. v. Löhneysen
Observation of Andreev Bound States at Spin-active Interfaces,
Phys. Rev. Lett. 109, 087004 (2012) - F. Hübler, J. Camirand Lemyre, D. Beckmann and H. v. Löhneysen,
Charge imbalance in superconductors in the low-temperature limit,
Phys. Rev. B 81, 184524 (2010) - J. Brauer, F. Hübler, M. Smetanin, D. Beckmann and H. v. Löhneysen,
Nonlocal transport in normal-metal/superconductor hybrid structures: Role of interference and interaction,
Phys. Rev. B 81, 024515 (2010) - D. Beckmann and H. v. Löhneysen,
Negative four-terminal resistance as a probe of crossed Andreev reflection,
Appl. Phys. A 89, 603 (2007) - D. Beckmann, H. B. Weber and H. v. Löhneysen,
Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures,
Phys. Rev. Lett. 93, 197003 (2004)
