Since the ferromagnetic side of a superconductor-ferromagnet junction is spin polarized, Andreev reflections are suppressed. Consequently, the induced superconductor order parameter in the ferromagnet is expected to decay rapidly, on the order of a few nm. Our scanning tunneling spectroscopy measurements on thin epitaxial (100)YBa2Cu3O7-δ-SrRuO3 (YBCO-SRO) bilayers, where SRO is a ferromagnet, indeed show that on most of the junction area the superconductor order parameter vanishes in the SRO over a distance less than 8 nm. However, we find localized regions, arranged along narrow (<10 nm) stripes, where the order parameter (superconductor-like gap structure) penetrates the ferromagnet more than 20 nm. This is attributed to "crossed Andreev reflections", taking place at domain boundaries, where an electron from one magnetic domain is retro reflected as a hole with opposite spin in an adjacent domain. This phenomenon, directly observed here for the first time, may account for the (not abundant) cases where a long-range proximity effect was found in superconductor-ferromagnet proximity systems.
Magnetic properties of SFS and SF ramp-type junctions with YBa2Cu3O7-δ (YBCO) electrodes (S), and the itinerant ferromagnet SrRuO3 (SRO - F), were investigated. We looked for a crossed Andreev reflection effect (CARE) in which an electron from one magnetic domain in F is Andreev reflected as a hole into an adjacent, oppositely polarized, domain while a pair is transmitted into S. CARE is possible in SRO since the width of its domain walls is of the order of the YBCO coherence length (2-3 nm). Our junctions behave as typical magnetic tunneling junctions, as the conductance spectra were always asymmetric, and a few showed bound state peaks at finite bias which shifted with field according to the classical Tedrow and Meservey theory. In many of our SFS junctions with a barrier thickness of 10-20 nm, a prominent zero bias conductance peak (ZBCP) has been observed. This peak was found to decrease linearly with magnetic field, as expected for Andreev and CARE scattering. In contrast, in SF junctions, the observed ZBCP was found to decrease versus field almost exponentially, in agreement with the Anderson-Appelbaum theory of scattering by magnetic states in F. Thus, transport in our SFS and SF junctions depends strongly on the size of the F layer. We also found that in both cases, the ZBCP height at zero field decreased with increasing magnetic order of the domains in F, in agreement with the CARE mechanism.
KEYWORDS: Interfaces, Quasiparticles, Superconductors, Diffusion, Superconductivity, Solids, Physics, Temperature metrology, Information operations, Thin films
The coupling between a HTSC thin film (S) and a normal conductor (N) in proximity is still not well understood. Most of the investigations done so far look at the penetration of superconductivity into N. We have extended this type of investigation to look at the other side of the coin, namely at the influence of N on S. Here, we report measurements of the critical temperature of S-N bilayer films as a function of the thickness of the YBCO (S) layer using Co doped YBCO as the Normal material. To understand the role of the S-N interface, bilayer having different interface morphologies were prepared using different growth modes of the films. We found that depending on the morphology of the S-N interface, the coupling between S and N layers can be turned on to depress the Tc of S by tens of degrees, or turned off so the layers appear almost totally decoupled. This novel effect is strongly correlated with the presence of different crystalline orientations on the interface. The range of influence of N on S is about 240 angstrom, rather than 20 angstrom expected from the coherence length (xi) s. These observations are explained using a theory of quasiparticle transmission into an anisotropic superconductor.
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