Evidence for crossed Andreev reflections in bilayers of (100)YBa2Cu3O7−δand the itinerant ferromagnet

Abstract

Scanning tunneling spectroscopy measurements on thin epitaxial SrRuO3/(100)Y Ba2Cu3O 7−δ ferromagnet/superconductor bilayers, reveal localized regions in which the superconductor order parameter penetrates the ferromagnet to more than 26 nm, an order of magnitude larger than the coherence length in the ferromagnetic layer. These regions consist of narrow (< 10 nm) and long strips, separated by at least 200 nm, consistent with the known magnetic domain wall structure in SrRuO3. We attributed this behavior to Crossed Andreev Reflections, taking place in the vicinity of the magnetic domain walls. PACS numbers: 74.45.+c, 74.50.+r, 74.87.Bz, In spite of a considerable research effort in the past years, a comprehensive understanding of the proximity effect (PE) in superconductor (S) ferromagnet (F) heterostructures has not yet been established. Such systems are of interest since they allow a direct investigation of the interplay between the two competing orders of superconductivity and magnetism. In an N/S proximity system, where N is a normal metal in good electrical contact with S, superconducting correlations are induced in N over a length scale of the normal coherence length, ξ N , while they are weakened in the S side over a scale of the superconducting coherence length, ξ S 1 . The mechanism underlying the PE at S/N interfaces is the Andreev Reflection (AR). Upon impinging on the interface from the N side, hole-like quasiparticles are retro-reflected as electron-like quasiparticles with inverse spin (maintaining phase coherence over ξ N = hD/k B T where D is the diffusion coefficient), while destroying Cooper pairs in the S side. Consequently, the PE is expected to be significantly suppressed when the N side is replaced by a ferromagnet due to spin polarization 2 . Theoretical works based on the Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) mechanism 3,4 , predict a rapid and nonmonotonic decay of the superconducting order parameter (OP) in F, of the form sin(x/ξ F )/(x/ξ F ) in the clean limit and exp(−x/ξ F )cos(x/ξ F ) in the dirty limit (where x is the distance from the interface) 5,6 . The corresponding coherence lengths in F where the exchange energy is E ex , are ξ F =hv F /2E ex (clean limit) and ξ F = (hD/2E ex ) (dirty limit), which are typically of the order of a few nm, much shorter than ξ N . For certain thicknesses of the F layer a 'π-state' may appear, in which the induced OP in F reverses its sign 5,6 . Many studies confirmed these predictions and clearly demonstrated damped OP oscillations in F and a corresponding dependence on the F thickness of the critical current in SFS junctions 7,8 . All of these effects occur on a length scale of a few nm, in agreement with estimates for ξ F . However, other experiments show a long range PE where the penetration depth of the induced order parameter in F is two orders of magnitude larger than ξ F 9,10 . The predictions concerning the S/F proximity systems result from the singlet pairing in S, and are independent of the symmetry (s-wave or d -wave) of the o...

Ferromagnetic SrRuO3 thin-film deposition on a spin-triplet superconductor Sr2RuO4 with a highly conducting interface

“…21,22 In another report, superconducting gap in a YBCO/SRO113(18-nm) bilayer persists only at the ferromagnetic domain-wall regions. 23 Our findings of highly conducting interface between SRO113 and SRO214 open up a possibility to develop high-quality FM/TSC junctions to explore the unconventional proximity effects.…”

Long-range proximity effect in La2/3Ca1/3MnOKalcheim1, Kirzhner2, Koren3 et al. 2011Phys. Rev. BSelf Cite

“…[13]. We note that the locations of such lines showed no correlation with any specific topographic feature.…”

“…Surprisingly, our tunneling spectra revealed spectroscopic features conforming to proximity-induced superconductivity even on LCMO layers as thick as 30 nm, an order of magnitude larger than the FFLO predicted ξ F . Moreover, these features were not confined only to narrow strips (as in our previous study [13] of SRO/YBCO bilayers) and both gaps and zero bias conductance peaks (ZBCP) were measured. Our data suggest, as discussed below, that proximity-induced triplet-pairing in LCMO plays an important role in the PE observed in our samples.…”

“…Subsequently, STM measurements [13] suggested that this long range proximity effect (LRPE) may take place only locally, along the domain walls (DWs) of the SRO. The tunneling dI/dV vs. V spectra revealed proximity superconducting gaps on the surface of SRO layers much thicker than the expected ξ F , but these were confined to strips of width consistent with the DW size of SRO.…”

Long-range crossed Andreev reflections in high-temperature superconductors

“…This anisotropy is behind the non-local nature of the electromagnetic response of HTcS [24,25]. In fact, some indirect evidence of CAR processes in HTcS coupled to ferromagnetic leads has been presented [26,27].…”