1. Superconducting Tunneling Through Spin Filter Barriers Mark Blamire, Avradeep Pal, Jason Robinson, Zoe Barber Cambridge University Department of Materials Science Device Materials Group

2. Non-magnetic tunnel barrier – spin transport determined by electrodes only – Magnetic electrodes => TMR – Non-magnetic electrodes => no spin polarised currents Superconducting state – no spin polarised currents Ferromagnetic Insulator tunnel barrier – Barrier height spin-split below the Curie temperature – Tunneling probability for one spin sign exponentially favoured – Almost 100% spin polarised current Tunnel barriers & spin filters E ex

3. GdN films A magnetic semiconductor / insulator Reactive sputtering from Nb & Gd targets Magnetic properties depend strongly on N stoichimetry Very sensitive to oxygen levels during growth Senapati et al., PRB 83, 014403 (2011).

4. Spin-filter Josephson junctions

5. Non-Ambegaokar-Baratoff temperature dependence

6. NbN/GdN/NbN junctions

8. 2 nd harmonic current-phase relation

9. I c (H) for different harmonics IcIc φ/φ 0 12 I=I 0 sinϕ 3 I=I 0 sin2ϕ H

10. NbN/GdN/NbN junctions Hysteretic I c (H) because of barrier magnetism SUST 26, 055017 (2013)

11. Extracting M(H) loop Add a hypothetical M(H) to H and calculate I c (H). Adjust M(H) parameters until fit optimised Distortion of first lobe (H 2+ ) minimal because GdN has large remanent moment H 2+ H 2- 2H 1

12. Effect of GdN thickness H 2- tends to zero as barrier moment increases H 1 and H 2+ halve once barrier magnetism established Eventually peak height ratio recovers to perfect Fraunhofer pattern Dominant 2 nd harmonic in current-phase relation

13. Models of spin filter Josephson junctions Symmetric spin mixers – angles α, β

14. Fit to data

15. Triplet pair generation by single spin-mixer? Singlet Triplet

16. fits

17. Tafuri Group data

18. So, what do we know? Spin filter Josephson junctions show reduced I c R n & non- Ambegaokar-Baratoff temperature dependence. ✓ Dominant 2 nd harmonic current-phase relationship. ✓ Asymmetric triplet pair formation requires asymmetric interfaces. Some evidence….. Fabricated S/FI/N junctions to explore single NbN/GdN interface

19. NbN/GdN/TiN non-superconducting

20. Zeeman-split S/I/N tunneling

21. Field-dependence offset Intrinsic Zeeman-splitting at NbN/GdN interface

22. Real device structure NbN ξ NbN ~ 5 nm Zeeman GdN NbN 3 nm

23. Zero-bias conductance peak

24. So, what do we know? Spin filter Josephson junctions show reduced I c R n & non- Ambegaokar-Baratoff temperature dependence. ✓ Dominant 2 nd harmonic current-phase relationship. ✓ Zeeman-split NbN/GdN interface. ✓ Evidence for some strongly unconventional superconductivity. ✓