STM to probe Odd-Triplet contributions to the Long-Ranged Proximity Effect

In conventional superconductors electrons are bound in singlet Cooper pairs, i.e. with opposite spin. More recently experiments on superconductor-ferromagnet-systems have shown Cooper pairs tunneling through ferromagnetic layers, indicating Cooper pairs of equal spin, thus corresponding to a long-range triplet proximity effect [1]. Most experimental evidence for triplet superconductivity comes from observations of the thickness dependence of the Josephson current through a ferromagnetic barrier, and there now is an increasing amount of direct spectroscopic evidence (e.g. our results measured on niobium/holmium multilayers [2]) to test the existing theoretical models [3].

This project aims to analyze the DOS of thin films of the ferromagnetic insulator europium sulfide on superconducting aluminum or vice versa, using a scanning tunneling microscope in spectroscopy mode at 280 mK and in varying magnetic fields. We observe significant broadening of the superconducting energy gap and a variety of sub-gap structures induced by the presence of the ferromagnet. We interpret our findings based on the diffusive theory [4,5] and a more advanced circuit theory model [4].

1. F. S. Bergeret, Phys. Rev. Lett. 86, 4096 (2001)
2. A. Di Bernardo, Nat. Comm. 6:8053 (2015)
3. F. Hübler, Phys. Rev. Lett. 109, 87004 (2012)
4. P. Machon, Phys. Rev. Lett. 110, 047002 (2013)
5. J. Linder, Phys. Rev. B 81, 214504 (2010)

Publications

1. A. Di Bernardo, S. Diesch, Y. Gu, J. Linder, G. Divitini, C. Ducati, E. Scheer, M.G. Blamire, J.W.A. Robinson, Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces, Nature Communications 6  (2015).                                       URL: www.nature.com/articles/ncomms9053.
2. S. Diesch, P. Machon, M. Wolz, C. Sürgers, D. Beckmann, W. Belzig and E. Scheer, Creation of equal-spin triplet superconductivity at the Al/EuS interface, In: Nature Communications 9 (2018)