University of Minnesota
School of Physics & Astronomy

Condensed Matter Seminar

Wednesday, November 18th 2015
1:25 pm:
Speaker: Norman O. Birge, Michigan State University
Subject: Spin-triplet supercurrents in ferromagnetic Josephson junctions

When a superconductor (S) and a ferromagnet (F) are put into contact with each other, the combined S/F hybrid system exhibits altogether new properties. There is a proximity effect where electron pair correlations from S penetrate into F, but the pair correlations oscillate rapidly and decay over a very short distance due to the large exchange splitting between the spin-up and spin-down electron bands in F. In the presence of non-collinear magnetization, Bergeret et al. predicted that spin-triplet pair correlations are generated, which are immune to the exchange field and hence persist over much longer distances in F [1]. Furthermore, these triplet pair correlations satisfy the Pauli Exclusion Principle in a new and strange way: they are odd in frequency or time. Several groups have now observed convincing evidence for such spin-triplet correlations in a variety of S/F and S/F/S systems. Our approach is based on measuring the supercurrent in Josephson junctions of the form S/F’/F/F’’/S, with non-collinear magnetizations in adjacent ferromagnetic layers [2]. I will discuss our latest results toward controlling the supercurrent in these junctions [3], as well as how various types of ferromagnetic junctions could be used as memory elements in a fully superconducting random-access memory [4].

Work supported by the US DOE under grant DE-FG02-06ER46341, by Northrop Grumman Corporation, and by IARPA via U.S. Army Research Office contract W911NF-14-C-0115.

[1] F.S. Bergeret, A.F. Volkov, and K.B. Efetov, Phys. Rev. Lett., 86, 4096 (2001).
[2] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., and N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010); C. Klose et al, Phys. Rev. Lett. 108, 127002 (2012).
[3] W. Martinez, W.P. Pratt, Jr., and N.O. Birge, arXiv:1510.02144.
[4] E.C. Gingrich et al., arXiv:1509.05368.

Faculty Host: Vlad Pribiag

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