Combining superconductors and ferromagnets: a new type of symmetry? – Norman Birge

Date: Thu. April 19th, 2012, 4:15 pm-5:15 pm
Location: Rockefeller 301

Physicists are constantly on the lookout for new symmetries in the ground states of quantum systems. Familiar examples include ferromagnets, which break spin-rotation symmetry, and superconductors, which break gauge symmetry. When a superconductor (S) and a ferromagnet (F) are put into contact with each other, interesting things happen, and the combined S/F hybrid system exhibits altogether new properties. There is a proximity effect where pair correlations from S penetrate into F, but this proximity effect decays over a very short distance due to the large energy splitting between the spin-up and spin-down electrons. Theory predicts that, under certain conditions, electron pair correlations will appear with spin-triplet rather than spin-singlet symmetry [1]. The two electrons in such a spin-triplet pair have parallel spins and are not energy split in F; hence they propagate long distances. Furthermore, these triplet correlations satisfy the Pauli Exclusion Principle in a new and strange way: they are odd in frequency or time. In this talk I will discuss our observation of these pair correlations in experiments with S/F/S Josephson junctions [2].

[1] F.S. Bergeret, A.F. Volkov, and K.B. Efetov, Rev. Mod. Phys. 77, 1321 (2005).
[2] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010); C. Klose et al., Phys. Rev. Lett. 108, 127002 (2012).