Disque Hall 919, S. 32nd Street and Chestnut Street, Philadelphia, PA 19104
Physics Colloquium: "Combining Superconductors and Ferromagnets: a New Type of Symmetry?"
Thursday, November 7, 2013
3:30 PM-4:30 PM
Norman Birge, PhD, professor, Michigan State University
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 electron 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. 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. At the end of the talk, he will discuss future prospects for using such junctions as elements in a superconducting memory.
Goran Karapetrov, PhD