Imaging Cooper pairing of heavy fermions in CeCoIn₅

The Cooper pairing mechanism of heavy fermion superconductors(1-4), long thought to be due to spin fluctuations(5-7), has not yet been determined. It is the momentum space (k-space) structure of the superconducting energy gap Delta(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure Delta(k) for any heavy fermion superconductor. Bogoliubov quasiparticle interference imaging(8), a proven technique for measuring the energy gaps of superconductors with high critical temperatures(9-11), has recently been proposed(12) as a new method to measure Delta(k) in heavy fermion superconductors, specifically CeCoIn5 (ref. 13). By implementing this method, we detect a superconducting energy gap whose nodes are oriented along k parallel to (+/- 1; +/- 1)pi/a(0) directions(14-17). Moreover, for the first time in any heavy fermion superconductor, we determine the detailed structure of its multiband energy gaps Delta(i)(k). For CeCoIn5, this information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that largest magnitude Delta(k) opens on a high-k band so that the primary gap nodes occur at unforeseen k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with d(x2-y2) gap symmetry. Such quantitative knowledge of both the heavy band-structure and superconducting gap-structure will be critical in identifying the microscopic pairing mechanism of heavy fermion superconductivity.