Point Contact Spectroscopy Study of CeCoIn$_5$: Evidence for Unconventional Pairing in a Multi-Band Heavy Fermion Superconductor

Point-contact spectroscopy was performed on single crystals of the heavy-fermion superconductor CeCoIn$_5$ in the temperature range 150mK to 2.5K. A pulsed measurement technique ensured minimal Joule heating over a wide voltage range. The spectra show Andreev-reflection characteristics with multiple structures which depend on junction impedance. Spectral analysis using the generalized BTK formalism revealed two nodal order-parameter components, with amplitudes $\Delta_1$=0.95 $\pm$0.15meV and $\Delta_2$=2.4$\pm$0.3meV which evolve differently with temperature. These observations indicate a highly unconventional pairing mechanism, possibly involving multiple bands [1]. Measurements are also done in a magnetic field to study the field-evolution of the order-parameter components we observed. \\ ~[1]~P.M.C. Rourke, \emph{et al}. cond-mat/0409562. \\ Work supported by: NSERC, CFI/OIT, MMO/EMK and the Canadian Institute for Advanced Research. Work at Brookhaven supported by: Division of Materials Sciences, Office of Basic Energy Sciences, US Department of Energy under contract no. DE-AC02- 98CH10886.