Effect of heavy electron mass $m^{\star}$ on Andreev reflection (AR) in heavy-fermion/superconductor (HF/S) point-contacts

We investigate the effect of $m^{\star}$ on Andreev reflection in HF/S point-contacts (PC) by measuring the differential electrical conductance at temperatures between 1.5 and 10\,K. An electrochemically etched Nb tip ($T_{\rm c}$\,=\,9.2\,K) is brought into contact with HF single crystals (CeCoIn$_5$, CeRhIn$_5$, YbAl$_3$) of varying $m^{\star}$. Our conductance signals show a clear superconducting gap structure expected for PC in the Sharvin regime which can be fitted by the Blonder- Tinkham-Klapwijk model. Preliminary results on CeCoIn$_5$ at temperatures above its $T_{\rm c}$\,=\,2.3\,K indicate an enhanced AR signal of similar magnitude as in Au/Nb PC which is in contrast to CeCoIn$_5$/Au PC as reported earlier\,[1]. We discuss this in the context of a two fluid model which considers heaviness and bandstructure of the HF quasiparticles and which may also explain the asymmetric conductance background observed in many heavy-fermions below a characteristic temperature $T^{\star}$. [1] W.~K. Park {\it et al.}, PRB {\bf 72}, 052509 (2005). $-$ This work was supported by the Deutsche Forschungsgemeinschaft, and U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the CMM at UIUC and the NSF-DMR-0503360 at UC.