Proximity effects in the superconductor / heavy - fermion bilayer system

– We have investigated the proximity effect between a superconductor (Nb) and a “heavy-fermion” system (CeCu6) by measuring critical temperatures Tc and parallel critical fields H ‖ c2(T ) of Nb films with varying thickness deposited on 75 nm thick films of CeCu6, and comparing the results with the behavior of similar films deposited on the normal metal Cu. For Nb on CeCu6, we find a strong decrease of Tc with decreasing Nb thickness and a finite critical thickness of the order of 10 nm. Also, dimensional crossovers in H ‖ c2(T ) are completely absent, in strong contrast with Nb/Cu. Analysis of the data by a proximity effect model based on the Takahashi-Tachiki theory shows that the data can be explained by taking into account both the high effective mass (or low electronic diffusion constant), and the large density of states at the Fermi energy which characterize the heavy-fermion metal. Introduction. – When a superconductor (S) is brought into contact with a non-superconducting conductor (N), superconductivity leaks into that material by the proximity effect [1]. For the thermodynamic properties of the S/N bilayer such as the critical temperature Tc or the upper critical field Hc2, this can be modelled as a spatial variation of the superconducting pair density. The spatial variation mainly depends on the electron diffusion constants for the Sand the N-metal, on the transparency of the interface for electrons and Cooper pairs, and on the pair-breaking mechanisms on the N-side of the interface. Until now, two classes of Nmaterials have been investigated. The first is formed by simple metals such as Cu or Au, where the physics is mostly understood. Pairs are broken at finite temperatures by thermal fluctuations, leading to dephasing of the constituents of the induced Cooper pair. This is translated (∗) Present address: Department of Nanoscience, Delft University of Technology Delft, the Netherlands.