Role of the external surfaces on the superconducting properties of superconductor/normal metal trilayers

Superconducting proximity effect is studied in superconductor/normal metal trilayers. The dependences of the superconducting transition temperature TcTc versus Nb thickness in Cu/Nb/Cu systems and versus Cu thickness in Nb/Cu/Nb ones are described by different values of the microscopical parameters. We attribute this difference to the influence of the external surfaces of the Nb/Cu/Nb hybrids on the superconducting properties of the system.