Pairbreaking in S-wave Superconductors by Two-channel Kondo Impurities

We propose dynamical defects with a rotational degree of freedom in a metal as a possible realization of two-channel Kondo systems. When thè = 0 state of the defect is blocked, the impurity exhibits a Kramers doublet ground state. We consider the pair breaking eeect of a nite density of this type of two-channel Kondo impurities in an s-wave super-conductor, using an auxiliary particle representation for the impurity, and the BCS model for the superconductor. Several experiments on mesoscopic systems indicate an apparently non-vanishing electron dephasing rate 1== at low temperatures T 1]. The origin of this dephasing saturation is of fundamental interest in condensed matter physics as well as relevant for the design of nanoscale devices involving quantum phase coherence. The strong dependence of the saturation value of on the type of material and on the sample preparation technique hints at a dephasing mechanism which is not intrinsic to the electron liquid. It has been proposed that dynamical structural defects in the crystal structure like two-level systems (TLS), coupled to the electron system, could cause dephasing at low T. In order to account for the experiments this requires either a distribution of level splittings , P() / 1== 2], or a sizable fraction of TLS with vanishing which produce a two-channel Kondo (2CK) eeect 3]. The 2CK eeect implies a non-vanishing T = 0 dephasing rate because of its nite zero-point en-tropy S(0) = k B ln p 2 4]. At present, there exists no microscopic model to explain a level splitting distribution P() / 1==. On the other hand, several experiments 5, 6] do show 2CK signatures 7, 8]. However, the realization of the 2CK eeect within the usual scenario of a heavy par