NMR study of super conducting Nb/Cu multilayrs

超导金属和正常金属Nb/Cu的多层结构为研究邻近效应引起的超导性提供了良好的机会。核磁共振特别适用于多层结构的研究，因为它可以选择性地测量相应层中超导准粒子的热激发谱。我们在低至0.1K的Nb/Cu超导薄膜上进行了^<93>Nb和^<63>Cu核磁共振，并利用场循环方法在零场中测量了核自旋晶格弛豫速率1/T_1。1/T_1的温度依赖性表明Nb层存在2DELTA=3.5K_BT_C的超导能隙，Cu层存在较小的2.8K_BT_C的超导能隙。低温下测得的^<63>Cu的1/T_1也表明超导Cu层中存在残余态密度（约为正常态值的50%）。在超导铌层中，相应的剩余态密度即使存在，也是非常小的。

The multilayr of superconducting and normal metals, Nb/Cu, provides a good opportunity for the study of superconducivity induced by the proximity effect. NMR is especially suitable for the study of multilayrs, since it can selectively measure the thermal excitation spectra of the superconducting quasi-particles in the respecive layrs. We have carried out ^<93>Nb and ^<63>Cu NMR in Nb/Cu superconducting thin film down to 0.1K,and measured the nuclear spin lattice relaxation rate, 1/T_1, in zero field by using the field cycling method. The temperature dependence of 1/T_1 has shown that the existence of superconducting energy gap with the magnitude of 2DELTA=3.5K_BT_C for Nb layrs, and a smaller gap of about 2.8K_BT_C for Cu layrs. The 1/T_1 of ^<63>Cu measured at low temperatures has also shown the existence of the residual density of states (about 50% of the value in the normal states) in the superconducting Cu layrs. The corresponding residual density of states, even if exist, is very small in the superconducting Nb layrs.