Gupta Chandan, Parab Pradnya, Bose Sangita
School of Physical Sciences, UM-DAE Center for Excellence in Basic Sciences, University of Mumbai, Kalina Campus, Mumbai, 400098, India.
Sci Rep. 2020 Oct 27;10(1):18331. doi: 10.1038/s41598-020-75351-8.
Superconductivity in 3D Nb-Cu nanocomposite granular films have been studied with varying thickness for two different compositions, Nb rich with 88 at% of Nb and Cu rich with 46 at% of Nb. For both compositions, the superconducting transition temperature (T) decreases with decreasing film thickness. For any thickness, doubling the Cu content in the films decreases the T by about 2 K. To explore if phase fluctuations play any role in superconductivity in these 3D films, the superfluid stiffness (J) of the films was measured using low frequency two-coil mutual inductance (M) technique. Interestingly, the measurement of M in magnetic fields showed two peaks in the imaginary component of M for both Nb rich and Cu rich films. The two peaks were associated with the pair-breaking effect of the magnetic field on the intra and inter-granular coupling in these films consisting of random network of superconductor (S) and normal metal (N) nano-particles. Furthermore, J was seen to decrease with decreasing film thickness and increasing Cu content. However, for all films studied J remained higher than the superconducting energy gap (∆) indicating that phase fluctuations do not play any role in superconductivity in the film thickness and composition range investigated. Our results indicate that an interplay of quantum size effects (QSE) and superconducting proximity effect (SPE) controls the T with composition in these 3D nano-composite films.
对两种不同成分的三维铌 - 铜纳米复合颗粒膜的超导性进行了研究，这两种成分分别是富含88原子百分比铌的富铌膜和富含46原子百分比铌的富铜膜，且膜厚度各不相同。对于这两种成分，超导转变温度（T）均随膜厚度的减小而降低。对于任何厚度，将膜中的铜含量翻倍会使T降低约2K。为了探究相位涨落在这些三维膜的超导性中是否起作用，使用低频双线圈互感（M）技术测量了膜的超流刚度（J）。有趣的是，在磁场中对M的测量显示，富铌膜和富铜膜的M虚部都出现了两个峰值。这两个峰值与磁场对由超导体（S）和正常金属（N）纳米颗粒随机网络组成的这些膜中的颗粒内和颗粒间耦合的对破坏效应有关。此外，J随膜厚度的减小和铜含量的增加而降低。然而，在所研究的所有膜中，J始终高于超导能隙（∆），这表明在研究的膜厚度和成分范围内，相位涨落在超导性中不起任何作用。我们的结果表明，量子尺寸效应（QSE）和超导邻近效应（SPE）的相互作用控制了这些三维纳米复合膜中T随成分的变化。
