Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University , 637371 Singapore.
Swiss-Norwegian Beamline, European Synchrotron Radiation Facility , 6 rue Jules Horowitz, Grenoble F-38043 Cedex, France.
ACS Nano. 2016 Jan 26;10(1):515-23. doi: 10.1021/acsnano.5b05450. Epub 2016 Jan 8.
The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature T, magnetic field H, or current I. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na2-δMo6Se6 featuring Na vacancy disorder (δ ≈ 0.2) are shown to behave as percolative networks of superconducting nanowires. Long-range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero (T,H,I) limit. However, a region of reentrant phase coherence develops upon raising (T,H,I). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy EJ at nonzero (T,H,I), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na2-δMo6Se6 is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated (T,H,I).
低维超导体的短相干长度与有用的高临界场或温度有关。不幸的是，这类材料往往对无序敏感，并受到超导序参量的相位涨落的影响，这种相位涨落在温度 T、磁场 H 或电流 I 下发散。我们提出了一种克服合成和涨落问题的方法：用纳米丝的不均匀复合材料来制造超导体。具有 Na 空位无序（δ≈0.2）的准一维 Na2-δMo6Se6 宏观晶体表现为超导纳米线的渗滤网络。通过各向异性一维细丝之间的横向耦合建立长程有序，但相位相干性在零（T、H、I）极限下仍然不稳定，容易受到涨落和局域化的影响。然而，在升高（T、H、I）时，会出现一个重新出现的相位相干区域。我们将这一现象归因于电子离域导致的横向耦合增强。我们对重新出现的相位相干性的观察与约瑟夫森能量 EJ 在非零（T、H、I）时的峰值相吻合，我们使用一个简单的各向异性无序超导体的分析模型来估计这个峰值。因此，Na2-δMo6Se6 为新一代具有内置抗相位涨落能力的纳米丝超导体提供了蓝图，这些超导体在升高的（T、H、I）时具有抗相位涨落的能力。
