Superconductivity in two dimensions has attracted renewed interest in the context of two-dimensional (2D) van der Waals materials. Key questions remain regarding the nature and manifestations of the superconductivity in these materials. One open question is whether superconducting fluctuations in such 2D systems can be described by the classic Aslamazov-Larkin (A-L) equation. While the A-L model has long been found to accurately describe the paraconductivity of some conventional 2D superconductors, its applicability in ultrathin limit near the superconductor-insulator transition (SIT) has not been established. Here, we report a systematic study of superconducting fluctuation and paraconductivity in ultrathin 2D amorphous Pb films near the SIT. Pb films were incrementally deposited, and the electrical measurements were performed in situ at each thickness in a dilution refrigerator, resulting in a series of sheet resistance curves [𝑅□⁡(𝑇)] across the SIT. Paramagnetic impurities (Cr) were then deposited on the same superconducting film, driving it back to the insulating state and yielding another set of 𝑅□(𝑇). Both types of 𝑅□(𝑇) curves were analyzed with the A-L equation. At all film thicknesses (and 𝑇𝐶's), we observed a background paraconductivity significantly greater than the A-L value. As the film thickness decreases and the SIT is approached, the paraconductivity increases precipitously, reaching more than an order of magnitude higher than that of the A-L value at the immediate vicinity of the SIT. Accompanying the presence of excess paraconductivity and its increase with decreasing thickness, the paraconductivity exhibits exponential scaling with 𝑇 above 𝑇𝐶, whose temperature range and slope show concomitant variations with decreasing film thickness. In contrast, suppression of 𝑇𝐶 by magnetic impurity results in much weaker increase of the excess paraconductivity and little change in the temperature range and slope of the exponential 𝑇 scaling. These observations are consistent with the presence of emergent localized superconducting pairing above 𝑇𝐶 in the strongly disordered, morphologically nominally uniform amorphous films. The spatially localized superconductivity likely causes excess paraconductivity beyond the A-L theory and percolative superconducting transitions. Such electronic inhomogeneities proliferate with decreasing film thickness, especially near the SIT.

中文翻译：

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超导体-绝缘体转变附近超薄非晶态 Pb 薄膜的超导涨落和副导性


二维超导性在二维 （2D） 范德华材料的背景下重新引起了人们的兴趣。关于这些材料中超导性的性质和表现，仍然存在关键问题。一个悬而未决的问题是，这种二维系统中的超导涨落是否可以用经典的 Aslamazov-Larkin （A-L） 方程来描述。虽然 A-L 模型长期以来一直被发现可以准确描述一些传统二维超导体的旁导性，但其在超导体-绝缘体过渡 （SIT） 附近的超薄极限中的适用性尚未确定。在这里，我们报告了对 SIT 附近超薄 2D 非晶 Pb 薄膜中超导涨落和顺导性的系统研究。Pb 薄膜逐渐沉积，并在稀释冰箱中对每个厚度进行原位电学测量，从而在 SIT 上产生一系列片状电阻曲线 [R□（T）]。然后，顺磁性杂质 （Cr） 沉积在同一层超导薄膜上，将其推回绝缘状态并产生另一组 R□（T）。两种类型的 R□（T） 曲线均采用 A-L 方程进行分析。在所有薄膜厚度（和 TC）下，我们观察到背景旁导性明显大于 A-L 值。随着薄膜厚度的减小和 SIT 的接近，旁导性急剧增加，达到比 SIT 附近的 A-L 值高一个数量级以上。 伴随着过量旁导性的存在及其随着厚度的减小而增加，旁导性表现出指数缩放，T 高于 TC，其温度范围和斜率随薄膜厚度的减小而变化。相比之下，磁性杂质对 TC 的抑制导致过量旁导性的增加要弱得多，并且指数 T 标度的温度范围和斜率变化很小。这些观察结果与在强无序、形态学上名义上均匀的非晶片中 TC 上方存在新兴的局部超导对一致。空间定位的超导性可能会导致超出 A-L 理论和渗流超导跃迁的过剩副导性。这种电子不均匀性随着薄膜厚度的减小而增殖，尤其是在 SIT 附近。