Critical States in Confined Superconductors: From mesoscopic phenomena to microscopic understanding

该项目解决了与研究影响无序低维超导体全局和局部量子相位相干性的微观参数有关的基本问题。在超薄超导体中，量子凝聚体受到无序和电子关联的影响，这两种效应都往往会破坏超导电性，并将材料驱动到绝缘状态。在超导体-绝缘体转变(SIT)附近，出现了电子不均匀，涉及到几个特征纳米级的长度。通过调谐超薄超导薄膜中表征良好的无序，该提议的目标是揭示出现的电子不均匀如何影响超导凝聚体的量子相干性。因此，我们将使用超导电流作为纳米尺度上的局域相位相干性的探测器。在超导条纹中注入超导电流，同时用扫描隧道显微镜探测局域态密度(LDOS)。循环到临界电流的局域超流强度图将与在没有电流的情况下测量的本征紧急不均匀进行比较。第一个目标是在超薄NBN薄膜中产生良好控制的无序，对其进行表征，并根据需要对其进行调整。这种紊乱的特征是在全球范围内通过低温下的电子传输，以及在局部通过高分辨率的透射显微镜。结合扫描隧道显微镜/光谱(STM/STS)和扫描力显微镜/光谱(AFM)测量将被用来绘制局域态密度和局域功函数(开尔文探针)。这些光谱图之间的空间相关性将允许将局部静电势与超导不均匀联系起来。第二个目标是建立一个专门的AFM-STM联合实验装置，能够找到不同尺寸(从微米到几十纳米)的纳米图案化超导器件，然后在很大的温度范围内进行超高真空下的局部性质的联合隧道和力谱测量。第三个目标是探索无序超导系统的近临界状态，并将其与基态进行比较。通过向不同厚度和宽度的纳米图形超导样品注入足够强的超电流密度，可以调节超导相变的邻近程度。同时，量子相干性将通过AFM-STM组合光谱仪在局部测量，并在全球通过载流超导纳米线的输运测量来测量。将探索接近渗流阈值的近临界状态的预期空间不均匀特征。

The project addresses fundamental issues related to the study of the microscopic parameters affecting the global and local quantum phase coherence of disordered low-dimensional superconductors. In ultra-thin superconductors, the quantum condensate suffers from the effects of disorder and electron correlations which both tend to destroy superconductivity and drive the material to an insulating state. Close to this Superconductor-Insulator transition (SIT), electronic inhomogeneities emerge, involving several characteristic nanometer-scale lengths. By tuning the well-characterized disorder in ultrathin superconducting films, the goal of the proposal is to reveal how the emergent electronic inhomogeneities affect the quantum coherence of the superconducting condensate. Thus, we will use supercurrents as a probe of the local phase coherence at the nanometer scale. A supercurrent will be injected in superconducting stripes while simultaneously probing the local density of states (LDOS) by scanning tunneling microscopy. The maps of the intensity of the local supercurrents circulating up to the critical current will be compared with the intrinsic emergent inhomogeneities measured in the absence of current.The first objective is to generate a well-controlled disorder in ultrathin NbN films, characterize it, and tune it on-demand. The disorder will be characterized globally by electron transport at low temperatures, and locally by high-resolution transmission microscopy. Combined scanning tunneling microscopy/spectroscopy (STM/STS) and scanning force microscopy/spectroscopy (AFM) measurement will be used for mapping the local density of states and the local work function (Kelvin probe). The spatial correlations between these spectroscopic maps will allow connecting the local electrostatic potential to the superconducting inhomogeneities. The second objective is to build a dedicated combined AFM-STM experimental set-up able to find nano-patterned superconducting devices of various sizes (ranging from micron to few tens of nanometers), and then to perform combined tunneling and force spectroscopies of their local properties in ultrahigh vacuum over a large range of temperature. The third objective is to probe the near-critical state of disordered superconducting systems and compare it to the ground state. The proximity of the superconducting phase transition will be tuned by injecting a strong enough super-current density into nano-patterned superconducting samples - nanowires of various thickness and widths. Simultaneously, the quantum coherence will be measured locally by combined AFM-STM spectroscopies, and globally by transport measurements on current carrying superconducting nanowires. The expected spatially inhomogeneous character of the near-critical state close to the percolation threshold will be explored.