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

该项目涉及影响无序低维超导体整体和局部量子相相干性的微观参数研究的基本问题。在超薄超导体中，量子凝聚体受到无序和电子相关性的影响，这两者都倾向于破坏超导性并将材料驱动到绝缘状态。接近这种超导体-绝缘体转变（SIT），电子不均匀性出现，涉及几个特征纳米尺度的长度。通过调整超薄超导薄膜中具有良好特征的无序，该提案的目标是揭示涌现的电子不均匀性如何影响超导凝聚体的量子相干性。因此，我们将使用超电流作为纳米尺度上局部相相干性的探针。在超导条纹中注入超电流，同时用扫描隧道显微镜探测局部态密度。将循环到临界电流的局部超流强度图与在没有电流时测量的固有非均匀性进行比较。第一个目标是在超薄NbN薄膜中产生良好控制的无序，表征它，并按需调整它。这种紊乱将通过低温下的电子传递来全局表征，通过高分辨率透射显微镜来局部表征。联合扫描隧道显微镜/光谱（STM/STS）和扫描力显微镜/光谱（AFM）测量将用于绘制局部态密度和局部功函数（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性联系起来。第二个目标是建立一个专用的AFM-STM联合实验装置，能够找到各种尺寸（从微米到几十纳米）的纳米型超导器件，然后在超高真空和大范围温度下对其局部特性进行联合隧道和力谱分析。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。通过注入足够强的超电流密度到纳米型超导样品（不同厚度和宽度的纳米线）中，可以调节超导相变的接近度。同时，量子相干性将通过联合AFM-STM光谱进行局部测量，并通过载流超导纳米线的输运测量进行全局测量。将探讨接近渗透阈值的近临界状态的预期空间非均匀性特征。

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.