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

• Applicant: Professor Dr. Michael Siegel
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 284075109

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.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partners: Dr. Tristan Cren; Professor Dr. Dimitri Roditchev