Würzburg ToCoTronics Colloquium

BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. In this talk, I will show the threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. I will also show how ultrathin superconducting films can be used to study superconductivity in large magnetic fields. Using spatially resolved spectroscopy, we characterize the vortex structure in the presence of a strong Zeeman field and find evidence of a paramagnetic Meissner effect originating from odd-frequency pairing contributions. These results illustrate two notable influences of reduced dimensionality on a BCS superconductor.

Figure:  (a) Constant-current STM topography of an 8.5 ML Al film on Si(111), (b) Superconducting gap spectra taken at T = 30 mK for samples with varying Al coverage, (c) Constant-contour dI/dV map in vector magnetic field, showing the Meservey-Tedrow-Fulde vortices for an 8.5 ML Al film.