Proximity effect and strong-coupling superconductivity in nanostructures built with an STM

We present high-resolution tunneling spectroscopy data at very low temperatures on superconducting nanostructures of lead built with scanning tunnel microscope. By applying magnetic fields, superconductivity is restricted to length scales of the order of the coherence length. We measure the tunneling conductance and analyze the phonon structure and the low-energy density of states. We demonstrate the influence of the geometry of the system on the magnetic-field dependence of the tunneling density of states, which is gapless in a large range of fields. The behavior of the features in the tunneling conductance associated to phonon modes are explained within current models.