Ballistic effects in a proximity induced superconducting diffusive metal

Abstract

Using a scanning tunneling microscope (STM), we investigate the local density of states (LDOS) of artificially fabricated normal metal nanostructures in contact with a superconductor. Very low temperature local spectroscopic measurements $(100\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ reveal the presence of well-defined subgap peaks at energy $\ensuremath{\mid}E\ensuremath{\mid}<\ensuremath{\Delta}$ in the LDOS at various positions of the STM tip. Although no clear correlations between the LDOS and the shape of the samples have emerged, some of the peak features suggest they originate from quasiparticle bound states within the normal metal structures (De Gennes St. James states). Refocusing of electronic trajectories induced by the granular structure of the samples can explain the observation of spatially uncorrelated interference effects in a nonballistic medium.