Observation of Coulomb Gap and Enhanced Superconducting Gap in Nano-Sized Pb Islands Grown on SrTiO_3

We report high-resolution scanning tunneling microscopy (STM) study of nano-sized Pb islands grown on SrTiO_3, where three distinct types of gaps with different energy scales are revealed. At low temperature, we find that the superconducting gap (\it\Delta_\rm s) in nano-sized Pb islands is significantly enhanced from the one in bulk Pb, while there is no essential change in superconducting transition temperature T_\rm c, giving rise to a larger BCS ratio 2\it\Delta_\rm s/k__\rm BT_\rm c \sim 8.31 and implying stronger electron-phonon coupling. The stronger coupling can originate from the interface electron-phonon interactions between Pb islands and SrTiO_3. As the superconducting gap is totally suppressed under applied magnetic field, the Coulomb gap with apparent V-shape emerges. Moreover, the size of Coulomb gap (\it\Delta_\rm C) depends on the lateral size of Pb islands (R) with \it\Delta_\rm C\sim 1/R^0.35, indicating that quantum size effect can significantly influence electronic correlations. Our experimental results shall shed important light on the interplay among superconductivity, quantum size effect and correlations in nano-sized strong-coupling superconductors.