Superconducting proximity effect in (7×7)R19.1∘ Ni nanoislands on Pb(111)

We have studied the proximity-induced superconductivity in $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ Ni nanoislands by combing scanning tunneling microscopy/spectroscopy (STM/STS) with density-functional theory calculation. Through depositing Ni onto Pb(111) substrate at 80 K, the monolayer Ni nanoislands with the $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ surface structure have been fabricated, where the termination of Ni atoms at the hexagonal close-packed site is energetically preferred and the filling of $3d$ orbitals from the charge transfer leads to the vanishing magnetic moment of Ni atoms. The topographic $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.{1}^{\text{o}}$ lattice as well as the asymmetric height contrast in atomic unit cell have been further corroborated by the STM simulations. With high spatial and energy resolution, tunneling conductance ($dI/dU$) spectra have resolved an isotropic superconducting gap with ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}\ensuremath{\approx}1.29$ meV, which is slightly larger than ${\mathrm{\ensuremath{\Delta}}}_{Pb}\ensuremath{\approx}1.25$ meV. The temperature dependence of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$ supports the substrate-induced superconducting proximity effect according to the same transition temperature ${T}_{c}\ensuremath{\approx}7.14$ K with the Pb(111). The line spectroscopy has spatially mapped out the small increase of ${\mathrm{\ensuremath{\Delta}}}_{Ni(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})}$, which could be explained by an enhanced electron-phonon interaction (${V}_{ep}$) under the framework of Bardeen-Cooper-Schrieffer theory as a manifestation of the hole doping of Pb(111) from the surface Ni atoms.