Epitaxial Growth of Nano-Sized Pb Islands on SrTiO3 Substrate: Competition Between Electron Correlation and Enhanced Superconductivity

The discovery of the interface-enhanced superconductivity in the single-layer film of FeSe epitaxially grown on SrTiO₃ substrates has triggered a flurry of activity in the field of superconductivity. It raised the hope to find more conventional high transition temperature (T_c) superconductors that are purely driven by the electron-phonon interaction at ambient pressure. Here the epitaxial growth of the Pb nano-sized islands on SrTiO₃ (001) substrates with the island volumes ranging from 286 to 4945 nm³ is reported by molecular beam epitaxy, followed by systematic scanning tunneling microscopic/spectroscopic (STM/S) investigation. The observed STS gap for the nanoscale islands highly dependent on the volumes of nano-sized islands, can be divided into three regions. By performing a detailed spectroscopic investigation, it is founded that superconductivity in the volume above 3700 nm³ (Region I) has a zero temperature energy gap (Δ(0)) and T_c of 6.8 meV and 9.8 K obtained by BCS fitting, showing the largely-enhanced Δ and slightly-increased T_c by comparing to the bulk Pb (1.4 meV and 7.2 K). As the volume in the range from 1300 to 3700 nm³ (Region II), a large Coulomb gap induced by electron correlation emerged and shows a volume-dependent behavior, suggesting the reduced size can enhance electron correlation in Pb islands. As the volume decreases down to Region III, enhanced electron correlation and Coulomb gap become more dominant and superconductivity is totally suppressed. The experiment reveals that an electron-electron interaction in nano-sized Pb islands can be significantly enhanced by reducing the island sizes and suppresses the superconductivity, thus demonstrates a competition between superconductivity and electron correlation as the volume varies.