Investigations of charging effects and high critical temperature superconductivity with low-temperature scanning tunneling microscopy

It is well known that under conditions corresponding to a voltage bias the tunneling characteristics of a double mesoscopic junction system, with junction parameters R{dollar}sb1{dollar},C{dollar}sb1{dollar},R{dollar}sb2{dollar},C{dollar}sb2{dollar} and e{dollar}sp2{dollar}/2C{dollar}sb{lcub}rm i{rcub}gg{dollar} kT (i = 1 or 2, and C{dollar}sb{lcub}rm i{rcub}{dollar} and R{dollar}sb{lcub}rm i{rcub}{dollar} are the junction capacitance and resistance respectively), is dominated by single electron tunneling or charging effects. Depending on the parameters, the tunneling characteristics associated with these charging effects can be identified as the Coulomb blockade in which a complete suppression of tunneling current near zero bias is found or the Coulomb staircase in which periodic oscillations in dI/dV are found. A low temperature scanning tunneling microscopy (STM) has been constructed and utilized to study superconducting granular lead films (T{dollar}sb{lcub}rm c{rcub}{dollar} = 7K) and high-T{dollar}sb{lcub}rm c{rcub}{dollar} cuprate superconductors. In the case of the lead films, the superconducting energy gap of lead was also found to coexist with the Coulomb staircase. Tunneling into high-T{dollar}sb{lcub}rm c{rcub}{dollar} cuprate superconductors has been carried out on films grown in situ using ozone as a source of oxygen. By associating the tunneling characteristics with low temperature STM images, a better understanding of the relationship between surface morphology and tunneling spectroscopy was reached. Charging effects were frequently observed when the STM tip was positioned in the granular regions of the high-Tc superconductor, whereas linear I-V characteristics were found in geometrically flat areas. Strong BCS-like tunneling characteristics were never obtained for these superconducting thin films.