The energy gap of pristine silicon clusters

During past years, scanning tunneling spectroscopy (STS) investigations of high temperature superconductors have revealed unusual characteristics of the superconducting gap. We present here the temperature dependence of the tunneling conductance spectra on YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+delta single crystals. We discuss similarities and differences between these two compounds. In particular we note that while in Bi2Sr2CaCu2O8+delta a pseudogap is observed above the critical temperature, even in overdoped samples, we find no indication of a pseudogap in optimally doped YBa2Cu3O7delta above Tc. This is consistent with other striking differences between the two systems and supports the interpretation of the pseudogap in terms of strong superconducting correlations.