Interface studies of high-Tc superconductors by field-ion and field-electron energy spectroscopy

Tunneling experiments may yield valuable data for an experimental foundation of the physical mechanisms responsible for electrical transport in oxide-based superconductors, such as the width of the band gap at the Fermi energy, EF, and its temperature dependence in the superconducting regime. Experiments using metal-insulator-(high-Tc)superconductor sandwiches or point-contact (STM) arrangements have already indicated gap widths ranging between 25 and 65 meV. Since field-electron-emission spectroscopy (FES), especially at low field strengths, is essentially confined to a small energy window (≅100 meV) at and below, EF, it should be a sensitive probe for the transition from the normal to the superconducting phase. However, one drawback for STM and FES experiments on oxide-based high-Tc specimens appears to be the presence of insulating ('dead') layers. In this paper, we report on successful attempts to establish quantum mechanically transparent tip surfaces for tunneling electrons. (orig.)