Scanning Tunneling Spectroscopy with Superconducting Junctions

This thesis contains two major topics, the restriction of tunneling to only a few channels in the scanning tunneling microscope (STM) and the interaction of local magnetic impurities with superconductivity. At a temperature of 15mK, the quantum back-action of the electromagnetic environment in an STM junction becomes prominent. It influences the tunneling process, and by that inevitably also the spectroscopy of physical phenomena. We demonstrate that the macroscopic tip shape strongly defines this back-action. It can be reduced by increasing the tip wire diameter. This increases the capacitance of the junction, and thereby significantly enhances the spectroscopic energy resolution. Modeling this effect with P(E)-theory, we extrapolate that the electromagnetic environment of the junction influences measurements in the STM up to a temperature of 1K. This result helps establish a direct correspondence between the P(E)-model and the energy resolution of the STM. We further study the tunneling process by constructing a single-channel junction of an Al adatom on an Al(100) crystal and the single apex atom of an Al tip. We provide proof that the transport in this junction is strongly limited to a single channel by analyzing Andreev reflection spectra over a wide conductance range up to the quantum of conductance. With this junction we show how the Josephson effect deviates from the many channel and low transmission model by Ambegaokar and Baratoff. We also present a model, based on the full Andreev bound state relation for the few channel limit, which accounts for transmission dependencies and multiple Cooper pair tunneling. Modeling the Josephson effect in our junction this model reproduces the experimental data in great detail. Regarding the determination of the Josephson coupling energy in STM-experiments, we expect at least 0.6% and up to 2.6% deviation from the linear model at a conductance of 0.1G0 and up to 17% at 0.5G0. In the normal conducting state of this junction the environmental back-action manifests as a transmission reduction around zero bias, known as the dynamical Coulomb blockade (DCB). Here we test the predicted vanishing of the DCB for transmissions towards unity. Our data support this expectation. These results suggest that the transport process becomes less sensitive to the environmental back-action with increasing channel transmission. Concerning pair breaking potentials in a multi-band superconductor, we study Fe-doped NbSe2 with a V-tip. We demonstrate that Yu-Shiba-Rusinov (YSR) resonances emerge not only in the energy-gap but also outside of it, at the position of coherence peaks, where they are significantly broadened. We demonstrate a correspondence of the YSR-state lifetime to the imaginary part of the superconducting order parameter. To do so we compare the experimental peak-width to peak-energy-position dependence with a T-matrix scattering model, taking into account the two-band superconductivity of NbSe2, with inter-band coupling and magnetic background scattering. Our results show that YSR-resonances can be used to probe the imaginary part of the superconducting order parameter. We suspect that many asymmetries observed in spectra of the superconducting gap are related to this effect. We collate some early results of the local Josephson critical current in NbSe2. We find local variations around the embedded Fe impurities suggesting that the order parameter is reduced by about 20%.