| Recent studies of the degenerate semiconductor, lead telluride, doped with the valence skipping element, thallium, have lead to propositions of negative U superconductivity. In this model, charge 2e fluctuations between the stable +1 and +3 thallium valence states stimulate formation of cooper pairs. Significantly, correlated with superconductivity in this material is a minimum in resistance, similar to that found in metals doped with trace magnetic impurities. This behavior has been attributed to a charge-Kondo effect, where scattering from impurities with valence fluctuations between +1 and +3 charge states mimics the effect of scattering from magnetic impurities, causing a minimum in resistance and an enhancement of the density of states at the Fermi level.;In this dissertation, I will describe experiments performed on three different sets of materials designed to illuminate aspects of superconductivity in materials that have been doped with valence-skipping elements. In the first project, I examine the effect of additional indium donors on samples of Pb 1-yTlyTe that would otherwise exhibit superconductivity and charge-Kondo behavior. In this work, I find that, as additional electrons are injected into Pb.99Tl.01Te by increasing the indium counterdopant concentration, the Kondo-like behavior and superconductivity are both suppressed near the same indium concentration. This is interpreted as a suppression of the degeneracy between +1 and +3 valence states by introducing additional electrons, which favor the +1 valence state, resulting in a suppression of charge-Kondo scattering and negative U pairing effects.;The second project extends this study to the related material indium-doped tin telluride, where the valence-skipping nature of indium has lead others to propose a similar negative U superconducting state. In contrast to thallium-doped lead telluride, where the host material, PbTe, is nonsuperconducting, SnTe superconducts when doped with high concentrations of vacancies on the tin site. This allows for comparisons between the superconducting state in SnTe without additional dopants, and SnTe that has been doped with the valence-skipping element, indium. By comparing the density of states at the Fermi level and the superconducting critical temperature in Sn.995-xInxTe, grown for this study, and Sn1-deltaTe, collected from the literature, I show that enhanced superconductivity in indium-doped material cannot be explained by changes in the density of states at the Fermi level. Instead, a stronger superconducting pairing strength is inferred, which could arise from negative U pairing effects at the mixed-valence impurities. Additionally, comparing the temperature of a structural phase transition to that of the superconducting transition temperature as a function of indium content rules out softening of phonon modes related to this transition as a source of the Tc enhancement.;The third study utilizes the high solubility limit of indium in tin telluride of up to 20%, compared to the limit of 1.4% thallium dopants in lead telluride, to examine the question of how interactions between valence-fluctuating impurities might affect superconductivity and the resistance minimum. In this case, resistivity measurements reveal an anomalously large resistance minimum in Sn988-x InxTe doped with 2.7% < x < 6.1%, which is strongly suppressed for x > 6.1%, where samples are found to superconduct. While a detailed theoretical model with which to compare these data is lacking, some possible explanations relying on interactions between indium sites are proposed. |
