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Pairing mechanisms in superconductors with valence-skipping dopants.

机译：超导体与价跳跃掺杂物的配对机制。

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摘要

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.
机译：最近对掺有价跳跃元素，的简并的半导体碲化铅的研究提出了负U超导的主张。在此模型中，稳定的+1和+3价态之间的电荷2e波动会刺激形成铜对。值得注意的是，与这种材料的超导性相关的是电阻的最小值，类似于掺杂有微量磁性杂质的金属中的电阻。此行为归因于电荷近藤效应，其中价态波动在+1和+3电荷状态之间的杂质杂散散射模仿了磁性杂质的杂散效应，从而使电阻最小，并提高了电势态密度。费米能级。在本文中，我将描述在三组不同的材料上进行的实验，这些材料旨在阐明掺有价跳跃元素的材料中的超导性。在第一个项目中，我研究了其他铟供体对Pb 1-yTlyTe样品的影响，否则这些样品将表现出超导电性和电荷-近藤行为。在这项工作中，我发现，随着通过增加铟的反掺杂剂浓度将其他电子注入Pb.99Tl.01Te中，在相同的铟浓度附近，近藤行为和超导性均被抑制。这被解释为通过引入有利于+1价态的额外电子来抑制+1和+3价态之间的简并性，从而抑制了电荷-近藤散射和负的U配对效应。这项研究涉及到相关材料铟掺杂的碲化锡，其中铟的化合价特性导致其他人提出了类似的负U超导态。与the掺杂的碲化铅不同，后者的主体材料PbTe不超导，而在锡位点上掺杂高浓度的空位时，SnTe超导。这允许在不添加其他掺杂剂的SnTe中的超导状态与已掺有价跃迁元素铟的SnTe之间进行比较。通过比较本研究中生长的Sn.995-xInxTe和从文献中收集到的Sn1-deltaTe中费米能级的状态密度和超导临界温度，我表明不能解释铟掺杂材料中增强的超导性通过费米能级的状态密度的变化。相反，可以推断出较强的超导配对强度，这可能是由于混合价杂质处的负U配对效应引起的。另外，将结构相变温度与超导转变温度的温度作为铟含量的函数进行比较，可以排除与该相变有关的声子模的软化，这是Tc增强的来源。第三项研究利用了高溶解度极限碲中的铟含量最高为20％，而碲化铅中的1.4掺杂含量为1.4％，以研究价数杂质之间的相互作用如何影响超导性和最小电阻。在这种情况下，电阻率测量显示掺杂了2.7％ 6.1％，则样品被发现超导，这被强烈抑制。尽管缺少用于比较这些数据的详细理论模型，但提出了一些可能的解释，这些解释依赖于铟位点之间的相互作用。

著录项

  • 作者

    Erickson, Ann Shaklee.;

  • 作者单位

    Stanford University.;

  • 授予单位 Stanford University.;
  • 学科 Physics Condensed Matter.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 129 p.
  • 总页数 129
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 工程材料学;
  • 关键词

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