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Multiband models of high temperature superconductors.

机译：高温超导体的多频带模型。

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摘要

In order to understand the complex electromagnetic properties of high temperature superconductors the details of the electronic band structure must be included. In this thesis we go beyond the simple free electron model in which the electromagnetic properties are determined by a single, spherical, Fermi surface (which is adequate for most conventional superconductors) to more realistic models containing multiple, anisotropic tight binding bands. Using these models we are able to calculate several electromagnetic properties, including the magnetic penetration depth, Josephson tunneling currents, the density of states and the optical conductivity, we find our results are in good agreement with experimental observations.; One of the banes of the field of high temperature superconductivity is that many experiments are carried out on materials that are less than ideal. Often the presence of impurities or less than ideal stoichiometries can have a large impact upon the measured properties. We are able to calculate the effect of impurities upon the above mentioned electromagnetic properties in both the unitary and Born scattering limits and find significant differences between them. We find that the presence of impurities in the chain band has little effect upon the critical temperature which is consistent with the experimental observations that Ni impurities (which go into the chains) and overdoped materials (in which the excess oxygen also goes in the chains) have little effect upon the critical temperature, while Zn impurities (which go into both the chains and planes) and underdoped materials (in which both the chains and planes are oxygen deficient) have a significantly lower critical temperature.; Another of the difficulties with the experimental results is caused by the presence of twin domains in the orthorhombic materials. The presence of twins in a material can mask some of the significant anisotropies that are observed in untwinned materials such as the magnetic penetration depth in the a and b directions and the c direction Josephson tunneling into a conventional superconductor. We calculate these properties in models that take into account the large anisotropies that are present in these materials. We find that the c direction Josephson tunneling can not only be non-zero, but can actually be quite large in a pure d wave superconductor if the band is anisotropic (i.e. orthorhombic). This result helps explain one of the few experiments that was considered inconsistent with d wave superconductivity.
机译：为了理解高温超导体的复杂电磁特性，必须包括电子能带结构的细节。在本文中，我们超越了简单的自由电子模型，在该模型中，电磁特性是由单个球形费米表面（对于大多数常规超导体而言是足够的）确定的，而更实际的模型是包含多个各向异性的紧密结合带。使用这些模型，我们能够计算出若干电磁特性，包括磁穿透深度，约瑟夫森隧穿电流，态密度和光导率，我们发现我们的结果与实验观察结果非常吻合。高温超导领域的基础之一是对不理想的材料进行了许多实验。通常，杂质的存在或小于理想的化学计量比可能会对测量的性能产生很大的影响。我们能够在单一和Born散射极限中计算出杂质对上述电磁特性的影响，并发现它们之间的显着差异。我们发现链带中杂质的存在对临界温度的影响很小，这与实验观察到的镍杂质（进入链中）和掺杂材料（其中过量的氧也进入链中）一致。临界温度几乎没有影响，而锌杂质（进入链和平面都进入）和掺杂不足的材料（链和平面都缺氧）的临界温度要低得多。实验结果的另一个困难是由于正交晶态材料中存在双畴。在材料中双晶的存在可以掩盖在未缠绕材料中观察到的一些显着各向异性，例如在a和b方向上的磁穿透深度以及进入常规超导体的c方向约瑟夫森隧穿。我们在考虑了这些材料中存在的大各向异性的模型中计算这些属性。我们发现，如果带是各向异性的（即正交的），则在纯d波超导体中，c方向的约瑟夫逊隧穿不仅可以为非零，而且实际上可以很大。该结果有助于解释被认为与d波超导性不一致的少数实验之一。

著录项

  • 作者

    O'Donovan, Chris.;

  • 作者单位

    McMaster University (Canada).;

  • 授予单位 McMaster University (Canada).;
  • 学科 Physics Condensed Matter.
  • 学位 Ph.D.
  • 年度 2001
  • 页码 135 p.
  • 总页数 135
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 O49;
  • 关键词

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