铜基和铁基高温超导体绝缘相的扫描隧道显微学研究

发布时间：2018-05-23 09:12

本文选题：铜氧化物高温超导体 + 铁基超导体　；参考：《清华大学》2014年博士论文

【摘要】：非常规超导材料的相图中存在一个重要的现象,即在其超导相附近往往存在着一个绝缘相。其中最显著的例子是铜氧化物高温超导体,其母体通常被认为是一个反铁磁关联的Mott绝缘体。最近在铁基超导材料中也发现了若干新奇的绝缘态。了解这些绝缘相的电子结构,将有可能对理解非常规超导电性形成的机制有所启示。本文将阐述我们应用扫描隧道显微镜技术对非常规超导体中的绝缘相进行研究所获得的结果。我们在原子尺度测量铜氧化物高温超导体的母体,Mott绝缘材料Ca2CuO2Cl2的电子结构,并首次获得了跨过整个Mott-Hubbard能隙的完整的电子谱。它呈现出电子-空穴对称,实空间均匀分布的特性。由杂质引入的载流子在Mott-Hubbard能隙中引发一个新的能态,此能态有很大的能量展宽,并在实空间高度局域化。我们的实验数据与Zhang-Rice单态模型大致吻合,但是对杂质所引起电子态的一些具体特征还需要进一步的理论解释。对于铁基超导体,我们测量了Cu掺杂的NaFeAs体系在超导体-绝缘体相变前后其局域电子结构的演化。我们发现Cu原子掺杂将系统地降低低能电子态的谱权重,并引入很强的局域杂质势。在绝缘相的电子谱中我们探测到一个低能能隙,它使得费米面附近的电子态密度进一步降低。尤其值得注意的是绝缘相电子谱的总体线型,以及其空间分布的不均匀性,都与极度欠掺杂的铜氧化物高温超导体十分类似。我们认为Cu原子掺杂对系统巡游电子态密度的压制以及它所引入的杂质势共同作用,从而在通常呈现金属性的铁基系统中导致了一个强耦合的绝缘态。
[Abstract]:There is an important phenomenon in the phase diagram of unconventional superconducting materials, that is, there is always an insulating phase near the superconducting phase. The most notable example is the copper oxide high temperature superconductor, the parent of which is generally considered to be an antiferromagnetic associated Mott insulator. A number of novel insulating states have also been recently found in iron-based superconductors. Understanding the electronic structure of these insulating phases may be helpful in understanding the mechanism of formation of unconventional superconductivity. This paper describes the results obtained by using scanning tunneling microscope (SEM) to study the insulating phase in unconventional superconductors. We have measured the electronic structure of Ca2CuO2Cl2, the parent material of copper oxide high temperature superconductors, at atomic scale. For the first time, we have obtained the complete electronic spectra across the entire Mott-Hubbard gap. It presents the characteristics of electron-hole symmetry and uniform distribution in real space. The carrier introduced by impurity induces a new energy state in the Mott-Hubbard gap, which has a large energy broadening and is highly localized in real space. Our experimental data are in good agreement with the Zhang-Rice single state model, but some specific characteristics of the electronic states caused by impurities need further theoretical explanation. For Fe-based superconductors, the evolution of local electronic structures of Cu-doped NaFeAs system before and after superconductor-insulator phase transition has been measured. We find that the doping of Cu atoms will systematically reduce the spectral weight of low energy electronic states and introduce a strong local impurity potential. We have detected a low energy gap in the electron spectrum of the insulating phase, which further reduces the density of electronic states near the Fermi surface. It is particularly noteworthy that the general line of the insulating phase electron spectrum and the heterogeneity of its spatial distribution are very similar to those of highly underdoped copper oxide high temperature superconductors. We consider that Cu doping compacts the density of states in the system and the impurity potential it introduces, which leads to a strongly coupled insulating state in the iron-based system, which usually presents gold properties.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM26

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