Solid state NMR studies of oxide superconductors and related materials

Yang, Shengtian

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https://hdl.handle.net/2142/20055 Copy

Description

Title

Solid state NMR studies of oxide superconductors and related materials

Author(s)

Yang, Shengtian

Issue Date

1991

Doctoral Committee Chair(s)

Oldfield, Eric

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Chemistry, Analytical
• Chemistry, Physical

Language

eng

Abstract

• Solid state nuclear magnetic resonance spectroscopy is used to obtain information on the nearest neighbor atom scale regarding various structural, electronic and magnetic properties of a variety of simple oxides, mixed metal oxides and oxide superconductors. Chapter 1 briefly reviews the basic theory of the NMR parameters of metals in the normal and superconducting state and the nuclear resonance properties of quadrupole nuclei. Chapter 2 introduces a new oxygen-17 labeling technique using $\sp{17}$O$\sb2$ as the source and the possible mechanisms involved in this exchange process. Chapter 3 describes the $\sp{17}$O NMR of lanthanide oxides, many of which are paramagnetic. These resonances fall in a range of 15,000 ppm and the individual shifts correlate with those in the solution state. Chapters 4, 5, 6, 7 reveal that for all the cuprate superconductors studied, there was a broad highly paramagnetically shifted resonance and one or two narrow ones in the diamagnetic region. The highly shifted features are assigned to CuO$\sb2$ oxygen sites and the peaks in the diamagnetic region to the non-CuO$\sb2$ oxygen sites. The CuO$\sb2$ oxygen sites in YBa$\sb2$Cu$\sb3$O$\sb7$, Bi$\sb2$Sr$\sb2$CaCu$\sb2$O$\sb8$ and Tl$\sb2$Ba$\sb2$CaCu$\sb2$O$\sb8$ have temperature independent resonance frequencies above T$\sb{\rm c}$. These resonances gradually decrease in frequency below T$\sb{\rm c}$. Their relaxation mechanisms are dominated by the s-wave Fermi contact interaction and manifest nearly ideal Korringa behavior. For La$\sb{1.85}$Sr$\sb{0.15}$CuO$\sb4$, La$\sb{1.85}$Ca$\sb{0.15}$CuO$\sb4$, and Bi$\sb2$Sr$\sb2$CuO$\sb6$, the CuO$\sb2$ $\sp{17}$O NMR resonances are temperature dependent for all the temperatures measured (4K-300K) and track the bulk magnetic susceptibility in these materials. The observed highly paramagnetically shifted feature in cuprates is absent in the bismuthate and plumbate superconductors, where the most highly shifted resonance is found in Ba$\sb{0.6}$K$\sb{0.4}$BiO$\sb3$, at $\approx$500ppm paramagnetically shifted from that of the parent compound, BaBiO$\sb3$ (350ppm). The relaxation behavior of the bismuthate and plumbate superconductors are also Korringa like, but the nuclear spin-lattice relaxation times are much longer than those in the cuprates, indicating that bismuthates are much less metallic. The plots of (T$\sb1$T)$\sp{1 \over 2}$ and the Knight shift with T$\sb{\rm c}$ suggest that the bismuthate and plumbate superconductors follow a relationship like the BCS relationship between N(E$\sb{\rm F}$) and T$\sb{\rm c}$, while the (T$\sb1$T)$\sp{1 \over 2}$ values of the cuprates do not vary significantly with T$\sb{\rm c}$.
• Chapter 8 shows that for the $\sp{135,137}$Ba NMR of YBa$\sb2$Cu$\sb3$O$\sb7$, the magnetic shift is small (0.06 $\pm$ 0.2%) and that the electric field gradient tensor is $\pm$(7.2, $-$0.7, $-$6.5) in units of 10$\sp{21}$Vm$\sp2$, with the largest component perpendicular to the crystallographic c-axis.

Type of Resource

text

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Copyright and License Information

Copyright 1991 Yang, Shengtian

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