Caves, Laurence Wilfred John (1992) An Investigation of high Tc superconductivity in the doped YBa2Cu3O7 and Ln2CuO4 (Ln=La, Nd) systems by neutron scattering and nuclear magnetic resonance. PhD thesis, University of Warwick.

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Abstract

Neutron scattering and nuclear magnetic resonance are excellent probes of the physical properties of the high Tc Superconductors. Both techniques have been used in this work.

La2-xBaxCu04 [214] has a maximum Tc of ~30K and was one of the first of the high Tc materials to be identified. The structure of the un-doped parent compound has been known for some time so any structural changes as a result of Ba doping might also be associated with the onset of superconductivity. A high resolution neutron powder diffraction study is reported for a range of doping and temperatures (especially in the region of Tc). The parent compound is confirmed to be tetragonal at room temperature; space group I4/mmm. For Ba doping near to x=0.15 a sequence of phase transitions is observed: tetragonal (I4/mmm) at room temperature, orthorhombic (Abma) below ~250K and returning to tetragonal (P42/ncm) below -100K, but co-existing with the orthorhombic phase to low temperatures. Refined structural cell parameters are given for each doping at selected temperatures. The reasons for these changes and their possible link with the onset of superconductivity is discussed.

Nd2-xCexCu04 has been reported to be an n-type superconductor (in contrast to other high Tc compounds) with a maximum Tc of ~24K. It has a similar structure to La[214] and has also been the subject of a high resolution neutron powder diffraction study. The range of doping was more restricted than in the Ba doped La[214] so as to match the range for which superconductivity occurs. The temperature dependence of the structure was looked at for each x in the vicinity of Tc. No structural phase transitions were resolved in any of the samples — all remained tetragonal (I4/mmm). A smooth decrease in lattice parameters a and c was observed as a function of temperature. Refined cell parameters are given at selected temperatures for each x. The contrast with La[214] and the possibility of phase separation are discussed.

The only magnetic rare-earth (R) to suppress superconductivity upon substitution for Y in YBa2Cu3CO7 [123] (Tc -90K) is Pr. It was initially suggested that this could be explained by a 4+ Pr valence resulting in Cu-O plane hole filling. A high resolution inelastic neutron scattering study was undertaken to determine the crystal electric field (CEF) spectra of Pr[123] and mixed Y/Pr[123]. A detailed theoretical analysis has determined that the true valence is Pr3+ and the reasons for the loss of superconductivity are considered in light of this. The refined CEF parameters are listed and are used to calculate both the magnetic susceptibility and specific heat of Pr[123].
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Zn/Ga have been shown to substitute onto the Cu-O planes/chains of Y[123]. Tc reduces more rapidly with doping for Zn than for Ga. High resolution NMR was used to determine the changes in shift and relaxation time of the 89Y resonance for a range of doping of each element. The two sets of data are in sharp contrast and the analysis of the results indicates that Zn doping on the planes produces fluctuating, localised magnetic moments which may be linked to the marked suppression of superconductivity.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Superconductivity, Neutrons -- Scattering, Nuclear magnetic resonance
Official Date:	September 1992
Dates:	Date Event September 1992 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Paul, Don McK
Format of File:	pdf
Extent:	xii, 131 leaves : illustrations
Language:	eng

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