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Title:Nuclear magnetic resonance studies of yttrium barium copper oxide in the superconducting state
Author(s):Barrett, Sean Eric
Doctoral Committee Chair(s):Slichter, C.P.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Condensed Matter
Abstract:In this thesis we report measurements of the $\sp{63}$Cu Knight shift in the superconducting state for the plane (Cu(2)) and chain (Cu(1)) sites in YBa$\sb2$Cu$\sb3$O$\sb7$. We also have measured the temperature and field dependent $\sp{63}$Cu(2) nuclear spin relaxation rates ($\sp{63}$W1$\alpha$) in the superconducting state.
Our determination of the $\sp{63}$Cu Knight shift below T$\sb{\rm c}$ compensated for the diamagnetic shielding of our sample by using $\sp{89}$Y NMR as an internal field marker. The $\sp{89}$Y resonance was observed in the superconducting state using the Carr-Purcell-Meiboom-Gill pulse sequence to enhance the signal-to-noise ratio.
We have interpreted our Knight shift data within a generalized Bardeen-Cooper-Schrieffer (BCS) pairing theory, and find that a spin-singlet pairing state is strongly favored by these data. The data are consistent with either an orbital s-wave or an orbital d-wave pairing state. While we can fit the Cu(2) data with a strong coupling energy gap, the Cu(1) data require a weak coupling gap.
During our measurements of the temperature dependence of the Cu(2) spin-lattice relaxation rates in the superconducting state ($\sp{63}$W1$\alpha$, where $\rm\vec Ho\vert~\vert\\alpha$), we discovered that the anisotropy ratio $\sp{63}$W1a/$\sp{63}$W1c, which was essentially independent of temperature in the normal state, drops sharply just below T$\sb{\rm c}$(77 K $<$ T $<$ Tc). The data which we have measured in the smallest fields possible (Ho $<$ 4.5 kGauss) show that as the temperature is lowered below T $\sim$ 77 K the anisotropy ratio $\sp{63}$W1a/$\sp{63}$W1c starts to increase, eventually exceeding the normal state anisotropy ratio. These low field data have been interpreted by several groups in terms of a generalized BCS pairing state. These groups successfully fit our data assuming a spin-singlet, orbital d-wave pairing state, but are unable to fit our data assuming a spin-singlet, orbital s-wave pairing state.
The application of magnetic field which penetrates the CuO$\sb2$ planes produces a sizeable linear dependence of the relaxation rate ($\sp{63}$W1c) upon the field below Tc.
Issue Date:1992
Rights Information:Copyright 1992 Barrett, Sean Eric
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9215775
OCLC Identifier:(UMI)AAI9215775

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