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Title:Nuclear magnetic resonance studies of YBa2̳Cu3̳07̳ in the superconducting state
Author(s):Barrett, Sean Eric
Doctoral Committee Chair(s):Slichter, C.P.
Department / Program:Physics
Subject(s):nuclear magnetic resonance
Knight shift
nuclear spin relaxation rates
superconducting state
Abstract:In this thesis we report measurements of the 63Cu Knight shift in the · superconducting state for the plane (Cu(2)) and Chain (Cu(l)) sites in YBa2Cu307. We also have measured the temperature and field dependent 63Cu(2) nuclear spin relaxation rates (63Wla) in the superconducting state. The accurate determination of the 63Cu Knight shift below Tc required a precise knowledge of the magnetic field strength inside the bulk of the sample. We have used 89y as an internal field marker to determine the amount of diamagnetic shielding present in our sample below T c· Observation of the 89y resonance in the superconducting state required several unusual NMR techniques, such as using the Carr-Purcell-MeiboomGill 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 temperature dependence of the Cu(l) Knight shift is slightly different from the temperature dependence of the Cu(2) Knight shift. It is possible to fit the data assuming either an orbitals-wave or an orbital d-wave pairing state, but in every case the energy gap seen by the Cu(2) apparently possesses a strong coupling temperature dependence, while the energy gap seen by the Cu(l) is much closer to the weak coupling gap assumed in the original BCS theory. During our measurements of the temperature dependence of the Cu(2) spin-lattice relaxation rates in the superconducting state (63W1 a.; where ~ . 1\ HoI 1-a.), we discovered that the anisotropy ratio 63W1aj63W1c, which was essentially independent of temperature in the normal state, drops sharply just below Tc (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- 77 K the anisotropy ratio 63W1a/63W1c 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 dwave pairing state, but are unable to fit our data assuming a spin-singlet, orbital s-wave pairing state. We also observe a sizeable field dependence of the spin-lattice relaxation rate in the superconducting state, which is much more pronounced when the magnetic field penetrates the Cu02 planes (63W1c) than when the field lies along the Cu02 planes (63W1a). These rates appear to be linearly dependent upon the field, indicating that flux lines may be contributing to the observed relaxation rates.
Issue Date:1992
Genre:Dissertation / Thesis
Rights Information:1992 Sean Eric Barrett
Date Available in IDEALS:2011-05-03
Identifier in Online Catalog:3478448

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