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N.M.R. study on single crystals of lead and thallium Schratter, Jacob Jack

Abstract

A study of the Knight shift and indirect coupling constants has been carried out on single crystals of lead and thallium. In lead, the exchange-narrowing model of Anderson and Weiss has been shown to be inapplicable. The angular variation of the second moment has established that the contribution from the second-nearest neighbours is very small, and values are obtained for the pseudoexchange and pseudodipolar coefficients for the first-nearest neighbours, which are in good agreement with the theoretical values of Tterlikkis, Mahanti, and Das. A small anisotropy of the Knight shift which has the expected angular dependence has been observed for the first time in a cubic metal, In thallium, the isotropic Knight shift decreases with increasing temperature, showing a nonlinear variation which is most pronounced between 50° and 150°K. This behaviour is correlated with the anomalous pressure dependence of the superconducting transition temperature at low pressures. From the superconducting data on pure Tl and Tl-Hg alloys we extract the volume dependence of the electronic density of states, and use it to derive the temperature dependence of the Knight shift. The results agree well with the observed data. We have measured the angular dependence of the linewidth in thallium and related it to the second moment to estimate the pseudodipolar and pseudoexchange contributions for the first two shells of neighbours. These values are very different, despite the fact that both shells have nearly the same radius, and this establishes experimentally for the first time an orientation dependence of the indirect coupling constants.

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