Abstract
We report an NQR-NMR study of the chain copper and apex oxygen in the high-temperature superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$. Above ${T}_{c},$ the temperature dependences of the Cu spin-lattice relaxation rate and the Knight shift are quantitatively described in the framework of the one-dimensional electron gas model, where the anisotropy and the correlation of the chain electron system are taken into account. Electric-field-gradient fluctuations due to charge carriers in the chains contribute partly to the chain Cu relaxation and dominate the apex oxygen relaxation. The different temperature dependences of the magnetic and quadrupolar relaxation rates of the chain Cu seem to confirm the separation of low-energy spin and charge excitations (spin-charge separation), expected in the framework of the one-dimensional electronic gas model. Finally, we present experimental evidence that below ${T}_{c}$ the Cu-O chains become superconducting through the proximity effect.
Published Version
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