Oxygen-17 NMR Spin-Lattice Relaxation and Knight Shift Behavior in Bismithate, Plumbate, and Cuprate Superconductors

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We have measured the 17O nuclear-magnetic-resonance spin-lattice relaxation times (T1) as a function of temperature for the CuO2 planar sites in Tl2Ba2CaCu2O8+x, YBa2Cu3O7−x, Bi2Sr2CaCu2O8+x, La1.85Sr0.15CuO4−x, La1.85Ca0.15CuO4−x, and Bi2Sr2CuO6+x, as well as for the oxygen sites in Ba0.6K0.4BiO3, BaBi0.25Pb0.75O3, BaSb0.25Pb0.75O3, and BaPbO3. The CuO2 planar sites in Tl2Ba2CaCu2O8+x, YBa2Cu3O7−x, and Bi2Sr2CaCu2O8+x exhibit close to ideal Korringa behavior in the normal state, characteristic of a good metal. In addition, the Knight shift of the CuO2 planes in these three materials is dominated by a spin contribution that is temperature independent in the normal state and vanishes in the superconducting state. The relaxation times of the BaO and SrO planar oxygens, as well as of the TlO planar oxygens, are much longer than those of the CuO2 planes, and are similar to the values found for the bismuthate and plumbate materials. In both lanthanum cuprates, the frequency shift of the CuO2 plane is temperature dependent in the normal state. Relaxation-rate data, when plotted as a function of ln(Tc), show a clear difference between the bismuthate (plumbate) and cuprate materials, and appear consistent with BCS-like behavior for the copper-free systems.

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Physical Review B



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