Existence of Large Antiferromagnetic Spin Fluctuations in Ce-Doped T'-Cuprate Superconductors

For the understanding of Ce-doped high-Tc cuprates with the so-called T'-structure, we have performed 63,65Cu and 139La NMR measurements in T'-Pr1.3-xLa0.7CexCuO4+d single crystals with x = 0.10 and 0.15. The Neel temperature (TN) was evaluated from the temperature (T) dependence of the 139La NMR line width. For x = 0.10, TN decreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For x = 0.15, TN of 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. 63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, 63,65Cu NMR spectra had a large electric field gradient. The T dependence of the 63Cu nuclear spin-lattice relaxation rate (1/T1) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in the appearance of superconductivity in the T'-cuprates.