Existence of large antiferromagnetic spin fluctuations in Ce-doped T'-cuprate superconductors

For the understanding of Ce-doped high-T_c cuprates with the so-called T'-structure, we have performed ^63,65Cu and ¹³⁹La NMR measurements in T'-Pr_1.3−xLa_0.7CexCuO_4+δ single crystals with x = 0.10 and 0.15. The Néel temperature (T_N) was evaluated from the temperature (T) dependence of the ¹³⁹La NMR line width. For x = 0.10, T_N 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, T_N 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. (author)