Tadashi Adachi

For the understanding of Ce-doped high-<italic>T</italic>_ccuprates with the so-called T′-structure, we have performed ^63,65Cu and ¹³⁹La NMR measurements in T′-Pr_1.3−<italic>_x</italic>La_0.7Ce<italic>_x</italic>CuO_4+δsingle crystals with <italic>x</italic>= 0.10 and 0.15. The Néel temperature (<italic>T</italic>_N) was evaluated from the temperature (<italic>T</italic>) dependence of the ¹³⁹La NMR line width. For <italic>x</italic>= 0.10, <italic>T</italic>_Ndecreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For <italic>x</italic>= 0.15, <italic>T</italic>_Nof 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 <italic>T</italic>dependence of the ⁶³Cu nuclear spin–lattice relaxation rate (1/<italic>T</italic>₁) 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.