¹⁷O NMR Measurements on Superconducting Na_0.35CoO₂·yH₂O

Abstract

An ¹⁷O NMR measurement was performed on nonoriented polycrystalline Na_0.35CoO₂·yH₂O with superconducting (SC) transition temperature T_c=4.6 K. A weak temperature dependence was observed in the Knight shift at the O site (¹⁷K). The spin part of ¹⁷K (¹⁷K_spin) is estimated from the plot of ¹⁷K against bulk susceptibility χ. The ¹⁷K_spin decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin–lattice relaxation rate 1⁄T₁ at the O site ¹⁷(1⁄T₁) shows a good scaling with 1⁄T₁ at the Co site ⁵⁹(1⁄T₁). This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between ¹⁷(1⁄T₁T) and ¹⁷K_spin and between ¹⁷(1⁄T₁T) and ⁵⁹(1⁄T₁T) show the development of incommensurate fluctuations at q∼0 other than q=0 below 30 K. A clear indication of ferromagnetic correlations at q=0 was not observed from the present ¹⁷O NMR studies.