Magnetic Properties of Superconducting Cobalt Oxides Na_xCoO₂·yH₂O

Abstract

Studies of the NMR Knight shift K of Na 0:3 CoO 2 . yH 2 O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c-axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant A spin estimated from the K-plot. It has also been found that even samples that do not exhibit a significant the Curie-Weiss-like increase of the uniform magnetic susceptibility with decreasing temperature T, exhibit superconducting transition, which indicates that the superconducting Na 0:3 CoO 2 . 1.3H 2 O is not necessarily in the region of proximity of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature T c of the samples prepared by mixing Na 0:7 CoO 2 , H 2 O (or D 2 O) and bromine for 4 h, and separating the resultant powder from the solution by filtration, depends on the elapsed time t after filtration. For these samples, the Curie-Weiss component of estimated at low temperatures increases with t, supporting the idea that the number of lattice imperfections possibly due to oxygen-vacancy formation increases with t. Even for such samples, the superconductivity appears, which seems to exclude the possibility of an anisotropic superconducting order parameter. These results are favorable for full-gapped superconductivity.

Observation of the in-plane ordering of D2O molecules in NaxCoO2·yD2O by neutron diffraction

“…No evidence has been found in the search for the ferromagnetic fluctuations in the superconducting bilayer phase, as reported elsewhere in detail. The present result is consistent with the results of studies of the NMR Knight shift [4,5].…”

Superconductivity of cobalt oxide hydrate, Na (H3O) CoO2· yH2O

“…The magnetic and reentrant superconducting phases appear in some sets of samples for a duration after the synthesis [75,76,[80][81][82] although it is unclear what exactly occurs during this period. With increasing time after the synthesis, SC2, NS, and SC1 phases appear, in this order.…”

Isotope Effect on Superconducting Transition Temperature of NaxCoO2·yH2O

“…[2][3][4] We have carried out various kinds of experimental studies and revealed by measuring, in particular, the 59 Co-NMR Knight shifts that the spin-singlet pairing is realized in this system. [5][6][7] We have also found that the rate of the T c -suppression by non-magnetic impurities is small. 8) These results are simply explained by considering the s-wave superconducting pairs.…”