⁵⁹Co NQR Study on Superconducting Na_xCoO₂·yH₂O

Magnetic Correlation of NaxCoO2and Successive Phase Transitions of Na0.5CoO2–NMR and Neutron Diffraction Studies–

“…Here, we emphasize that in the region of x≤0. 6 19) the Co sites with ν Q =~4 MHz may become important for the discussion of the physical properties related to the superconductivity. Now, in the ordered phase of Na 0.5 CoO 2 (T<87 K), the moment values µ 1 and µ 2 at the two Co sites with ν Q ~4.1 and 2.8 MHz, respectively, can be estimated from the neutron diffraction data to be µ 1 ~0.34±0.03 µ B and µ 2 ~µ 1 /3 at 9.8 K. The possible magnetic structure is either one of those shown in Figs.…”

“…22) In any case, it seems to be important for the complete understanding of the superconductivity of Na 0.3 CoO 2 ⋅yH 2 O, to construct a consistent explanation of various kinds of experimental results. [12][13][14][15][16][17][18][19][20][21][22][23] …”

59Co-NMR Knight Shift of Aligned Crystals and Polycrystalline Samples of Superconducting Na0.3CoO2·1.3H2O

“…In this sense, it is interesting that many theoretical groups have suggested the unconventional nature of the superconductivity with triplet Cooper pairs, [6][7][8][9] though the pairing state induced by the interband interaction may not necessarily be triplet one. 10) In order to investigate the pairing state of the present system, we have carried out NQR-and NMR-studies and reported the T dependences of the NQR longitudinal relaxation rate (1/T 1 ) 11) and the Knight shift 12) : As for the relaxation rate, we reported that a very tiny coherence peak was observed in the 1/T 1 T-T curve. However, because the existence of the coherence peak has not been confirmed in the succeeding measurements carried out for other pure and impurity doped samples, 13) a possibility cannot be excluded that the superconducting order parameter does not have a fully gapped order parameter.…”

Upper Critical Field for Cobalt Oxide Superconductors

“…However, the existing experimental results often contradict each other [4,5] and it remains unclear whether singlet or triplet pairing is responsible for superconductivity in Na x CoO 2 · yH 2 O. If the singlet pairing actually takes place, the resonating valence bond (RVB) state, constructed in terms of electron singlets, would be a straightforward explanation of superconductivity in the cobalt oxide [6,7].…”