June 11, 2003
Layered Co oxide NaxCoO2.yH2O with the superconducting transition temperature Tc =4.5 K has been studied by 59Co-NMR. The Knight shift K estimated from the observed spectra for powder sample exhibits almost temperature(T)-independent behavior above Tc and decreases with decreasing T below Tc. This result and the existence of the coherence peak in the spin-lattice-relaxation-rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s-wave symmetry is realized in NaxCoO2.yH2O. Differences of the observed behaviors between the spectra of the non-aligned sample and the one aligned in epoxy adhesive by applying the external magnetic field are discussed.
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July 1, 2005
The superconducting state of NaxCoO2.yH2O with three CoO2 layers in a unit cell has been studied by 59Co-NMR. The Knight shift measured for a peak of the NMR spectra corresponding to the external magnetic field H along one of the principal directions within the CoO2 plane, exhibits a rapid decrease with decreasing temperature T below the superconducting transition temperature Tc, indicating that the spin susceptibility is suppressed in the superconducting phase, at least, for...
June 2, 2003
We report 59Co NMR studies on the magnetically oriented powder samples of Co-oxide superconductors NaxCoO2yH2O with Tc ~ 4.7 K. From two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated by the magnetic field dependence of second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie-Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility chi. From the analy...
March 22, 2008
In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist two superconducting regions of nuQ3 separated by the nonsuperconducting region, where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for a sin...
December 17, 2004
Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H // ab plane and H //c-axis, respectively. For both directions of H, the shift K decreases below the superconducting transition temperature Tc(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The upper critical fields Hc2(T) d...
March 11, 2004
The spin contributed Knight shift $^{59}K_{s}$ of $^{59}\mathrm{Co}$ in the triangular lattice superconductor, $\mathrm{Na_{0.35}CoO_{2} \cdot 1.3H_{2}O}$, has been estimated by NMR experiments in an oriented 2-dimensional powder sample. The Knight shift in the paramagnetic state was estimated by $\delta\nu/(\gamma\mathrm{_{N}}H_{i})$ vs. $(\gamma\mathrm{_{N}}H_{i})^{-2}$ plot at various temperatures. A Knight shift vs. magnetic susceptibility diagram revealed that at least 0...
February 14, 2006
Studies of the NMR Knight shift K of Na0.3CoO2.yH2O 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 Aspin estimated by t...
May 28, 2003
Layered Co oxide NaxCoO2.yH2O with a superconducting transition temperature Tc =4.5 K has been studied by 59Co NQR. The nuclear spin relaxation rate 1/59T1 is nearly proportional to temperature T in the normal state. In the superconducting state, it exhibits the coherence peak and decreases with decreasing T below ~0.8Tc. Detailed comparison of the 1/T1T values and the magnetic susceptibilities between NaxCoO2.yH2O and NaxCoO2 implies that the metallic state of the former sys...
November 17, 2005
The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-...
May 9, 2006
We report a ^{59}Co Knight shift measurement in a single crystal of the cobalt oxide superconductor Na_{x}CoO_2\cdot1.3H_2O (T_c=4.25 K). We find that the shift due to the spin susceptibility, K^s, is substantially large and anisotropic, with the spin shift along the a-axis K^s_a being two times that along the c-axis K^s_c. The shift decreases with decreasing temperature (T) down to T\sim100 K, then becomes a constant until superconductivity sets in. Both K^s_a and K^s_c decr...
June 9, 2008
We report ^{59}Co NMR studies in single crystals of cobalt oxide superconductor Na_{0.42}CoO_{2}\cdot1.3H_{2}O (T_c=4.25K) and its parent compound Na_{0.42}CoO_{2}. We find that both the magnitude and the temperature (T) dependence of the Knight shifts are identical in the two compounds above T_c. The spin-lattice relaxation rate (1/T_1) is also identical above T_0 \sim60 K for both compounds. Below T_0, the unhydrated sample is found to be a non-correlated metal that well co...