Mai Yokoi

59Co-NMR Knight Shift of Superconducting NaxCoO2·yH2O

Layered Co oxide Na x CoO 2 · y H 2 O with the superconducting transition temperature T c =4.5 K has been studied by 59 Co-NMR. The Knight shift K estimated from the observed spectra of a powder sa...

Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0:5CoO2 —NMR and Neutron Diffraction Studies—

Data taken by various methods including NMR/NQR and neutron scattering are presented for Na x CoO 2 , particularly Na 0.5 CoO 2 . Attention has also been paid to the x dependence of the electronic nature of this system. The pseudo-gap-like behavior observed in the region of x < x c ∼0.6 is emphasized. For samples with x ≤0.6, signals from two kinds of Co sites with different quadrupole frequencies ν Q of ∼4.1 and ∼2.8 MHz were observed, and the former ν Q , which is nearly equal to that of the superconducting system Na 0.3 CoO 2 · y H 2 O, becomes dominant with decreasing x to 0.3. For Na 0.5 CoO 2 , the Co sites with the larger ν Q have larger magnetic moments. They align antiferromagnetically at T c1 ∼87 K with their direction within the plane, while the Co sites with the smaller ν Q have smaller moments. They align in the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T -dependence, indicating that the existence of these sites is not due to macroscopic ...

59Co NQR study on superconducting NaxCoO2.yH2O

Layered Co oxide Na x CoO 2 · y H 2 O with a superconducting transition temperature T c =4.5 K has been studied by 59 Co NQR. The nuclear spin relaxation rate 1/ 59 T 1 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 ∼0.8 T c . Detailed comparison of the 1/ T 1 T values and the magnetic susceptibilities between Na x CoO 2 · y H 2 O and Na x CoO 2 implies that the metallic state of the former system is closer to a ferromagnetic phase than that of the latter. These experimental results impose a restriction on the mechanism of the superconductivity.Layered Co oxide Na x CoO 2 · y H 2 O with a superconducting transition temperature T c =4.5 K has been studied by 59 Co NQR. The nuclear spin relaxation rate 1/ 59 T 1 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 ∼0.8 T c . Detailed comparison of the 1/ T 1 T values and the magnetic susceptibilities between Na x CoO 2 · y H 2 O and Na x CoO 2 implies that the metallic state of the former system is closer to a ferromagnetic phase than that of the latter. These experimental results impose a restriction on the mechanism of the superconductivity.

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

The temperature ( T ) dependence of the 59 Co-NMR Knight shifts K of Na 0.3 CoO 2 ·1.3H 2 O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H ∥ a b plane and H ∥ c -axis, respectively. For both directions of H , the shift K decreases below the superconducting transition temperature T c ( H ) with decreasing T , indicating that the superconducting electron pairs are in the singlet state. The upper critical fields H c2 ( T ) determined from the K ( H )– T curves are found to be consistent with the values reported from the resistivity measurements for both directions of H .

Impurity Effects on the Superconducting Transition Temperature of NazCoO2·yH2O

Superconducting transition temperature T c of Na z Co 1- x M x O 2 · y H 2 O has been studied for M = Ir and Ga, which can substitute for Co. It has been found that T c is suppressed by the M-atom doping. The decreasing rates |d T c /d x | are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the T c suppression.

Magnetic Properties of Superconducting Cobalt Oxides NaxCoO2·yH2O

Studies of the NMR Knight shift K of Na 0.3 CoO 2 · y H 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 –χ 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 χ 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...

Co-NMR Knight Shift of NaxCoO2·yH2O Studied in Both Superconducting Regions of the Tc–νQ3 Phase Diagram Divided by the Nonsuperconducting Phase

In the temperature ( T )–ν Q3 phase diagram of Na x CoO 2 · y H 2 O, there exist two superconducting regions of ν Q3 separated by the nonsuperconducting region, where ν Q3 is usually estimated from the peak position of the 59 Co-NQR spectra of the ±5/2↔±7/2 transition and well-approximated here as \(\nu_{\text{Q3}}\cong 3\nu_{\text{Q}}\), ν Q being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59 Co-NMR Knight shift ( K ) for a single crystal in the higher-ν Q3 superconducting phase and found that K begins to decrease with decreasing T at T c for both magnetic field directions parallel and perpendicular to CoO 2 -planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide ν Q3 regions, but is suppre...

Magnetic and Metal–Insulator Transitions in β-Na0.5CoO2 and γ-K0.5CoO2 –NMR and Neutron Diffraction Studies–

Co-oxides β-Na 0.5 CoO 2 and γ-K 0.5 CoO 2 have been prepared by the Na de-intercalation from α-NaCoO 2 and by the floating-zone method, respectively. It has been found that successive phase transitions take place at temperatures T c1 and T c2 in both systems. The appearance of the internal magnetic field at T c1 with decreasing temperature T indicates that the antiferromagnetic order exists at T ≤ T c1 , as in γ-Na 0.5 CoO 2 . For β-Na 0.5 CoO 2 , the transition temperatures and the NMR parameters determined from the data taken for magnetically ordered state are similar to those of γ-Na 0.5 CoO 2 , indicating that the difference of the stacking ways of the CoO 2 layers between these systems do not significantly affect their physical properties. For γ-K 0.5 CoO 2 , the quantitative difference of the physical quantities are found from those of β- and γ-Na 0.5 CoO 2 . The difference between the values of T c i ( i =1 and 2) of these systems might be explained by considering the distance between CoO 2 layers.

T–3νQ Phase Diagram of Hydrated Cobalt Oxides NaxCoO2·yH2O

The superconducting transition temperatures of Na x CoO 2 · y H 2 O prepared by various methods have been found to exhibit the systematic dependence on the nuclear quadrupole frequency ν Q . In the...

NMR Studies of Successive Phase Transitions in Na0.5CoO2 and K0.5CoO2

59 Co- and 23 Na-NMR measurements have been carried out on polycrystalline and c -axis aligned samples of Na 0.5 CoO 2 , which exhibits successive transitions at temperatures T = 87 K (= T c1 ) and T = 53 K (= T c2 ). 59 Co-NMR has also been carried out on c -axis aligned crystallites of K 0.5 CoO 2 with similar successive transitions at T c1 ∼60 K and T c2 ∼20 K. For Na 0.5 CoO 2 , two sets of three NMR lines of 23 Na nuclei, explained by considering the quadrupolar frequencies ν Q ∼1.32 and 1.40 MHz, have been observed above T c1 , as is expected from the crystalline structure. A rather complicated but characteristic variation of the 23 Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below T c1 and the 59 1/ T 1 – T curves of Na 0.5 CoO 2 and K 0.5 CoO 2 are also discusse...