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Register a new userRaman spectroscopy study of NaxCoO2 and superconducting NaxCoO2.yH2O
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The Raman spectra of the parent compound NaxCoO2 (x=0.75) and the superconducting oxyhydrates NaxCoO2.yH2O with different superconducting temperatures (Tc) have been measured. Five Raman active phonons around 195 cm-1 (E1g), 482 cm-1, 522 cm-1, 616 cm-1 (3E2g), 663 cm-1 (A1g) appear in all spectra. These spectra change systematically along with the intercalation of H2O and superconducting properties. In particular, the Raman active phonons (A1g and E1g) involving the oxygen motions within the Co-O layers show up monotonous decrease in frequency along with superconducting temperature Tc. The fundamental properties and alternations of other active Raman phonons in the superconducting materials have also been discussed.
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The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed and compared to experimental results from Co3O4 single crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer of Co3O4 easily forms that leads to the observation of an intense phonon around 700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved crystals from optical floating zone ovens do not show such effects and have a high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96, 167204 (2006)]. We discuss the relation of structural dimensionality, electronic correlations and the high frequency phonon cut-off of the thermolectric cobaltite.
We synthesized Nax(H3O)zCoO2yH2O samples with various Na/H3O ratios but with the constant Co valence of s = +3.40, and measured their magnetic properties to draw phase diagrams of the system. The superconductivity is very sensitive to the Na/H3O ratio. With varying x under fixed s of +3.40, magnetically ordered phase appears in the intermediate range of x sandwiched by two separated superconducting phases, suggesting that the superconductivity is induced by moderately strong magnetic interactions. In the vicinity of the magnetic phase, transition from the superconducting state to the magnetically ordered state was induced by applying high magnetic field. This transition is of the second order, at least, above 1.8 K. The upper-critical field is expected to be much higher than the Pauli limit for a phase located far away from the magnetic phase regarding the Na/H3O ratio.
Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $sim$+3.48, which was compared with a previously obtained one of $sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.
Here we present results of accurate wet-chemical redox analyses, revealing that the oxidation number of cobalt in the newly discovered CoO2-layer superconductor, NaxCoO2 yH2O (x ~ 0.36, y ~ 1.3) is ~ 3.46. This value is significantly lower than the one (~ 3.64) expected on the basis of the value determined for the Na to Co stoichiometry ratio, x. The lower-than-expected value for the Co oxidation state may be alternatively explained by presence of oxygen vacancies or excess protons. Moreover, the result implies that rather than an electron-doped Co(IV) lattice the phase should be considered as a hole-doped Co(III) lattice.
We synthesized powder samples of Na$_{x}$CoO$_{2}cdot y$H$_{2}$O changing the volume of the water in the hydration process, then investigated their superconducting properties,. It was proved that the volume of water is one of key parameters to obtain a single phase of Na$_{x}$CoO$_{2}cdot y$H$_{2}$O with good superconducting properties. The transition temperature, $T_{c}$, of the sample changed gradually while it was stored in the atmosphere of 70% humidity. Superconducting behavior under high magnetic field was very sensitive to $T_{c}$. $H_{c2}$ of a high quality sample with high $T_{c}$ seemed very high.
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