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Register a new userHard x-ray spectroscopy in NaxCoO2 and superconducting NaxCoO2 - yH2O: A view on the bulk Co electronic properties
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Authors
Philippe Leininger
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The electronic properties of Co in bulk Na0.7CoO2 and the superconducting hydrated compound Na0.35CoO2 - y H2O have been investigated by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) using hard x-rays. The XAS spectra at the Co K-edge were measured in both compounds with two different polarization directions. The changes in the XAS spectra upon hydration and their polarization dependence are well accounted for by linear muffin- tin orbital calculations in the local density approximation. The underlying electronic structure indicates the strong hybridization between the Co 3d and O 2p states in both compounds, while the electron localization is enhanced in the hydrated compound due to the increase of the Co-Co interplanar distance. The Co K pre-edge further highlights the splitting of the d band as a result of the crystal field effect and demonstrates the Co valency increase when Na0.7CoO2 is hydrated. The RIXS spectra measured at the Co K-edge show an energy loss feature around 10 eV in both compounds in fair agreement with the calculated dynamical structure factor. The RIXS feature is associated to a damped plasmon excitation.
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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.
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.
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.
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.
We synthesized the five batches of the samples of the novel P3 type superconductor, Na$_{x}$(H$_{3}$O)$_{y}$CoO$_{2}cdot y$H$_{2}$O, by the soft chemical process starting from $alpha$-NaCoO$_{2}$. The chemical and structural properties varied rather widely from batch to batch, with a result that $T_{c}$ varied from 4.6 K to 3.2 K. The magnetic susceptibility above $T_{c}$ shows upturn at low temperature as in the case of the P2 phase. The $T_{c}$ seems to be well correlated to the lattice parameters.
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